Choosing an HVAC System for a Cold Climate
Hello,
for my new future house in Climate Zone 4a (zip 20817), both my builder and architect suggested to install a gas furnace in the basement with heat pump and ducts in the attic. It took me a couple of days on GBA to understand that instead all electric is the way to go, that I should avoid ducts/mechanical in the attic, and request a proper manual J and D sizing of the heating/cooling/ducting system.
However, I am still confused on what a good heating/cooling/ventilation system should look like for a 2 story house with basement (about 1600 Sqft/floor) in my climate zone. Is a multizone heat pump with dehumidifier and ventilation a good solution? Is a separate ventilation duct system required in my climate zone? What am I missing?
Thanks!
Mauro
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Replies
On a new build, going with a cold climate ducted minisplit is the way to go. We're in zone 4a and did a retro with ducted mini-splits. Check out Fujitsu. Also going one to one is preferable and not multi zone. A separate ducted hrv is also something to go ahead and design for your new build as well.
Hi Mgensler,
thanks for the suggestion. Would ducted minisplits be more efficient that a ducted heat pump also in non open floor-plan? Or really the house needs to be designed around the minisplit system?
I guess I found my answer here
https://www.greenbuildingadvisor.com/article/tweaking-plans-for-a-minisplit-system
Ok so how different is a ducted minisplit from a heat pump with inverter? It seems to me that an inverter heat pump combined with an air handler basically works as a minisplit that can handle longer ducts.
can distribute air to the entire house, and efficiency not necessarily is going to be lower than minisplits.
As far as I know (and someone please correct me) there are basically no differences between a "mini-split" (ducted/ductless) and a "variable refrigerant, high static pressure central unit". In fact all these can be mixed and matched with the same outdoor compressor (although try to avoid that, stick with 1-to-1 if you can)
http://www.mylinkdrive.com/USA/M_Series
Ductless (aka wall mounted)
Ducted minisplit (aka horizontal ducted):
VRF high-static (aka "multi-position air handler), which is what I think you mean by "air handler"
Also, if I was doing a new house consider something like the CERV2 which handles ventilation, dehumidification and some small levels of heat and cooling.
Hello James,
your suggestion of CERV2 made me think.
At 1:38 in this video https://www.youtube.com/watch?v=s2XKR6JSU9k it is been shown that CERV2 can be installed as stand alone, in line with ducted minisplit or feed into central system. As a stand alone it would need a dedicated piping system, With a ducted minisplit it will share the piping of the minisplit but that would not allow to reach the entire house (at least in my 2 story + basement house). So only in a central system the CERV can reach the entire house and not require an extra piping system. That sounds to me as a big advantage. So this is leading me towards system A: "variable refrigerant, high static pressure central heat pump + ERV system" instead than system B "multiple ducted minisplits + ERV with separate duct systems". I understand that pressure drops with long ducts will affect efficiency of system A but I expect the cost of hardware+installation for A to be significantly lower than the cost for system B.
Makes sense to me. If you are designing the ducts from scratch and use a proper duct design approach then you can get that all to work. My experience shows that even when working with a trusted and experienced contractor a third-party duct design (specifying materials/topology/registers and flows) will help everyone (including the mechanical contractor). Don't forget airflow noise parameters.
Only thing to consider is that the ducts for the CERV (or any ERV) would be significantly smaller than those for central-air, so a dual duct system might make sense. A dual duct system would then also simplify air pressure issues with two positive pressure devices (ie two blowers in the same system can cause design issues).
You want to compare the capacity tables for the different units. 4A is a heating-dominated climate so you want to pay the most attention to the heating capacity. We installed the Fujitsu ARU12RLF. It's a 12,000 btu unit and it outputs 17,200 btu at an indoor temp of 65 F and outdoor of 5 F. It's also has an HSPF of 11.5. Fujitsu also makes a medium static unit (more inline with what you are thinking?) ARU12RGLX that has about the same performance at 5 F but more static pressure . It really comes down to your floorplan design. Ours was an existing house that already had 3 furnaces. We mounted one minisplit vertically for the basement, one hung from the basement joists for the 1st floor, and a ductless for the kitchen/dining as it was an open floor plan.
mgensler I am still at a stage where I am trying to decide which type of system works best. Would you opt for an ERV with separate ERV duct system and ducted minisplit or a central heat pump with inverter which shares the same ducts system with ERV ?
Lot of things go into your fuel choice decision.
If your home will have a city gas connection for some other appliance gas may well be the low cost option given today's pricing.
If comfort is the biggest factor in you choice gas is a clear winner as it will be blowing much warmer air around your house.
If no city gas is available a heat pump is likely the low cost option.
If you goal is net zero and you will have photovoltaic panels heat pump is your best choice.
Walta
Hi Walter,
I do not have any other gas appliance and I will be installing PV in a couple of years at most. I did a quick search and gas to electric cost seem comparable (maybe a bit cheaper for gas but depends on the specific rate I pick). (See below). Since heating cost is comparable I rather avoid having gas in the house for safety reasons and for the sake of simplicity. I guess that no gas can also have an effect on home insurance cost.
Cost comparisons are so variable based on location (climate) but more importantly utility rate structure. They all do things a bit different on the accounting side. Efficiency wise, there’s no contest. There are no efficient furnaces when you compare them to heat pumps.
I would agree with most of Walters comments except about comfort from gas. A properly sized and installed modulating heat pump is superior in comfort to almost any heat source. Since these are modulating units, they are providing heat all the time, the air delivered is just the right temperature to keep the temperature stable. They also tend to be much quieter.
Having lived in older houses with oversized single or two stage furnaces, a modulating heat pump is way more comfortable.
Once you have proper Man J and S for your new design, you can search through here to compare units:
https://ashp.neep.org/#!/product_list/
In a two story home with a basement, you want either a single high static unit in the basement feeding the whole place or a combination of high static for the main floor+basement and a slim ducted unit for the 2nd floor.
To search, you can select "single zone centrally ducted" for a high static air handler, and "single zone compact ducted" for a slim unit.
If you are going with a single air handler in the basement, an important detail is to have a largish return near the ceiling on the 2nd floor. This does take up floor space to run, but without it cooling will suffer, so don't skimp on it.
I forgot to say in my first post say NO equipment or ductwork in your attic.
My opinion putting HVAC in the attic is a great deal for developers and contractors and the worst possible idea for the home owner as it will double the operating costs if the attic is vented and only slightly better if the attic is conditioned.
Be smart find the room inside your home for the equipment and ducts.
Akos do not get me wrong my variable speed heat pump is great but will never blow 130° air since it is delivering cooler air it must move 4X the air. A heat pump register placement is a deal breaker. I miss my gas furnace it was a treat to warm and dry my boots on a register.
Walta
A new heat pump should use a temp > 85 degrees, so maybe only 2x the volume compared to the furnace. My experience has been that ~100 degree air moving at less than half the velocity sure feels warmer than blasted by 130 degree air. Of course, everyone's mileage will vary.
Unless I missed it in the discussion thus far, it should be pointed out that the OP's first concern should be to build a tight and very well insulated house with very good windows, generally in the "superinsulated" class. Then the heating system needed will be of much smaller size, the locations of introduction of heat almost of no concern, and there will be essentially no cold spots anywhere. The introduction of simply "warm" heat pump air, compared to blasts of exceedingly warm air, will be hardly noticed, as the house will be very comfortably isothermal.
I like your idea
From my experience, all-electric is indeed a winner – more efficient and eco-friendly. Ducts in the attic? Nope, it's not fun dealing with leaks and losses. And kudos for the manual J and D idea – right on point!
For your setup, a multizone heat pump sounds rad! Especially with that dehumidifier – no more swampy feels. Here, at solutions heating and cooling, you can find all the necessary services. Good luck!