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.