We are building in New Mexico, elev 6000'. The heated space is about 2200sqft, the passive solar house is built using adobe bricks, is well insulated, etc. We plan to install a 4 panel solar system for heating as the primary source for heat. This will include 2-80gal tanks.

I have a couple of questions:

First, The radiant floor system has been quoted to include a Polaris boiler. This can also be used as a HWH. However, reading in this forum, condensing boilesr such as Peerless and Munchkin have been suggested as alternatives. They are cheaper by almost half compared to the Polaris. We have the solar tanks, so the additional water the Polaris has may not be needed.

Second, My original thought was to use the solar tanks as a preheater for the boiler. If they were hot enough, the boiler would not come on. Most likely only at night. Then the boiler would use a heat exchanger to indirectly heat a glycol based liquid in the floor piping. But I have also seen where the floor can be heated directly or again indirectly by the solar tanks, using the boiler to supplement any heat the tanks would need via circulating pumps.

I have not seen anywhere where it is discussed as to the better configuration. Are heat exchangers less efficient than heating direct. I like the idea of keeping the floor isolated from the other liquids. What is the consensus thought on this.

Bill

You must keep the floor separated from potable water. Only hacks promote "open direct" systems. the minor loss of effectiveness with a heat exchanger is more than outweighed by the protection and long term insurance a heat exchanger provides. Plus if you are going to glycol the system, you have no choice in the matter.

if you have a coil in the solar tank for the heating system, you don't need a pump, and the heating system can be configured to draw heat from the solar tank when needed. I like this configuration, with an electric on demand boost heater for DHW, so you never maintain the tank unless it's with solar, and you can use every single solar BTU collected even if it's not warm enough to heat the home or hot water by itself.

Rob,

I agree with the heat exchangers. I felt this is the best way to go.

Unless I did not understand what you said, you suggest not providing any backup to the solar for heating? I would think several days of cloudy weather would diminish the heat storage to almost zero, and throw in some very cold nights it could get uncomfortable. I agree the on-demand heaters a an option, especially with new construction. I always thought it was necessary to have a source of heat backing up the solar.

Rob is right, Keep your heating isolated from the domestic water via heat exchanger, an electric water heater 40-50 gallons prefed by the solar storage tank, If you use duel coil solar storage tanks you have a lot of options. A condensing boiler can be used as a boost the heating and as well for domestic, also if you plan on getting a lot of your heat load off soloar it may be more effective to add panels and use a small electric boiler as injector for the occasional heat bump.

I'm sorry, I'm not saying you don't need backup for the heating.

the total system we usually recommend is:

storage tank heated by solar, and only by solar. volume is potable, coils for input and heating system extraction.

boost heater for DHW, typically electric on demand.

Primary source for heating separate. this is whatever makes sense in your area for your heat load. If passive solar and active solar really will meet most of your load, it could be almost anything. more commonly, a higher efficiency source is desired using an energy source that is economical in your area.

OK, I think I understand what you guys are advising.

The passive aspects of the house are only guesses at this time and real time experience will tell more. My contractor has suggested not jumping in there with an expensive boiler until we get a feel for what the true demands may be. May last home has a two panel one tank system and that was adequate for all our hot water needs as late a December. Though we did not use it for primary heat, it did support three radiators. So now we intend to double the system as it will be the primary source of heat along with anything the passive side provides.

I like the idea if the condensing boiler. It is economically attractive and as Dan said it too can be used for DHW as well as a booster. LP gas is the cheapest power source for our area, and will be used for the boiler.

Thanks guys,

Bill

the condensing boiler is probably the most expensive possible option. it's a great choice but if you do not have a large heat load in the end, it might be hard to cost justify. of course cost justification is not the only reason to go for efficiency, but if cost is a parameter, that's a tough choice to make blind on a home you think might meet a lot of its load with the sun.

to use a mod/con for domestic means you are either using one with a domestic "on demand" circuit in it (which may not like being fed preheated solar water, and will have limited domestic output, one shower at most in most cases, maybe two low flows with preheat). Or you are maintaining a tank. If your solar meets most of your domestic needs, the cost of the boost heater energy is a non-factor and an electric on demand would be the cheapest and easiest and most effective choice on the DHW side.

Posted By NRT.Rob on 26 May 2010 10:57 AM
the condensing boiler is probably the most expensive possible option. it's a great choice but if you do not have a large heat load in the end, it might be hard to cost justify. of course cost justification is not the only reason to go for efficiency, but if cost is a parameter, that's a tough choice to make blind on a home you think might meet a lot of its load with the sun.

to use a mod/con for domestic means you are either using one with a domestic "on demand" circuit in it (which may not like being fed preheated solar water, and will have limited domestic output, one shower at most in most cases, maybe two low flows with preheat). Or you are maintaining a tank. If your solar meets most of your domestic needs, the cost of the boost heater energy is a non-factor and an electric on demand would be the cheapest and easiest and most effective choice on the DHW side.

Rob, I get what you are saying, and it may well be a good idea and wait and see what the exact demand will be.

That said, which would be the better way to go if we need to boost the temps in the solar tanks for floor heating purposes?

depends on the load. options include tank heaters, gas tankless on demands in some rare cases, another electric booster, mod/con boiler, conventional boiler...

the more heat you need, the more it matters. mod/con boiler is the most efficient choice there in the gas world.

What is the mod part stand for of the mod/con assuming the con part is condensing?

Modulating.

Again,
I would look at something akin to tanks in line.
40 gallon electric domestic Prefed by solar storage(s) tank, tanks would have duel coil heat exchangers, options to extract heating loop, solar panel loop, heat dump...
then the heat loop can have an injector from a mod con or electric boiler (not expensive) to bring up the heat as needed for the in floorwhen there is not solar capacity.

Posted By Blueridgecompany.com on 26 May 2010 11:57 AM
Again,
I would look at something akin to tanks in line.
40 gallon electric domestic Prefed by solar storage(s) tank, tanks would have duel coil heat exchangers, options to extract heating loop, solar panel loop, heat dump...
then the heat loop can have an injector from a mod con or electric boiler (not expensive) to bring up the heat as needed for the in floorwhen there is not solar capacity.

I think you have hit the proverbial nail on the head. Thanks for the input...Bill

Bill:
You might want to look at the Equinox system I just installed one and really works great; it's a plug and play system.

www.sunequinox.com

Posted By Blueridgecompany.com on 26 May 2010 11:57 AM
Again,
I would look at something akin to tanks in line.
40 gallon electric domestic Prefed by solar storage(s) tank, tanks would have duel coil heat exchangers, options to extract heating loop, solar panel loop, heat dump...
then the heat loop can have an injector from a mod con or electric boiler (not expensive) to bring up the heat as needed for the in floorwhen there is not solar capacity.

Dan,

You mentioned as have others, as well as configuration options I have seen, concerning coils in the storage tanks. If I plan to use indirect/closed heating, ie heat echangers with solar and raidiant floor containing glycol, I thought that needing coils in the tanks would not be necessary. Would not the coils be used in a system where the storage tanks are not used for potable water or that the water in the coils is used in an open/direct system?

I guess I am a bit confused about the need or use of the coils. THey are heat exchangers themselves and if I am installing an indirect system, then the coils are not needed. Is this right?

Bill

Interesting, I was looking at this as part of a solution for our needs. It seems that all the Plumbers around here are OK with new ideas as long as it has been proven by at least two prior generations. My best guess is that a tank of no more that 120 gallons would be needed for DHW, installed so that Solar is primary, the propane tank can be used to dump excessive heat.

On the subject of not using potable water as a heat source, I will not as my systems will be using Glycol.

But the gain from the DHW Hot in a highly insulated house with a re-circulating pump could be quite significant depending on the level of pipe insulation.

All Hot Water systems provide some heat, no matter what the end purpose.

Posted By Como on 02 Jun 2010 02:46 AM
On the subject of not using potable water as a heat source, I will not as my systems will be using Glycol.

But the gain from the DHW Hot in a highly insulated house with a re-circulating pump could be quite significant depending on the level of pipe insulation.

All Hot Water systems provide some heat, no matter what the end purpose.

I guess there are always two ways to look at things. I have not sorted it all out yet, But I see the solar side of the system heating two potable storage tanks (120 gals ea.) via an external heat exchanger. But with all solar systems you need a backup heat source, and in New Mexico, I believe it is code (not sure here).  The cost for a boiler is such, that a standard HWH gas or electric (gas in my case) could be used as the backup. My previous system used the solar as a preheater for winter use and this worked fine for just hot water and a bit of radiator heat in the bathrooms. Spring, Summer and Fall, the HWH was bypassed.

I am thinking the HWH would be the less expensive initial plan until we determine what the exact heating needs will be.

Bill 

external heat exchangers have some advantage in modularity of components for lifecycle replacement, and control over sizing.

however they require additional, expensive pumps (though they can usually be small and/or low energy and/or variable speed) and pumping tanks kills stratification. stratification is a big deal when it comes to extracting usable heat out of a solar tank. for that reason, dual coil arrangements with a bottom coil for heat input and a top coil for heat extraction are much more ideal. there can easily be 30 degree differentials top to bottom in a solar tank. Pump it, and you're at the average, in the middle. much less useful for heating.

I would vastly prefer single solar input tanks with two coils. the only argument I've seen against them that was compelling to me was "staged tanks" where you load one up with heat until it's usable for DHW, then flip to a secondary tank. but if you do one of a few "boost heat" methods, there is little benefit to that IMHO, since you can use any heat even if it doesn't cross a magic threshold of "usability".

Posted By NRT.Rob on 02 Jun 2010 08:27 AM
external heat exchangers have some advantage in modularity of components for lifecycle replacement, and control over sizing.

however they require additional, expensive pumps (though they can usually be small and/or low energy and/or variable speed) and pumping tanks kills stratification. stratification is a big deal when it comes to extracting usable heat out of a solar tank. for that reason, dual coil arrangements with a bottom coil for heat input and a top coil for heat extraction are much more ideal. there can easily be 30 degree differentials top to bottom in a solar tank. Pump it, and you're at the average, in the middle. much less useful for heating.

I would vastly prefer single solar input tanks with two coils. the only argument I've seen against them that was compelling to me was "staged tanks" where you load one up with heat until it's usable for DHW, then flip to a secondary tank. but if you do one of a few "boost heat" methods, there is little benefit to that IMHO, since you can use any heat even if it doesn't cross a magic threshold of "usability".

Rob,

If dual coil tank is used, then one would use a direct system with the solar collectors as well as the radiant system, correct? This would save the expense of the additional equipemt such as the heat exhangers and pumps required for an indirect systems but increased expense with the dual coil tank. Also, you would need a drain-back system as well to prevent solar fluid freezing, correct?

Bill