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anyone using a modulating gas furnace?
Last Post 26 May 2010 09:19 AM by greenfin. 20 Replies. |
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jerkylips
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Posts:246
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| 19 Feb 2010 03:57 PM |
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Our builder bid out a couple options - one was geothermal, another was a 98% modulating gas forced air system. Geothermal was roughly $20,000 more before the tax credits, $10,000 more after the tax credits. The next step is to do the heat loss calcs & see where they come out, but we haven't finalized our window selection yet.
Obviously we'll have to wait to see the numbers, but with the goal of getting heat loss as low as possible, there's definitely a possibility that the payback will be very long for geothermal. I'm starting to wonder if it makes more sense to go with something like this system and possibly have some extra budget for other things if needed.
The thing that appeals to me about this system (I didn't know much about them until recently) is that it constantly adjusts the output to match the need. In our case, with low heat loss, it could be running at only 30,000 btu's at most times. Anyone have experience with these? |
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Jere
New Member
Posts:71
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| 20 Feb 2010 02:19 PM |
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Jerkylips
Someone from another forum has a York Affinity modulating furnace... check out Jeff from Wisconsin reply at the link below. How did the HVAC contractors size your system if heat loads weren't done?
http://www.ownerbuilderbook.com/forum/Sizing-furnace-wzones-t4276.aspx
Jere
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Brock
Advanced Member
Posts:521
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| 20 Feb 2010 04:56 PM |
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We have a Bryant evolution and it has a high, med and low setting, it automatically steps through them based on settings, the default was 30 min on low, 30 min on med then on to high. I set it to only run in low and haven't had any issues and as you said that is the most efficient for heating, ours is suppose to be 95% in low. |
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| Green Bay, WI. - 4 ton horizontal, 16k gallon indoor pool, 1.8kw solar PV setup, 3400 sq ft |
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jerkylips
Basic Member
Posts:246
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| 20 Feb 2010 08:25 PM |
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Posted By Jere on 20 Feb 2010 02:19 PM
Jerkylips
Someone from another forum has a York Affinity modulating furnace... check out Jeff from Wisconsin reply at the link below. How did the HVAC contractors size your system if heat loads weren't done?
http://www.ownerbuilderbook.com/forum/Sizing-furnace-wzones-t4276.aspx
Jere
yeah, that was my question too.. I guess they do a "preliminary" for purposes of the bid. I'm reading "preliminary" as "guess". Once we have the rest of the plan finalized (windows, insulation choices, etc) they will do a real heat loss calc. HVAC guy told me that there is about a $200 difference between the 40,000 btu & 60,000 btu models (40,000 is the smallest they make in that model, I guess) so the differences should be minimal. My brother is an HVAC installer & he told me that if we can heat a 2000 sq ft house with a 60,000 btu furnace we're doing pretty good. We'll see.. |
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Como
New Member
Posts:61
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| 21 Feb 2010 01:09 PM |
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A new house should need a lot less than 60,000 |
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jerkylips
Basic Member
Posts:246
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| 24 Feb 2010 04:07 PM |
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Posted By Como on 21 Feb 2010 01:09 PM
A new house should need a lot less than 60,000 your response is pretty vague - elaborate? |
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Como
New Member
Posts:61
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| 24 Feb 2010 04:59 PM |
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I am aiming fro less than 10btu sq ft with 10,500 HDD
So even where I am, unless it is a McMansion, 60,000 is a lot. |
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Dana1
Veteran Member
Posts:1438
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| 24 Feb 2010 05:01 PM |
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Posted By jerkylips on 24 Feb 2010 04:07 PM
Posted By Como on 21 Feb 2010 01:09 PM
A new house should need a lot less than 60,000
your response is pretty vague - elaborate?
I can't speak for Como, but a "low heat loss" house in most of the lower 48 comes in at a design-day heat load well under 60,000BTU/hr, probably half that or less. My not-so-low heat loss antique in central MA (~7000 HDD climate) has a design-day heat load of under 30,000BTU/hr, and there are newer houses 2x the size of mine (I'm ~2200 square feet) in the neighborhood with lower design day heat loads. To hit a heat load of 60K at my place would take a frighteningly cold minus 70F-ish out! Were I to build it from scratch with better insulation, better windows (with less total glazed area) and better thermal breaks on framing (or SIP construction) it wouldn't break the budget to get it under 20,000- in fact I'll probably hit that within the next decade of ongoing improvements. So unless you live in a large house north of Whitehorse Yukon I guess I'd be agreeing with Como- a new house built to code won't need a 60K burner to keep up, and if built with "low heat loss" in mind should need a lot less than 60,000. What did your heat loss calc come in at, and how confident are you in those numbers? I haven't used a modulating gas furnace, but those with ECM drive continously variable blowers are allegedly QUITE comfortable. To hit 98% efficiency the exit air at the heat exchanger has to be quite cool though (~80F), but with either low air velocity or proper location of registers it won't have a wind-chill effect on the occupants. |
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jerkylips
Basic Member
Posts:246
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| 24 Feb 2010 05:38 PM |
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well, I suspect that the HVAC bid was on "rule of thumb", even though I gave some additional details of the sealing & insulation that we're doing. I will demand a manual j before committing to anything. At the very least, I think that the bid was 'worst case scenario', cost-wise.
As for the house, it's just under 2000 sq ft ranch (I don't consider that a McMansion, not sure if others do). It's a walkout ranch with another 1400 sq ft in the basement that will be finished off at some point, but probably not for a couple years.
We're at about 8000 HDD here.
I appreciate all of the input. I was planning to dig deeper on this anyway, but these are good numbers to be able to throw back at the hvac guy.
It does bring up another potential issue, though. I'm pretty sure that furnace starts at 40,000 btu. If the heat loss calc does come in at 20-30k btu, it would appear to be oversized.. |
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Brock
Advanced Member
Posts:521
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| 25 Feb 2010 09:42 AM |
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Yes and no. This could just be me but my experience with gas furnace’s is that most HVAC people tend to oversize them. My guess is you don't really want a forced air system having to run 24x7 to keep the house at temperature. We were in the same boat when we built. Even though we had radiant geothermal they put in a natural gas furnace (we wanted forced air AC as well) and the smallest efficient furnace was what we got, and it is 60-120k btu's. Again I have our set to only run in 60k mode and never had a problem, even when it was -10F and 30 mph winds out of the north.
I don't know how small they make gas furnaces, but they seem to be so close in price between sizes they the HVAC people tend to oversize them. I think part of that comes from people setting their thermostats down to 55F when they are gone and expecting the house to warm back up to 70F in less than an hour with the furnace running. |
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| Green Bay, WI. - 4 ton horizontal, 16k gallon indoor pool, 1.8kw solar PV setup, 3400 sq ft |
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jerkylips
Basic Member
Posts:246
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| 25 Feb 2010 10:14 AM |
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Posted By Brock on 25 Feb 2010 09:42 AM
Yes and no. This could just be me but my experience with gas furnace’s is that most HVAC people tend to oversize them. My guess is you don't really want a forced air system having to run 24x7 to keep the house at temperature. We were in the same boat when we built. Even though we had radiant geothermal they put in a natural gas furnace (we wanted forced air AC as well) and the smallest efficient furnace was what we got, and it is 60-120k btu's. Again I have our set to only run in 60k mode and never had a problem, even when it was -10F and 30 mph winds out of the north.
I don't know how small they make gas furnaces, but they seem to be so close in price between sizes they the HVAC people tend to oversize them. I think part of that comes from people setting their thermostats down to 55F when they are gone and expecting the house to warm back up to 70F in less than an hour with the furnace running. Hey Brock, What you're saying was my initial thought too, but I've been second-guessing it. The good thing about these modulating furnaces is that they gradually ramp up based on need. According to the Luxaire site, they start at 30% of max & increase in 1% increments. I'd guess that most times it would run at or near the 30%. That would mean operating at 18,000 btu for a 60,000 btu unit or 12,000 btu for a 40,000. This may be a stupid analogy, but I think of those cars that have V8 engines but the onboard computers turn off 4 of the cylinders under light load, so the gas mileage increases. Under normal circumstances it will probably operate well under the 60,000 btu's, but I still have the "horsepower" if I need it. 40,000 is the smallest size they make in this unit, so if I wanted to go smaller I'd have to go a different direction. On another note, I had another company bid the hvac system. I gave them all of the "stats" about orientation, insulation, sealing, etc. They came back with a recommendation of an 80,000 btu system! Our last house was a 50+ year old 1000 sq ft ranch with essentially NO insulation. When we had to replace the furnace about 2 years ago, we put in a 45,000 btu 95% goodman. The house was comfortable all winter, & that was one of the coldest in many many years. If 45,000 btu's kept that house comfortable, 80,000 for our new house seems crazy.. (for the record, goodman has gotten kind of a bad rap over the years but we were very happy with ours - ordered a package deal online & got the furnace & AC for $2100!) |
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Dana1
Veteran Member
Posts:1438
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| 25 Feb 2010 02:51 PM |
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Posted By Brock on 25 Feb 2010 09:42 AM
Yes and no. This could just be me but my experience with gas furnace’s is that most HVAC people tend to oversize them. My guess is you don't really want a forced air system having to run 24x7 to keep the house at temperature. We were in the same boat when we built. Even though we had radiant geothermal they put in a natural gas furnace (we wanted forced air AC as well) and the smallest efficient furnace was what we got, and it is 60-120k btu's. Again I have our set to only run in 60k mode and never had a problem, even when it was -10F and 30 mph winds out of the north.
I don't know how small they make gas furnaces, but they seem to be so close in price between sizes they the HVAC people tend to oversize them. I think part of that comes from people setting their thermostats down to 55F when they are gone and expecting the house to warm back up to 70F in less than an hour with the furnace running.
That 24/7 cycle literally NEVER happens, even on 20% UNDERSIZED systems! Peak loads only occur for 1-3% of all hours in a heating season, typically only 2-5 hours in any one day, usually during the wee hours before dawn of the coldest nights of the year. By noon of that very same day the load is usually more like 50-60% of the peak, and often it's below 20%. Longer cycles of lower air volume/velocity blower are much more comfortable than rapid cycles with inherent overshoot of oversized systems (the "scorched air" scenario.) AFUE testing assumes oversizing by a factor of 1.7 at which the low mass of hot air furnaces means it's still on the flat-part of the curve- the efficiency is still pretty close to the steady-state efficiency, most of the season. Manual-J tends to oversize by 25-35% of reality, and oversizing Manual-J by 15% means it'll still pretty much deliver it's AFUE numbers (maybe even slightly better.) But oversizing Manual-J by 100% (which is all too typical), means you're nearly 3x oversized for the actual peak load, and both efficiency & comfort suffer. In CA under Title 24 2008 over or under sizing furnaces by more than 15% of Manual-J is actually prohibited (!). Of course, the heat loss numbers are only as good as whoever is doing the measuring, and that can be fudged, but it's still way better than what has been traditionally practiced in the HVAC trades. On highly efficient building envelopes hot air furnaces are all oversized- they just don't make 'em small enough. When looking at multi-stage or modulating furnaces, the more important number is the minimum modulation or lowest-stage firing rate. Some will have low-fire down in the 30K range, but I'm trying to think of any that are below that. The NTI Matrix goes down to ~15K, and is a full-on ventilation & hot water heating appliance as well but IIRC doesn't have any air-conditioning capacity. The high-firing rates of multi-stage furnaces buy you quicker recovery from deep setbacks, but is generally less efficient than running them only in low-fire mode- the lower the better. If your true design-day heat load is under 20K it may be easier/better/cheaper/more-efficient/more comfortable to use a small hydronic coil running off the home's hot water heater in a variable-speed air handler, making it somewhat like an NTI-Matrix without the heat recovery ventilation. But this again becomes a design problem for the HVAC folks, and they can oversize that too... (sigh). |
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jerkylips
Basic Member
Posts:246
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| 25 Feb 2010 05:59 PM |
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Posted By Dana1 on 25 Feb 2010 02:51 PM
Posted By Brock on 25 Feb 2010 09:42 AM
Yes and no. This could just be me but my experience with gas furnace’s is that most HVAC people tend to oversize them. My guess is you don't really want a forced air system having to run 24x7 to keep the house at temperature. We were in the same boat when we built. Even though we had radiant geothermal they put in a natural gas furnace (we wanted forced air AC as well) and the smallest efficient furnace was what we got, and it is 60-120k btu's. Again I have our set to only run in 60k mode and never had a problem, even when it was -10F and 30 mph winds out of the north.
I don't know how small they make gas furnaces, but they seem to be so close in price between sizes they the HVAC people tend to oversize them. I think part of that comes from people setting their thermostats down to 55F when they are gone and expecting the house to warm back up to 70F in less than an hour with the furnace running.
That 24/7 cycle literally NEVER happens, even on 20% UNDERSIZED systems!
Peak loads only occur for 1-3% of all hours in a heating season, typically only 2-5 hours in any one day, usually during the wee hours before dawn of the coldest nights of the year. By noon of that very same day the load is usually more like 50-60% of the peak, and often it's below 20%.
Longer cycles of lower air volume/velocity blower are much more comfortable than rapid cycles with inherent overshoot of oversized systems (the "scorched air" scenario.)
AFUE testing assumes oversizing by a factor of 1.7 at which the low mass of hot air furnaces means it's still on the flat-part of the curve- the efficiency is still pretty close to the steady-state efficiency, most of the season. Manual-J tends to oversize by 25-35% of reality, and oversizing Manual-J by 15% means it'll still pretty much deliver it's AFUE numbers (maybe even slightly better.) But oversizing Manual-J by 100% (which is all too typical), means you're nearly 3x oversized for the actual peak load, and both efficiency & comfort suffer. In CA under Title 24 2008 over or under sizing furnaces by more than 15% of Manual-J is actually prohibited (!). Of course, the heat loss numbers are only as good as whoever is doing the measuring, and that can be fudged, but it's still way better than what has been traditionally practiced in the HVAC trades.
On highly efficient building envelopes hot air furnaces are all oversized- they just don't make 'em small enough. When looking at multi-stage or modulating furnaces, the more important number is the minimum modulation or lowest-stage firing rate. Some will have low-fire down in the 30K range, but I'm trying to think of any that are below that. The NTI Matrix goes down to ~15K, and is a full-on ventilation & hot water heating appliance as well but IIRC doesn't have any air-conditioning capacity. The high-firing rates of multi-stage furnaces buy you quicker recovery from deep setbacks, but is generally less efficient than running them only in low-fire mode- the lower the better. If your true design-day heat load is under 20K it may be easier/better/cheaper/more-efficient/more comfortable to use a small hydronic coil running off the home's hot water heater in a variable-speed air handler, making it somewhat like an NTI-Matrix without the heat recovery ventilation. But this again becomes a design problem for the HVAC folks, and they can oversize that too... (sigh).
I just plugged in my info into a heat loss calculator on builditsolar.com . It came up with around 30,000 btu. Anyone have real-world experience on how accurate that calculator really is? |
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Bruce Frey
Basic Member
Posts:366
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| 27 Feb 2010 05:45 AM |
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Posted By Dana1 on 25 Feb 2010 02:51 PM
Manual-J tends to oversize by 25-35% of reality
I don't follow residential HVAC very closely, but is there some data on this? Is that an intentional, built in, safety factor? Bruce |
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Dana1
Veteran Member
Posts:1438
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| 01 Mar 2010 10:45 AM |
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Posted By Bruce Frey on 27 Feb 2010 05:45 AM
Posted By Dana1 on 25 Feb 2010 02:51 PM
Manual-J tends to oversize by 25-35% of reality
I don't follow residential HVAC very closely, but is there some data on this? Is that an intentional, built in, safety factor?
Bruce
I don't know, but strongly suspect that it's a built-in safety factor. Air leakage isn't as easy to estimate with simple measurements, and whole-wall R-values can vary considerably between best/typical/worst-case installation, etc. Assuming worst-case on the numbers would be prudent from a burner-sizing point of view, and could easily be 25% worse than "typical" losses. The efficiency hit of being only 35% oversized is very small, so it's not an unreasonable fudge-factor (if only installers weren't then adding their own 30-100% fudge-factor on to of it.) I don't know of any hard data on this, but it's been my (very limited and unscientific) experience, as well as the heating pros I've worked with that using the appliance itself in-situ to measure the heat loss via fuel use correllated against weather data most heat-loss calcs overshoot by ~25-35%, and that undersizing by 10-15% per Manual-J won't lead to loss of comfort, even against the 25-year extremes of super-cool weather events. |
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jerkylips
Basic Member
Posts:246
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| 01 Mar 2010 07:40 PM |
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Posted By Bruce Frey on 27 Feb 2010 05:45 AM
Posted By Dana1 on 25 Feb 2010 02:51 PM
Manual-J tends to oversize by 25-35% of reality
I don't follow residential HVAC very closely, but is there some data on this? Is that an intentional, built in, safety factor?
Bruce When we were in our old house, the heating guy came over & did the manual j calc to determine furnace size. He told me that there is a 20% "fudge factor" built into the program to prevent undersizing. If a contractor is adding their own cushion onto that, I can see where you could get very oversized.. |
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erwinzeez
New Member
Posts:1
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| 29 Apr 2010 07:21 AM |
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Posted By jerkylips on 25 Feb 2010 05:59 PM
Posted By Dana1 on 25 Feb 2010 02:51 PM
Posted By Brock on 25 Feb 2010 09:42 AM
Yes and no. This could just be me but my experience with gas furnace’s is that most HVAC people tend to oversize them. My guess is you don't really want a forced air system having to run 24x7 to keep the house at temperature. We were in the same boat when we built. Even though we had radiant geothermal they put in a natural gas furnace (we wanted forced air AC as well) and the smallest efficient furnace was what we got, and it is 60-120k btu's. Again I have our set to only run in 60k mode and never had a problem, even when it was -10F and 30 mph winds out of the north.
I don't know how small they make gas furnaces, but they seem to be so close in price between sizes they the HVAC people tend to oversize them. I think part of that comes from people setting their thermostats down to 55F when they are gone and expecting the house to warm back up to 70F in less than an hour with the furnace running.
That 24/7 cycle literally NEVER happens, even on 20% UNDERSIZED systems!
Peak loads only occur for 1-3% of all hours in a heating season, typically only 2-5 hours in any one day, usually during the wee hours before dawn of the coldest nights of the year. By noon of that very same day the load is usually more like 50-60% of the peak, and often it's below 20%.
Longer cycles of lower air volume/velocity blower are much more comfortable than rapid cycles with inherent overshoot of oversized systems (the "scorched air" scenario.)
AFUE testing assumes oversizing by a factor of 1.7 at which the low mass of hot air furnaces means it's still on the flat-part of the curve- the efficiency is still pretty close to the steady-state efficiency, most of the season. Manual-J tends to oversize by 25-35% of reality, and oversizing Manual-J by 15% means it'll still pretty much deliver it's AFUE numbers (maybe even slightly better.) But oversizing Manual-J by 100% (which is all too typical), means you're nearly 3x oversized for the actual peak load, and both efficiency & comfort suffer. In CA under Title 24 2008 over or under sizing furnaces by more than 15% of Manual-J is actually prohibited (!). Of course, the heat loss numbers are only as good as whoever is doing the measuring, and that can be fudged, but it's still way better than what has been traditionally practiced in the HVAC trades.
On highly efficient building envelopes hot air furnaces are all oversized- they just don't make 'em small enough. When looking at multi-stage or modulating furnaces, the more important number is the minimum modulation or lowest-stage firing rate. Some will have low-fire down in the 30K range, but I'm trying to think of any that are below that. The NTI Matrix goes down to ~15K, and is a full-on ventilation & hot water heating appliance as well but IIRC doesn't have any air-conditioning capacity. The high-firing rates of multi-stage furnaces buy you quicker recovery from deep setbacks, but is generally less efficient than running them only in low-fire mode- the lower the better. If your true design-day heat load is under 20K it may be easier/better/cheaper/more-efficient/more comfortable to use a small hydronic coil running off the home's hot water heater in a variable-speed air handler, making it somewhat like an NTI-Matrix without the heat recovery ventilation. But this again becomes a design problem for the HVAC folks, and they can oversize that too... (sigh).
I just plugged in my info into a heat loss calculator on builditsolar.com . It came up with around 30,000 btu. Anyone have real-world experience on how accurate that calculator really is?
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Clark
Basic Member
Posts:142
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| 01 May 2010 09:17 AM |
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When I was planning the construction of my ICF house (2200 sf ranch with full basement) I, too, pondered the choice and size of my HVAC system. I used HVAC-Calc to do my load analysis and came up with between 20,000 and 30,000 BTU depending on various factors I was considering, e.g., window type, fireplace option, etc.
I have to admit, at the time I was nervous about choosing a modulating boiler (16,000- 46,000 BTU output), thinking it might be undersized. My fears seem laughable now. On sub zero mornings, the boiler will fire on a 30% duty cycle to keep the house at 70 degrees. I think that the only time it modulates up to maximum output is when heating my indirect hot water supply.
I have radiant heat in the basement floor, but forced air on the main level. I paired my boiler with a variable speed air handler with an hydronic heating coil and evaporator cooling coil. The air handler circulates the air continuously at a very low volume and ramps up when there is a call for heat. I love this system because it provides all the advantages of forced air (ventilation, filtering, cooling) without any of the disadvantages (uneven heat, noise.)
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dmaceld
Advanced Member
Posts:904
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| 18 May 2010 02:05 AM |
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Hey jerky,
I know this is coming in rather late but I haven't reviewed the GBT heating forums for awhile. My house is 2000 sq ft, inside the walls, with 6" ICF walls. I use the crawl space for supply plenum and the attic for return. Roof is ~R40 SPF. My heating/cooling is a 3 ton Daikin air to air heat pump with one 36,000 Btuh air handler in the crawl space. This past winter we had night after night of temps between 0 & 10°F, and lots of days and nights below 20 - 30°F around the clock. My wife is a bit cool blooded so our thermo is set on 74F constantly. There are no auxiliary heat strips in the heat pump system. The heat pump kept the house at 74, well maybe 72, even on the coldest nights. I turned on the pellet stove one time just for ambiance on a cool rainy Sunday afternoon. The Daikin spec shows the heat pump output drops to about 27,000 Btuh with outdoor temp at 5F and indoor at 75F. The COP at those temps is about 2.5. The Daikin is a variable speed compressor so it puts out only the the amount of heat needed to maintain the set temperature. Typically during the heating season the HP will come on late in the afternoon and turn off late the next morning. It does not cycle on and off like a gas furnace does. By late morning the house would have enough heat from life activities and solar gain to no longer need input from the HP.
If you want a heating/cooling system that responds linearly with the heating/cooling demand variation throughout the day and season take a look at the Daikin system. At a COP of 3 to 4 it can be less expensive than natural gas. There are some other heat pumps that will operate down to 0°F also, but they aren't constantly variable like the Daikin.
Another thing to keep in mind while you're sizing the heating/cooling system is heat from life activities. Cooking, TV, computers, etc., all add heat to the house. With an energy efficient house that heat can be a significant proportion of the total heat supply. Nearly all the electricity that comes into the house, with the exception of what may be used to heat DHW which goes down the drain, gets converted to heat in the house.
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jerkylips
Basic Member
Posts:246
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| 18 May 2010 09:12 PM |
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thanks for the reply! the furnace install started last week so a little late for this. We did look into geothermal & air source heat pumps but decided against it for a couple reasons. The recent trend is that natural gas prices have been steady or going down, while electric is going up at a pretty steady rate. Between the higher initial costs & the higher electric rates in our area, it was pushing the payback time out a little past my comfort zone. I know that it could change dramatically, but in looking at the extra costs involved, we felt it made more sense to put the money into insulation & air sealing. We're scheduled to be in the house in about 5 weeks, so we'll see how well that plan worked..
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