HVAC Recommendations and Costs for a tight well insulated home
We are at the stage of determining the HVAC system for our new near passive home 2 story home (Upstate NY zone 5a). The Manual J run by my builder (total heating load ~14,000, cooling ~20,000) is attached along with the floor plans. We do not want a visible unit in the main living area. We are concerned about getting even heating and cooling in the master suite and upstairs where noise transmission is an issue (we both work from home). Backup heat will be with a wood stove but the building inspector will probably want another source if we use a heat pump.
The HVAC contractor has recommended:
• Mitsubishi 3 zone ducted heat pump system (main living area, master suite, second floor). The installed cost for this would be $18,679. This would provide enough heat down to around 0 degrees. OR
• The second alternative would be a Unico Small Duct, High Velocity system with a hot water coil that would be heated with a water heater (provided by others). The installed cost of this system is $14,981.
• Plus an American Aldes Premium Fully-ducted Heat Recovery Ventilation System at $4,995 installed
Questions:
1. Given the floor plan how many zones should we do? One, two or three?
2. Would we be better off (cost and comfort) with one Mitsubishi cassette in the main living area and two wall mounted units for upstairs and the master suite?
3. How do we get the heat from the wood stove around the house when the temperature drops?
4. What is your opinion about the recommended products and installed costs?
5. Are there better alternatives given our floor plan and constraints (no visible unit in living area, sound transmission issues)?
Thank you, GBA has been an invaluable resource
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Replies
Elizabeth,
1. Your floor plan is stretched out on the first floor and broken up into many rooms on both floors. Such a plan doesn't lend itself well to single-point-source heating. If you try to heat the house with just the wood stove, I imagine that your master suite could be cold. (By the way, I don't see the chimney upstairs -- where is it?)
2. I'm not a fan of Unico high-velocity systems, which really have no advantages at all except for the use of small diameter ducts, which are only sensible for retrofit situations where it's hard to run ductwork. In general, oversized ductwork is almost always better than small diameter ductwork.
3. It's a little late in the design process to be uncertain of your heating system. Before determining the size of your mechanical room and the type of framing used for your house, you need to know (a) what type of heating appliance you will be using, (b) where the heating appliance will go, and (c) where all of your ductwork will go.
Hi Martin,
If we were designing for the best energy efficiency this would be a 2 story box with the bedrooms upstairs. The plan is designed for our age and lifestyle (work from home) and is stretched out because we need to have all our living space on the first floor (aging baby boomers). The second floor is primarily our offices where we will spend much of the daylight hours. The chimney runs through a chase in the front office.
The framing is double 2x4 walls filled with dense packed cellulose and triple Intus windows. The mechanical room is centrally located on the second floor. We are building on a slab and will use open web trusses between the first and second floor, will use soffits where needed, and a utility chase on the second floor which will be sealed above with plywood to form a sealed box (vented attic).
Given the plan doesn't lend itself to a single heat source, what is your recommendation? Should we consider 2 or more mini-splits and boost the air circulation (inline fans) or just go with a conventional gas fired forced air system with air conditioning?
Thanks
Elizabeth,
If you have access to natural gas, I advise going with a natural gas furnace.
If you don't have access to natural gas, I advise going with ductless minisplits.
If it were my house, I'd start out with as few ductless minisplits as possible (because I'm cheap). If you're uncomfortable, you can always add electric resistance space heaters in the colder rooms.
However, if you really want very even comfort in every room, with no possibility of temperature variations, you could go with one of the U.S. made air source heat pumps that operate at low temperatures; these are ducted units that work with conventional ductwork. I think one option is the Lennox XP14 (https://www.greenbuildingadvisor.com/sites/default/files/XP14_expanded_pg3.pdf). The specifications include heat output specs down to -15 degrees F.
Hi Martin,
We have natural gas available. What is the backup heat with the air source heat pumps when the temperature goes very low?
Is there a great deal of difference in upfront costs and operating costs between the gas furnace and the air source heat pump? Which would you go with?
Thanks
Elizabeth,
I don't know how many levels of backup heat you want. You have a wood stove you can use when the electricity goes out or when you have extended spells of -25°F.
I've lived in the Northeast Kingdom of Vermont for 37 years. When the temperature dips to -25°F or -35°F in the early morning, it almost always rises to -8°F or -5°F by 2:00 p.m. It occasionally stays below zero for 24 hours or more, but an air-source heat pump can operate at -15°F, so it will be providing heat at that temperature.
Electric-resistance space heaters are cheap; pick of three or four at WalMart if you want. Or you can install electric-resistance baseboards at a very low cost.
As I wrote in my last answer, if I had access to natural gas, I'd go with a natural gas furnace.
the cheapest upfront with guaranteed year round comfort would be the gas furnace with high efficient AC. However you can swap the ac out for a ASHP as Martin describes. Lennox HPs are getting pretty good, and an XP14 or XP16 will only be a small add over an AC unit, but will provide cheaper cooling and heating, especially during shoulder months. Depending on your electric rates, you can set the HP to have shut off times where when it drops COP as the temp drops, the gas furnace can kick in and take it from there. Around here (zone 6 with electric rates around .08-.1) that is around 15-20 degrees typically. Our gas rates are really low as well. This setup is very common around here. I am not sure what your home exact size is, but around here this setup for install for around 15k for a 3000 sqft house including ductwork and HRV (none dedicated ducting).
Hi Jesse,
The house is about 2,700 sf. needs 14K heating, 20K cooling.
Are you saying that with the Lennox HP we would also have a gas furnace to provide heat when the temperature drops. Wouldn't that mean two devices, sounds expensive?
I thought you needed a dedicated duct system for the HRV.
$15,000 sounds good for both the HVAC and HRV. Where are you located?
"traditional" hvac systems are usually split systems in 2 parts. You have an outdoor AC compressor which provides cooling during the summer, and the inside gas or electric furnace. The HP basically replaces the outdoor AC unit, but still needs an air handler on the inside to push the air around (furnace accomplishes this). Can you can eliminate the gas furnace and go with a blower coil and electric heat strip backup, and the equipment will reduce in cost slightly typically. However the cost to run this setup vs a gas furnace will most likely be higher. The advantage the HP has over the straight AC/furnace combo is it allows the HP to provide heating offset and cooling needs, thus reduces the cost to run. The good thing about a furnace is, if your electric rates go sky high, you can use your gas furnace to heat during the winter and not use your HP at all. These setups do cost more then a furnace/AC typically, but around here we are talking $500-1500 more depending on how efficient the HP is. Typically its not worth paying more for the 17-19 SEER units when the 14-16 SEER units are nearly as efficient and cost less. You probably will not recove the upfront costs in energy savings.
You also do not NEED dedicated HRV ducting, its just highly recommend on this site. Dedicated ducting probably would not add too much to the cost.
I just want to chime in as the builder of this project. Me have fairly expensive electricity and cheap gas, so I have always approached heat pumps with skepticism. In this case, the annual heating energy use of the house will be so low that savings due to efficiencies will be relatively small. As such, the HVAC decision is more about first cost and comfort in my mind. As noted above, the house really isn't open enough to rely on a point source heating/cooling system so some of the comfort issue is heating/cooling distribution. The other comfort issue is not oversizing the system with such a low load. I have one question regarding standard heat pumps (ie split system). On he heating side, since they work best at warmer temperatures, aren't they inherently oversized at those conditions. I looked at that Lennox spec sheet, and it puts out some good BTUs at warmer temperatures, right when this house will only need a few thousand. From this sense, aren't heat pumps almost always mis-sized (without inverter technology)?
I do not have their spec sheet infront of me, but we typically use a 2 stage unit. I believe a 2 stage 2 ton will run in the first stage most of the time closer to 10k btu. One thing that is very interesting to me in your heating climate, is the higher cooling load.
Also note in super insulated and tight homes, shoulder months tend to require next to no conditioning at all. Even in my zone 6, circulating the air via an ERV/HRV and relying on interal loads for heat is usually plenty from September through end of October, even early November. Same thing from March through April.
Martin,
Do you agree with Jesse that we don't need dedicated ducting for the HRV if we do a ducted HVAC gas furnace or HP system? That seems to contradict everything I've been reading on GBA.
emphasis is on the word need. Its highly recommended on this forum, but not required. Your benefits come from efficiency.
Elizabeth,
Jesse is right. While I strongly recommend dedicated ventilation ductwork, you don't need it.
What do we lose (utility cost, comfort, etc.) by not doing dedicated ducting? Making decisions are a lot easier when you know all the tradeoffs.
Thanks
Elizabeth,
You have to be sure that during the swing seasons, you aren't using an energy-hog fan to distribute ventilation air; you also need to be sure that the large ducts don't interfere with uniform fresh air delivery, especially to remote bedrooms.
Hi Martin,
What is the optimal duct size for an HRV? For a central air HVAC? Forgive my innocence, but isn't there an energy efficient variable fan that could handle both the HRV and HVAC air flow?
Elizabeth,
Q. "What is the optimal duct size for an HRV?"
A. Most ventilation duct runs require only 4-inch-diameter ductwork; a large house may require a 6-inch-diameter trunk.
Q. "For a central air HVAC?"
A. Duct sizes vary depending on many factors, but it's not unusual for a main HVAC trunk to measure 8"x16" or 8"x24". Individual registers usually get 6-inch-diameter or 8-inch-diameter ducts.
Q. "Isn't there an energy efficient variable fan that could handle both the HRV and HVAC air flow?"
A. Yes. Ideally, you should specify a furnace with an ECM blower. However, it isn't always desirable to use a furnace fan to distribute air from an HRV, since HRVs already include blowers. Consult your HRV manufacturer for advice on control strategies and fan operation when hooking up an HRV to space-heating ductwork.
Thanks Martin,
I think the smaller ducts required for the HRV is why the HVAC guy recommended a Unico system.
Has anyone come up with a single system that integrates HVAC with HRV? Seems like there might be a market, given that more and more homes will be tightly sealed and need both.
Elizabeth,
Q. "Has anyone come up with a single system that integrates HVAC with HRV?"
A. Yes. Such systems are common. Visit the website of your HRV manufacturer, and look at the installation instructions. All HRV manufacturers include details for such installations.
Hi Martin,
I meant a single piece of equipment that provides fresh air, heating and A/C through a single ducting system.
Elizabeth,
In Europe, they have a device that almost delivers what you want -- but of course they don't include air conditioning, since residential air conditioning is rare in Europe. The nickname for such a device is the "magic box." For more information, see A ‘Magic Box’ For Your Passivhaus.
If you don't want to choose the type of appliance mentioned in my "magic box" article, the closest you'll come in North America is the following arrangement:
1. A forced-air furnace
2. A split-system air conditioner with a cooling coil in the furnace plenum
3. A central-fan-integrated supply ventilation system equipped with a motorized damper and a FanCycler control.
Of course, you still need a knowledgeable HVAC contractor to put the pieces together and commission the system.
Thanks Martin,
I will pass your recommendations along.
Given the low amount of heating needed, would a hydronic forced air system have any advantages (heat from a gas hot water heater instead of a furnace)? It would mean one less piece of equipment to maintain (hot water heater does both domestic hot water and heating).
Elizabeth,
I'm not recommending that equipment; I was just describing equipment that comes close to fulfilling your question about "a single piece of equipment that provides fresh air, heating and A/C through a single ducting system."
I'm not necessarily a fan of "a single piece of equipment," even when it exists.