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Solar heating options

Hello, I'm in Westchester NY, climate zone 4. I am considering getting a PV system installed. The system will be approx 10k and will cover most if not all of my electrical use during the year.

My current heating is provided by a standard 3 pass boiler (oil), 50 gal indirect hot water tank and Hydronic air-handlers a 4 ton and 1.5 ton which heats 3000 sq/ft. I guess my question is whether there is a potential for me to heat the house by retrofitting the solar heating of water with the hydro-air air-handlers instead of using the current boiler?

I have been reading about a similar system mentioned on the net with no other real specifics for me to elaborate on other than that it can provide hot water for the house, radiant heat or a hydro-air system through the PV with a natural gas backup. I don't know how much power this type of system would draw to produce enough hot water for heating purposes and therefore how much I would have to up the size of the PV system and reduce my electrical to make it possible. Even more important is whether something like this even exist?

Any input would be greatly appreciated.

Asked by Eric Schroeder
Posted Feb 21, 2013 11:57 PM ET
Edited Feb 22, 2013 8:00 AM ET


16 Answers

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Solar houses do exists, plenty. Check the www

You have to make sure that the energy demand is known. Without this information no planner can help you finding a solution.

"Oil of Emmental", watch the video:


Energy demand is expressed in kW (for max. demand)
in kWh (for total demand).

Check pvgis (www) for the potential solar radiation and harvest on your site.

Since electric batteries are still expensive a ST system might be more suitable for a 100% coverage.

Answered by Hein Bloed
Posted Feb 22, 2013 4:42 AM ET
Edited Feb 22, 2013 4:47 AM ET.


In your climate, solar thermal equipment is a very poor investment. The sun doesn't shine much when you need the heat. Every dollar invested in PV will yield more energy savings than a dollar invested in solar thermal.

If you have any extra dollars floating around, don't spend the money on solar thermal. Instead:

(a) Improve your thermal envelope (air sealing work and improved insulation), or

(b) Buy a bigger PV system.

Answered by Martin Holladay, GBA Advisor
Posted Feb 22, 2013 6:02 AM ET


How much do you currently spend for heating ?? ( $$ and or KW/BTU ?? )

What is the total KW of the 10K$ PV system proposed ?

HEIN : please "Since electric batteries are still expensive a ST system might be more suitable for a 100% coverage." explain ST system ? ( solar thermal ?) why do you suggest it in this case?

Answered by Jin Kazama
Posted Feb 22, 2013 11:31 AM ET


What Martin said- the solar thermal required to run at hydro-air temps would be ungodly large and expensive, with a very low financial return.

In addition, whether you overbuild the PV array or not, at current electricity and oil prices, ductless heat pump technology costs half (or less) to run than your existing oil-fired hydro-air system, but whether it is a viable whole-house solution to your heating situation depends on the layout and how much you can improve the thermal performance of the building. Starting with the building is always step-1, but heating even one large zone with a better-grade ductless mini-split can take a large chunk out of the heating oil bill. (And they provide some of the highest efficiency air-conditioning available.)

The hydro-air capacity seems on the large size for a 3000' house too, at ~22 BTU per square foot of conditioned space, for a load that is probably no more than 15BTU/ft and could be under 12BTU/ft. How much oil do you run through in a year? (And if you have a mid-winter oil bill with a "K-factor" number stamped on it, what's the number?) That would give some idea as to how much you might expect to get out of weatherizing, and if you can get the peak load low enough you may be able to use a variable-speed air source heat pump with your duct systems at nearly (but not exactly) the same efficiency as a mid-grade ductless. I suspect the boiler is sized for the hydro-air output to guarantee that the exit air at the registers is always above 110F too, which means it's probably at 100KBTU/hr burner or higher, which loses some efficiency when serving peak loads that are under 50KBTU/hr (which your house almost certainly is, or could be with some envelope upgrades.)

Answered by Dana Dorsett
Posted Feb 22, 2013 3:29 PM ET


Jin Kazama asks:

"HEIN : please "Since electric batteries are still expensive a ST system might be more suitable for a 100% coverage." explain ST system ? ( solar thermal ?) why do you suggest it in this case?"

My understanding was that the OP wanted to cover 100% of the thermal demand with solar energy, hence my recommendation to go for a ST (solar thermal) system.
A 100% thermal coverage with solar energy is cheaper with a ST system compared to the alternative PV system.

The OP has edited his posting in the mean time.

Answered by Hein Bloed
Posted Feb 22, 2013 5:50 PM ET


Hi everyone. Just wanted to say how much I appreciate the input from all of you and how helpful this sight has been for me in the past. It's now been one and a half years since the we finished the renovation to our house. Many of the helpful suggestions from this sight were implemented in the house, especially in regard to the envelope. We took advantage passive solar with a number of windows and used dense pack cellulose, spray foamed rim joists, foam board, air-tight sheet rock, air-tight electrical boxes, Roxul, and acoustical caulk where there might be any vertical loss through the ceiling or walls. The solar system suggested was a 10KW system and I just wanted to make sure I didnt miss anything new before I install the PV and before I convert the boiler to natural gas from oil. Thanks Martin I figured as much. My average so far has been about $3600 for the year in Fuel oil @ $3.50 a gal with about 800 to 1000 used for the year. Last winter was very mild though. I believe I could do better but unfortunately my wife has poor circulation and needs a little extra heat. :-) My electric bill is around $2500 to $3000 for the year at I believe 0.20 cents per KH/hr? We just switched the house to 95% LED lighting, so I'm sure that number will come down ( by the way Cree makes some nice lights). we had a manual J done for the house prior though we found out later it wasn't strictly followed. The boiler is a Biasi B10-4 which can be used for oil or gas and one of the reason I purchased it, besides the good rep. We also didnt have the option for gas yet, but in anticipation and a year and a half of heavy lobbying the neighbors and ConEd we are just getting a 4" main installed on the street. The boiler was sized at 60 KBTU as per the manual J. Dana, I considered heat pumps but was talked out of it by the contractor. The air-handlers were over-sized only by a half a ton from the manual J due only to a lack of a 1 ton for the smaller zone in the basement and the contractors paranoia about the 3.5 being enough. I will concentrate on my attic which is the weak point of the houses envelope for a number of reasons. My intention is to create a 1" channel between the underside of the roof sheathing from sofit vent to ridge poll vent between the rafters to isolate the attic from the outside then closed cell foam over that channel the rest of the rafter depth to try to make the attic as air tight as possible. At least there will a channel for air movement from sofit to ridge vent. I will focus on the envelope some more then. Thanks

Answered by Eric Schroeder
Posted Feb 24, 2013 4:00 PM ET
Edited Feb 24, 2013 10:47 PM ET.


Just read Dr. Joseph Lstiburek article about roof venting. I was wondering about the 2" channel he states as minimum space size. Which of his rules are more important since I live in zone 4. The minimum 2" space or that the insulation above should be equal to or greater than what is in the walls. If I create a 1" channel between my 2x8 rafter I will have more space for insulation which will pretty much equal what I have in the walls. If I increase the channel to a minimum of 2" plus the material over it to make the channel I will have that much less R value between the rafters and probably a little less than the walls? Am I understanding this correctly? I also started to use a 1" gap on a number of bays already. I Would hate to have to start over. What about the dead bays? I have a number of bays that dont start at a soffit vent, but terminate at the ridge vent where the hip intersect. Should I just foam right against the sheathing since there would be no air movement anyway? All of those bays are north facing. Just thinking out loud.

Answered by Eric Schroeder
Posted Feb 24, 2013 11:38 PM ET
Edited Feb 24, 2013 11:41 PM ET.


The Biasi B10-4 has an output of 110,000BTU/hr out,,which would be 3x oversized for my (not so super-insulated 2400' antique, located in US zone 5) house, not 60K, according to the literature:


Unless you have a gia-normous house your heat load at Westchester 99% outside design temps won't be anywhere near the output rating of the boiler. It may not be able to hit it's AFUE numbers without adding heat-purging economizer controls (eg. Intellicon 3250 HW+) . But even if it's running only ~75% as-used efficiency as a gas-burner it'll be cheaper than or comparable-to running heat pumps at current Westchester electricity and gas pricing.

Roofs need larger deck ventilation gaps than walls due to lower convection forces and far more vapor-tight exterior cladding. Leaving only 1" is asking for trouble, especially on north facing bays (==colder average winter temp, and lower convection drive than on aspect that get more direct sun.) With roof pitches of 12:12 or higher you might get away with 1.5", but it still should be fully vented, which is tough in hipped roof designs. For 2x8 raftes on a hipped roof it's usually better to go unvented, with 1-2" of closed cell foam directly on the roof deck (as a semi-permeable non-wicking condensing surface) and filling the rest with fiber, with no interior vapor retarder. That ends up being ~R30 at center-bay with R7 bridging at the rafters, which isn't terrible, even though it doesn't quite meet code. It's significantly better than R21-R23 at the center with a 2" gap, with R5 of rafter bridging, and it works very well. http://energycode.pnl.gov/EnergyCodeReqs/?state=New%20York (R30 is usually acceptible in cathedralized ceilings where R38 is generic code for attics.

For a sanity check on the flash'n'fill approach to roof decks, see:


See Table 3 on page 11, specifically the columns marked 1" ccSPF + spray fiberglass, and 2" ccSPF + spray fiberglass. Since the simulations were done at the full code R values, even with 1" of closed cell you'd be in better shape than the simulation implies, since you'd have a higher foam-R/fiber-R fraction.

Mind you, the IRC has not caught up with this approach, and demands a higher foam-R still, but I trust Straub more than most practitioners to run a WUFI simulation correctly. A flash-inch will do just fine.

If you've been using R25s (uncompressed loft ~8") it'll be just fine compressed into the 6-1/4" nominal space under a flash-inch in a 2x8 rafter. If you've been using low-density R22s (nominal uncompressed loft ~ 6-3/4") it may not be springy enough to really fill the space without compressions or voids. A compressed batt of any density has a higher R/inch than the full-loft tested value, but a lower total value due to the smaller depth. See: www.owenscorning.com/around/insulation/CompressionChart.xls

Ideally you'd be using something rated ~R4/inch or better, like rock wool (Roxul is being sold at box stores now) or high density "cathedral ceiling" batts, which have much better performance over the full temperature range (rather than the ASTM C518 test temp range to which it's labeled), and much better air-retardency characteristics. R28 rock wool batts will compress readily into 6-1/4" space with good spring-back. I'm not sure who is making high-density fiberglass for 2x8 cavities (Owens Corning makes R30Cs for 2x10 framing, but that would be too much to crunch in.)

Alternatively using damp-blown cellulose works, and most damp-spray products meet thermal ignition barrier requirements for the foam at 3" or more (as does the damp-sprayed Spider fiberglass simulated in the RR-1001 document.).

Answered by Dana Dorsett
Posted Feb 27, 2013 5:21 PM ET


BTW: Chapter & verse on unvented roof stackups at full IRC R-values- see section 5.3:


Note, an inch of closed cell foam is only ~R6, but you're not going to the full R49 prescription for zone 4A under IRC2012 either. The simulation in RR-1001 for the zone 4 and zone 5 locations are sufficient for demonstrating the low-risk. When it's time to re-roof adding 2-3" of rigid polyiso above the roof deck would then give you HUGE margin, and bring you closer to the IRC2012 code-R (which is higher than currently required in NY.)

Answered by Dana Dorsett
Posted Feb 27, 2013 5:51 PM ET


Hi Dana, forgot that the manual J sized the boiler around 50 and due to placement of a Modine in the garage to temper it due to two bedrooms above and the possible dormering of the attic in the near future to add plus 600 sq/ft, the contractor pushed the B10-4. I think 3x oversized is a little bit of a stretch, no?
About the 1" channel the roof is approx. 6/12 pitch. Due to the design and a decorative overlapping the original peak there are very few complete bays which actually face North/West. There are a number of dead heads. the side of the roof that would mostly have the channels installed would be would be South/East exposure. If I did continue to install the 1" channel with 1" xps foam board and then spray-foam over that, other than the lower R-value what exactly would "looking for Trouble" be? What are the potential risks?
And if I did spray 1-2" closed cell directly to the sheathing and then fibrous insulation after isnt that a trade off in that spraying directly to the sheathing prevents me from seeing any damage or potential leaks in the roof? My architect recommended to avoid that. Wouldnt the channels even at 1" provide some sort of cooling durong the summer and some type of drying if wet due to leak? Could I use 2" channels and just fur out the rafters deeper to increase R-value? Also, I understand what you are recommending, but an article by Martin entitled " How to Build an Insulated Cathedral Ceiling" he states: " Some builders aren’t satisfied with commercially available vent baffles, so they make their own site-built baffles. According to the International Residential Code (IRC), “A minimum of a 1-inch space shall be provided between the insulation and the roof sheathing and at the location of the vent.” Such a vent space can be created by installing 1 inch by 1 inch “sticks” in the upper corners of each rafter bay, followed by stiff cardboard, thin plywood, OSB, fiberboard sheathing, or panels of rigid foam insulation. Many experts advise that 2 in. deep vent cavities are even better." "Even better" is the statement, not that less would be "trouble". Has that opinion since changed that much? I'm not doubting you, just trying to understand. Thanks again for taking the time to respond.

Answered by Eric Schroeder
Posted Feb 28, 2013 11:46 PM ET
Edited Feb 28, 2013 11:52 PM ET.



Answered by Eric Schroeder
Posted Feb 28, 2013 11:46 PM ET
Edited Feb 28, 2013 11:48 PM ET.


Code-min is indeed still 1" when there is both soffit & ridge venting ( http://publicecodes.cyberregs.com/icod/irc/2012/icod_irc_2012_8_par098.htm ), but in practice there's an argument for more, especially with dead-headed bays, particularly on sections of roof that get little direct sun, and run cooler. (On the sunny sides the average temp of the roof deck is warmer, which helps it purge moisture more quickly.) Minimum is minimum, not optimum. J. Lstiburek is a big proponent of 2" clearances on roof decks everywhere, but I'm pretty comfortable with 1.5" if it has a clear vent path from soffit to ridge. On your dead-end bays, all bets are off, which makes the foamed-roof deck option preferable.

The hazard of undersizing the clearance is mold & rot getting started on the roof deck and rafters.

With or without spray foam on the underside of the roof deck you still won't be seeing leak-damage until you strip the roof. Closed cell foam on the underside of a roof deck tends to isolate the damage to the roof deck near the leak point. That's arguably better than letting leaks drip through, damaging the interior gypsum and making the fiber insulation & rafters moldy.

Answered by Dana Dorsett
Posted Mar 1, 2013 4:05 PM ET


BTW: If it's heating loads other than just the house, and Manual-J on the house was 50K it's probably not 3x oversized the way it would be for my house, but it could be close. Dormering out 600' of attic isn't going to add very much to the total heat load, unless you really cheap out on R-values and have a lot of code-min window area. If a Manual-J came up with 50K, there's no need for a boiler with 110,000BTU/hr output like the Biasi B10-4. (Oversizing by more than 15% from a Manual-J would be a code violation in California, and the B10-4 is 120% oversized for your stated Manual-J!)

Measure the burn-times on it- if a cast iron boiler is not running at least 10 minutes per burn it's unlikely to be hitting it's AFUE numbers. If it's under 5 minutes on any burn it's a short-cycle, and it would be worth installing heat-purge boiler controls to manage thermal mass of the boiler to it's best potential, which will boost the efficiency significantly (gaining back double-digit percentages in fuel use reduction are typical at 2x oversizing). Fancier newer boilers come with a lot of that function built into the controls as a means of keeping the shoulder-season efficiency from falling off the low-duty-cycle-high-standby-loss cliff.

Answered by Dana Dorsett
Posted Mar 1, 2013 4:19 PM ET


Thanks, Dana. I will check to see if it is short cycling. Though the 600 sq/ft might not add much I believe the hydronic Modine in the garage does unfortunately require a lot to run. This tempers the garage all winter since the bedrooms are above. Also, I do believe I would probably spray foam over the dead heads since they are not very many and north facing. It's interesting how a half inch could make that much of a difference in the air channel. Thanks for the insight again.

Answered by Eric Schroeder
Posted Mar 1, 2013 4:47 PM ET



Answered by Eric Schroeder
Posted Mar 1, 2013 4:47 PM ET
Edited Mar 1, 2013 4:48 PM ET.


The Swiss Solarhouse designer "Jenni" has re-designed it's plans , half of the original storage tank capacity seems to be enough:


The design plans are available. NY seems to be better suited for full ST coverage than Switzerland.

Why not going for the tried and tested, Eric Schroeder?
How high or low is the thermal energy demand you're trying to cover? I've asked that before ...

Answered by Hein Bloed
Posted Mar 2, 2013 5:02 PM ET

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