Gas Heat vs. Heat Pump in Spray Foamed New Construction

I should have known the issues with installing a typical gas furnace in a spray foamed house, but it slipped my mind, and was reminded when the HVAC guy bid only electric heat. When I called him we got into the discussion of the tightly sealed house needing a 90% gas furnace or a heat pump (or electric strips). We plan to be on propane. Two story 2800 ft conditioned space, 2 units, air handlers both upstairs. He didn't seem too thrilled with the 90% furnace, something about foaming around the exhaust as it goes through the roof deck being a possible fire hazard, but said it is definitely an option.

I have never been in a spray foamed house, so have no experience with the heating requirements in one down here (ZIP 78382). This house will be on pilings 4' above grade so foamed all around. HVAC guy is saying they are so tight my heating load will be minimal. Heat pump upgrade is $700-800/unit which seems very reasonable to me. Is it worth it to go with the 90% gas over a heat pump?

We also discussed a separate dehumidifier that calls for the blower on low speed to circulate dried air throughout the house. I'm definitely excited about that! Sounds like about $3k for that unit though.

Thanks!
Travis

 

For optimal comfort & efficiency oth the heating and cooling loads need to be calculated, not assumed, and not using some idiot's rule of thumb such as "a ton of cooling per 500 square feet" or "25 BTU/hr of heat per square foot", Manual-J methods preferred. Try to keep the oversize factor for either heating or cooling capacity to under 1.5x (ASHRAE recommends 1.4x for heating.) The 1% & 99% outside design temps in your area are pretty much the same as for Corpus Christi, or about 94F & 37F repectively, which isn't very challenging for a heat pump.

As crummy as rules of thumb are, if you have to go there, a typical 2800' house on the Gulf Coast would have a cooling load of about a ton per 1400', or about 2 tons for the whole house, and a heat load of about 8-10 BTU/hr per square foot or ~25,000 BTU/hr for the whole house, maybe a bit less if it's tight, with low-E glass windows. So if the load is served by separate heat pumps, a 1.5 tonner up stairs and a 1 ton down stairs is likely to be in the optimal range. (2.5 tons/2 tons = 1.25x oversize factor for cooling.) It's common to see quotes for systems with 2-3x that much capacity, and that's always wrong, even for fully modulating or 2 stage systems.

It's almost impossible to find condensing gas furnaces small enough to be right-sized for the heating loads of just half your house (maybe a pair of the smallest Dettson Chinook series would do it), but heat pumps or hydro-air handlers (running off a condensing water heater) can be right-sized for relatively small heating loads like yours.

This graphic plotting house size against square feet/ton ratio was compiled by an outfit in Decatur GA, on Manual-Js performed on real houses (almost all in the gulf coast states) in the course of their HVAC consulting business:



For a tighter house there will be less latent cooling load than the average home in south TX due to the lower infiltration rates, which allows you to get the most out of high SEER equipment without ending up "cold but clammy" without having to resort to whole-house dehumidifiers.

For the $3K you'd be spending on a whole house dehumidifier could buy & install a ducted heat pump water heater, which provides a substantial amount of dehumidification. Heat pump water heaters take the heat of vaporization of the water removed from the indoor air and store it as sensible heat in the water inside the insulated tank. Unless you are doing your space heating with a hydro-air coil, a heat pump water is really a total win on net energy use, since it actively lowers the cooling and dehumidification loads. The Rheem's Performance Platinum Hybrid series come pre-configured to be ducted to utilize the bigger air volumes of adjacent rooms (or the whole house, or the great outdoors) for pulling heat & moisture out of the air. (This technology has come a LONG way in the past decade- definitely ready for prime time.)

If either the first floor or second floor has an open floor plan, it may be worth heating/cooling/dehumidifying that zone with a right-sized Daikin Quaternity series ductless mini-split heat pump. That mini-split is unique in the industry in that it's proprietary technology split coil in the indoor unit allows it do dehumidify with or without sensible cooling (though it can't heat and dehumidify simultaneously), with independently settable humidity and temperature settings. So when your heat pump water heater is keeping up with the latent load a Quaternity automatically adjusts it's sensible heat ratio to to a high-SEER cooling mode, and even when there is no sensible cooling load it will still pull moisture out of the air as-needed without chilling the place.

I'm sure this is all off-the-charts different from the proposals you've seen to date, but this is an opportunity moment to get it right that only comes around once in a couple of decades. Even though most of his discussion is targeted at a heating-dominated climate audience, the principles behind Nate Adams' Home Comfort 101, HVAC 101 , and HVAC 102 chapters & short videos are worth reviewing before making a $5000 (or greater) mistake that leaves you more broke, oversized, and less comfortable right out of the gate.

 

Thanks for the reply Dana. And of course as you say, "this is all off-the-charts different from the proposals you've seen to date"

Do you have a good website that makes calculating a Manual J as painless as possible? I'll probably have to do it myself which is fine. I haven't watched the videos yet that you linked to but will this weekend.

They are quoting a 4 ton downstairs and a 2 ton upstairs. Downstairs is 2000 ft and upstairs is 800 ft.

I like the idea of the heat pump water heater for sure, but the issue I see in our small town is finding someone with experience to service them. I can probably find someone in Corpus Christi that has experience with them, but then they are 35 miles away. But it may be worth getting a bid from a company there with more overall experience. And we were planning to place the water heater in a room upstairs that will be drywalled but not living space, so it will be mostly closed off from the rest of the house. I see you mention that it is ducted so I'll have to look into that, because otherwise the cooling and dehumidification would be isolated to one room.

I'm a fan on mini-splits but not for my primary residence. If this was a second home/cabin, etc. I would consider them seriously. Just personal preference, that's all.

So how would a gas tankless water heater and gas cooktop work in this sealed up house when an regular gas furnace will not? That is what is currently spec'd for thos two appliances.

 

Not a true Manual-J, but Manual-J-ish and easy to use is LoadCalc. It will usually oversize by 25-30% even with aggressive assumptions, but if you avoid upsizing from there it's not terrible.

Somewhat more complicated to use and a true Manual-J would be CoolCalc. There are ways to screw this up, but if you take your time you should be able to get it right.

Unless your house is a greenhouse, with a gazillion square yards of unshaded west facing glass that would yield truly insane oversizing. Your cooling loads could hit those levels if you leave some windows & doors open all day & night though.

There is a minimum size to the room that works, but ducting to a larger space is legit. Both of the major box store chains carry them, and they are such an net win for gulf coast states I'm sure somebody local has a clue.

Ducted mini-splits are really no different than modulating systems like the Carrier Infinity Greenspeed series, but can be had in smaller, more appropriate sizes. (The smallest Greenspeed is a 2-tonner, whereas Fujitsu's AOU/ARU x x RGLX series can be had anywhere from 3/4 ton to 4 tons, in half ton increments from 1-ton & up.)

Unfortunately Daikin's proprietary coil design that allows the Quaternity to provide dehumidification to a setpoint without sensible cooling is only available in wall-blobs, not mini-duct or ceiling cassettes.

A gas tankless can be installed outdoors in your location. It can also be installed indoors "direct vented/sealed combustion", with the combustion air ducted into the sealed unit, with zero risk of exhaust getting into the house. If kept to 140F or less (like almost all condensing residential tankless units) it can use cheap plastic venting. A gas cooktop will need a fairly powerful hooded exhaust vent to keep it from putting a lot of moisture and other exhaust products inside the house.

 

BTW: The likely gross oversizing of the proposed AC will almost certainly mean they run at such a low duty cycle that the separate dehumidifier becomes essential. Right-sized modulating ducted mini-split AC will run almost constantly during the heating season, and the sensible heat ratio can be lowered as need by putting the system in "DRY" cooling mode when it gets too sticky. While it isn't effective as a whole house dehumidifier on days when there is literally no sensible load, it's still not bad compared to typical 1-2 stage split AC systems.

 

Have you been working with a architect. Need to get a recommendation for a contractor that can run a load and not a guess like other have said. There are variable refrigerant flow central air heat pumps Bosch makes one that are very efficient. Water heater have to decide how much room and location your going to put it. Electric, gas or heat pump get the cost of kw electric, therm of nat gas and gallon of lp including taxs, delivery and efficiency of what your going to use
Kw= 3400 btu
Lp gallon 91000 btu
Ng therm 100,000 btu

 

Dana,

I did the CoolCalc calculation and came up with the numbers below. The only question I had was on the ducting. I selected on both floors that the ducting would be in conditioned space, since the underside of the floor is closed cell and the walls and underside of the rafters is open cell. So all ducting would be within that envelope.

Downstairs:
Heating BTUH: 35,579
Cooling BTUH: 26,890
CFM: 1,174
Sensible cooling: 23,247
Latent cooling: 3,643
SHR: 0.865

Upstairs:
Heating BTUH: 9,362
Cooling BTUH: 10,115
CFM: 332
Sensible cooling: 8,045
Latent cooling: 2,071
SHR: 0.795

We do have a lot of windows so I was pretty liberal in selecting for that, although I wish it used an actual area measurement rather than low, medium, and high. What CoolCalc didn't account for is that we have 10' wide porches on all 4 sides of the first floor, so all of those first floor windows are shaded. Second floor is standard eve width.

Didn't catch that you said DUCTED mini-splits. Didn't know those existed. Something else to look into.

We worked with an architect/drafting guy to develop plans but that's about as far as it went since we took those plans to our builder. And we are working with the builder's recommended HVAC guy, so anything further than that is up to me. Electricity is about $0.10/kW but need to get a price on propane. I've got a full tank sitting there from the previous property owner

 

The architect may be capable of or may have a preferred an engineer for making the load calculations using a professional tool. The initial CoolCalc numbers feel a bit padded, but not as insanely padded as the builder's HVAC guy's proposals. The heating load on the first floor seems unusually high- probably due to floor losses (perhaps due to an open pier foundation ?). Bumping up the floor R to R25-R30 instead of the code-min R13 might be might be "worth it" for wintertime comfort, and knock something off the heat numbers. (TBD)

The ~ 10K load upstairs doesn't seem crazy, but the ~27K down stairs is probably overstated. As you pointed out, with 10' deep porch roofs all around only the east & west facing windows get direct sun, and then only for a brief period.

Run the numbers using LoadCalc as well, which gives a good summary breaking down the load by component type (even if the load numbers aren't super accurate.)

With 10 cent electricity a pretty good duct 1 speed heat pump with an HSPF of 10 (= 10 BTU/watt-hour, or 10,000 BTU/kwh) is going to cost about $10/MMBTU ( million BTU) With a modulating right-sized ducted minisplit it would be more like $7-8/MMBTU.

A 95% efficiency gas furnace delivers about 87,000 BTU/gallon, to it takes bout 11.5 gallons/MMBTU. If delivered propane is well under a buck a gallon it might break-even with the operating cost of a heat pump (or ducted mini-split heat pump), but that would be less than half the average statewide cost of residential propane in TX. As in most markets it's not even close.

 

I do need to check with our architect on his recommendation for an engineer. We drew up plans almost two years ago so that part of the process is almost gone from my mind and I don't think of talking to him. This house is on wooden pilings with a floor elevation 4-5' above grade and just enough screening to keep rodents out and to provide for aesthetics. Need lots of ventilation to keep moisture levels in check. The insulation guy bid 2" of closed cell foam under the floor (R15) so we could up that number somewhat.

I haven't had time to run the numbers yet using LoadCalc but still plan to. Busy time at work.

I appreciate the calculation on electric vs. gas operating cost. That along with the purchase expense of a 95% efficiency gas furnace makes that decision much easier. Thanks again.