So I just purchased this 'Passive Solar' home in Charlton, MA 01507. I'm having trouble figuring out how to finish the HVAC of the home, as well as wether or not to do Solar Thermal and Electric. Here are some basics of the specs of the house:

• Slab on Grade 58' x 30'
• Passive Solar Design
• House facing almost directly south (Solar consultants say its in the perfect direction)
• Corrugated Metal/Steel Roof
• 1st level is 1700sqft or so of concrete, with radiant. This is being driven by an Steibel Electron Hydro Shark 2 Electric Modulating boiler system.
• 1st level is full ICF foundation.
• Looking at putting R-45 or higher Spray Foam in ceiling (on back of metal roof) and R-30 in 2nd level cavity walls.
• South Face of the home is all sliding windows.
• Currently no heating 2nd level
• Currently no domestic hot water system

Photos:
http://www.flickr.com/photos/reeboo...153381257/

What I'm concerned with the most is the Passive Solar, with Electric Boiler. In my climate/area is that going to be enough to heat the home? I'm being told my HVAC specialists as well as Energy Star that my electric bill will be 3x as much to heat the house, in compared to using a HE Boiler. I don't think they are factoring in the Passive Solar tho. Does anyone know how much BTU energy could be absorbed daily? I'm not sure if I'm asking the right question here either. Other people have told me its going to depend on the performance of the Passive Solar design, but how can I determine that?

I need to make a decision soon wither to keep the current system, or install a $20k HE Boiler w/ Indirect Water tank that would run the radiant floor, new baseboards upstairs and hot water ($20k install). In order to do that I have to get a Propane Tank (no natural gas) and I've heard Propane is getting really expensive! I'm also told I need to put an ERV fresh air system in ($5k). So if anyone has any suggestions, this is driving me nuts really. I don't plan on putting A/C in the house either, I'm hoping just some fans and the ERV system would handle the cooling, as well as the Passive Solar design of the home should keep it cool enough?

Here are a few photos, you can follow the link above to see all of them:



Thanks,
Ryan

Electricity at standard residential retail in central MA has been running 15-20cents/kwh. Do they have special rates for heating customers? (I've not looked into it- I'm on Nat'l Grid.)

A gallon of propane burned in a mod-con boiler at 90% efficiency delivers about 24kwh- worth of heat to the system.

So a gallon of propane at the current MA average price of $2.87 is the equivalent of $2.87/24= ~12 cents/kwh electricity.

That's cheaper, but it'll likely take a coupla centuries to pay for a $20K boiler upgrade on fuel pricing alone if the place is a super-insulated passive home. A heat pump system would pay off quicker, but it may be worth running it on the electric boiler for a season to determine the actual passive solar performance, since this house is SO well insulated. (One good measurement trumps all estimates.) At that point you could calculate the payback on various alternate backup schemes, but a high-efficiency propane boiler just won't be there. OTOH a smallest-in class ~80-85% efficiency propane boiler (or hot water heater) might, at a fraction of the up-front cost of a high-efficiency condensing propane burner.

Keeping the place cool without AC might be possible if you use exterior shading to minimize the solar gain in a high-mass/high-R house. (I can do that with my not-so-high-R house in Worcester.) But an ERV alone will not deal with the substantial latent loads we get in MA. It will need to have at least some mechanical dehumidification to keep the indoors relative humidity under the recommended max of 60% (50% for those with dust mite allergies.) When summertime dew points are high, you'll have to run the ERV at the minimum duty cycle or it'll be introducing humidity into the house that the dehumidifier has to then take out. I have central air, but use it seldom- fewer than 10 hours/year (under 6 hrs most years.)

In the winter you can boost the ventilation rates on the ERV, which will help dry out the house. Keeping the interior RH at the recommended 30% minimum will reduce the likelihood of mold conditions inside the wall cavities, and getting it down that low with an ERV will take many-X the air exchanges than necessary for healthy air.

In central MA, in a very tight very well insulated building without AC and summertime solar gain well controlled you're probably better off with an HRV than an ERV, since you can run it at a lower duty cycle and still control indoor humidity in winter. HRVs are basically the same as ERVs, but operate without a dessicant wheel for humidity transfer. ERVs are more useful in hot humid climates with more than 1000cooling degree-days (CDD) of sensible-load, but in Charleton you're at well under 500CDD- you wouldn't be running a lot of air conditioning even if it weren't well insulated, even less since you are, as long as you kill the passive gain with exterior shading/shades. (Indirect scattered light can be significant in a passive solar house. Even if shaded by overhangs you may need exterior shades to bring the passive gain down May-October.)

Summer night-time ventilation schemes tend to increase indoor humidity in this area. The number of days when it's hot enough that you want to dump heat, yet dry enough that nighttime ventilation can be your primary cooling strategy are fairly few. If you're keeping it below 60% RH indoors the humidity load that you introduce at night can be a larger load than running a small window air-conditioner to handle the sensible load. (At least that's the way it is in my house just a few miles N of you.)

I'm on National Grid as well, I'm not sure. I'm wondering too if they have On and Off-peak rates?

So the propane is cheaper, but really not by much at this point.

As far as insulation, would you say that if I do R-30 in the 2nd level walls and R-45 in the ceiling that would be considered 'well' insulated or do I need more insulation? The original plans called for R-45 ceiling R-19 in 2nd level walls. The Reward ICF foundation is claiming an insulating value of R-32 for the 1st level.

Someone told me about possibly using Mitsubishi A/C wall hung units that are really energy efficient.

You can certainly do some heating with a mini-split heat pump at attractive rates.

Your heat load is likely fairly small. One inexpensive choice would be to take the fireplace out and install a gas stove in its place (or a wood stove for that matter) When you need supplemental heat, fire it up. This should be very low cost (except the cost of the tank and piping). IF you decide to install a gas boiler later, you already have the tank and piping. This gives you a cheaper fuel source then electric, with a simple system.

This will not be as efficient overall as a heat pump. Something like this would work in a modern home. The efficiency is not great compared to a boiler though. http://www.hearthstonestoves.com/gas-stoves/stove-details?product_id=29#stove_information
 
Having a setup with both gas and electric heat sources gives you a lot of flexibility.

Posted By ReeBooT on 20 Apr 2010 06:28 PM
...The Reward ICF foundation is claiming an insulating value of R-32 for the 1st level.

Yup, that is their guesstimated 'Effective' R-Value. In MA, you won't see that in the winter. I believe that the static R-Value is claimed to be R-22.

What's the difference between a Gas Fireplace and Gas Stove? The Fireplace that is in there is a Gas/Wood combo, I can use either or.

Posted By ReeBooT on 20 Apr 2010 06:28 PM
I'm on National Grid as well, I'm not sure. I'm wondering too if they have On and Off-peak rates?

So the propane is cheaper, but really not by much at this point.

As far as insulation, would you say that if I do R-30 in the 2nd level walls and R-45 in the ceiling that would be considered 'well' insulated or do I need more insulation? The original plans called for R-45 ceiling R-19 in 2nd level walls. The Reward ICF foundation is claiming an insulating value of R-32 for the 1st level.

Someone told me about possibly using Mitsubishi A/C wall hung units that are really energy efficient.

Nat'l Grid does offer off-peak rates for residential customers.  But depending on what your background peak-hours load is like it may/may-not pay off for you.  But it's worth investigating- the distribution charges alone drop by over 5cents/kwh for off-peak, and the supplier charges may have similar breaks.  Your peak heating load occurs during the wee hours of the morning, so it's probably fine for heating. 

But cooling peaks will tend to occur at  peak rates but a strategy of pre-chilling during off peak hours the night before predicted swelter can reduce the afternoon & evening cooling loads, should you go with a mini-split heat pump for supplemental heat & AC.  A mini-split will use roughly half the power of electric boiler when it's above 25F, which could make it worthwhile to use  as your primary backup heat during the shoulder seasons, and your AC/dehumidifier during the cooling season.

The R32 claims on the ICFs are only remotely valid when the diurnal temperature swings are going between load/no-load.  The total amount of energy used during winter will be closely linked to it's R22 steady-state R value.  But even then the thermal mass moderates the peak load, which allows the designer to size the system for something between the average than the peak daily loads.  (Even on "design day" when it's 0F at dawn, the average daily temps are typically above 10F.)  Still, R22 clear-wall R-values aren't bad- most stick-built houses in Worcester county still aren't quite that high- it takes 2x6" framing with blown insulation (not batts) and and inch exterior foam to get to R22, so while less than super-insulated, it's above-average for this area, if a bit low for a solar home.  Since average daily temps in summer are usually quite moderate, having the massive wall to moderate the load toward the daily average keeps the active cooling to latent-load only.  The monthly-average temps for July & August in Worcester are still below 70F despite peaks well above 80F.  With a high-mass well insulated house, as long as you've killed the passive gain it takes a heat wave before there's any sensible AC load.

R30 on the second story with R45 ceiling is definitely worthwhile from both a heating & cooling P.0.V., and R19 batts/no-foam would be way sub-par for getting the most out of the passive solar.  If you go with 2x6" studs, blown cellulose or fiberglass + 1" of exterior foam sheathing would get you to the R22 range, but making that 2" of exterior foam would get you to R28+ clear-wall.  Alternatively, 8.5-9" SIPs get you there.  (By contrast, my 1920s antique house with full-dimension 2x4 framing & dense packed cellulose only runs ~ R14 for clear-wall R-value.  When I re-side the place it'll be getting 3" of iso on the exterior to bring it above R30.)

The answer is it depends, traditional fireplaces are often break even as far as heat produced vs heat lost. Wood stoves are designed for the heat output, not just aesthetics. Some look like a fireplace but heat like a woodstove.
 Basically you need to find out if what you have, and if it can be used efficiently as a heat source for the house. Having both gas and wood capability is interesting. Makes me think it could function with wood when you are home, but be its own backup with gas when you are away? The other thing that is interesting is that with lots of thermal mass, you may be able to run the woodstove at higher heat outputs, with the slab slowly absorbing the excess heat.

I bet you will have ti live int he place a while to figure it out.
Eric