HVAC For Tight Envelope House
Alright…help me out if you can. I’m a home owner designing a 3,000 sf house in West Virginia, breaking ground next year. I’m deep into systems research and have benifited greatly from this site.
Question concerns what type of heating system I should plan to install. I keep hearing of stories like “My buddy built a house that was super insulated. He has an electric furnace and his electric bill is like $300 a year”. If this is at all reality why would I think about investing in geothermal? It sounds like I should put money into super tight and insulated and save money upfront (I know this is contrary thinking) with an efficient gas furnace. I have to shave money somewhere.
Thoughts? Eric
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Replies
Eric,
I agree with you analysis. It's always better to invest in an above-average envelope with thick insulation, good air sealing, and excellent windows. The type of heating system becomes unimportant once your heating load is drastically reduced. This is the philosophy behind the Passivhaus standard that was first developed in Germany.
For most homeowners, it's better to invest $10,000 more on insulation, air sealing, and windows than it is to pay the $10,000 incremental cost of a ground-source heat pump.
Would you live in a sealed plastc bag. Watch out for indoor air quality. Your home can make you sick.
Yes, Mr. or Ms. ExpansionValve, I'd be happy to live in a totally sealed home — or one that was as close to perfectly sealed as possible. I'd get my fresh air from a well-designed mechanical ventilation system.
Depending on where you are in West Virginia you may be in a place with hot humid summers like we have here in North Carolina. A conventional air source heat pump installed with a zoned bypass air handler system may be the perfect match for your home to optimize de-humidification over cooling in the summer.
These systems use one air handler and compressor to handle two (or more) zones by tying all the returns together and using electric zone dampers in the supply ducts to control air supply to separate parts of the house.
As the dampers close when zones are satisfied, a weighted damper is opened by the pressure of the restriction which allows some of the conditioned supply air to mix with the return air and cycle through the heat exchanger a second time. this pre-cools (or, in winter pre-heats) the air going into the coil and allows the dew point to be met more easily resulting in more complete condensation of the latent moisture in that air.
When your attic insulation and windows are very good you don't need a lot of cooling but you still need de-humidification so this optimizes de-humidification in homes that generally don't need all that much cooling. It also results in warmer air at the supply registers in the winter months so you don't feel that cool air discomfort that is often associated with using a heat pump on it's heating cycle and it improves efficiency if you have the guest wing shut down when the kids are away.
The control wiring and ductwork on these systems is somewhat more challenging but since many of these tight houses require so little heating and cooling it can be difficult to purchase small enough equipment to properly match the manual J load calculation. Our last house had two zones on a three ton heat pump, two 1.5 ton units (or a one ton and a two ton) would have been difficult to find in the SEER rating we were looking for.
As Martin said, you still need a fresh air system. If you are in a place with chilly winters you probably will be looking at a heat recovery ventilator.
By the way I've been a plumber for twenty-plus years and I've heard or expansion tanks, pressure reducing valves, and pressure relief valves but I've never heard of an expansion valve, I thought it was the policy of this site that people use their legal names.
I don't think many HVAC contractors know how to deal with a tight house and provide proper ventilation. I too built a tight house and specified a mechanical ventilation system. Unfortunately, the idiot contractor my builder used was totally ignorant of an effective system, refused to follow the National Standard, and fought continuoulsy about correcting it. He was fine with it not bringing in fresh air for months at a time due to a high humidity over-ride. I tried to get help from the State Licensing board, to no avail. After 5 years of fighting them, I interviewed a dozen or so contractors before I found a competent contractor, and spent $5,000 correcting all the problems left me by the first one.
Hi Eric, My house is in Ottawa, Canada. It's a super insulated air-tight ICF constructed bungalow with a 9-foot high radiant heated basement of 1850 sq ft and a high-efficiency natural gas furnace as the primary heating source and to provide central A/C. Four adults have lived in the home since it was built in 2001 and to manage the fresh air and humidity conditions, I have the largest HVR by Venmar, which supposedly can handle 3000 sq.ft. (about equivalent to the 3000 sq.ft. that we use daily).
If I kept the windows and doors closed and go to a once-a-day full air change, the cost heating and cooling costs are very low in comparison to regular housing stock. However I choose to have the normal or slighly higher fresh air change to a minimum of once every three hours, and therfore I pay about 60-70% of the cost of regular housing stock. I still save 30-40%, I don't have to skimp on heating or cooling and the integrity of the building envelope is not affected by moisture.
The biggest and single issue I have is the capacity and performance of the HRV, which is not as efficient and overall very cost-effective. The best HRV I've seen uses a heat pump, which is great for a heat and cooling recovery but lousy from a total energy perspective because of the cost of running the AC motor. I'm waiting for one with a variable DC motor before I replace my HRV.
Regards,
Gerry