I'm building an 800sf super-insulated, and very tight house in Zone 4 (Portland OR). Roughly ~10k btuh heating load. The first floor has kitchen/living room and is open plan. Second floor has bath, hallway and 2 small bedrooms. Most windows are on south side, and will be shaded from summer sun. I am currently leaning towards a single ductless mini-split on the first floor for both heat and cooling, though mostly heating, since it doesn't get that hot here for most of the summer.

One drawback is that I'll need a separate DHW system, which either costs ~$1k for tankless, or less for larger tank, but that takes up much more room in this already tiny place. Alternatives that can combine DHW and space heating include radiant ceiling and/or walls or floor heated by a pellet stove, mod-con, or some sort of heat-pump, or forced air of some sort. All of which seem like more expense and design complexity than a ductless mini split, and none provide cooling. While I am enamored with the comfort aspects, and double duty DHW bonus of hydronic radiant heat, I'm not seeing it make sense financially in my case.

The other issue I've been reading about is the 'cold (or hot) bedroom' problem that single point heating (and cooling) causes, when doors are closed. Some folks say just leave the doors open, and it's fine, at least for heating. Others say bedrooms might be quite cold, and you need to add electric resistance heaters as a backup. Some folks say that the HRV will move the heat/cold around, while others point out that HRV is always bringing in colder air from outside, so it won't do much to warm the cold bedroom, and I can only assume this carries over to cooling situations in reverse.

So what's a guy to do? Well, I've got 2 options so far- 1 is to add a completely separate duct system for air mixing, powered by a duct fan, and controlled by a thermostat. Downside: having to run HRV and air mixing ducts is a pain, and costs money. Upside: seems like it would be less prone to conflicts between HRV and air mix system.



Option 2 would be to combine the ducting from the HRV and the air mix vent. A damper would be included to keep the HRV fresh air from leaking out through the duct fan when it's not running. Might need a second damper to keep the duct fan from back-pressuing the HRV fresh air line when the duct fan is running? What do you folks think?

Continuing to research- came across the Twinfresh Comfo ERV that does 32CFM (which is about what I need in an 800sf w/ 2 bedrooms). So here is option #3- ERV on main floor, powered mixing duct to move both fresh air and heated/cooled minisplit air around the house. Ideally duct fan would run on a timer as well as thermostat, so in timer mode it would assist the twinfresh in providing fresh air to the whole house, but in thermostat mode, it would balance out temp difference between rooms.

I also added a bath fan and stove vent hood, which would probably also require makeup air in a tight house? Especially if I get a gas stove?

As usual, all your thoughts are most welcome.

In finishing stages of a 2 story 24x32 house (1300+ sq ft conditioned space) in climate zone 6 (NH). Well insulated (r40+ walls, r70+ roof) and tight. Can give you some advice on my experience...

Heat - I wouldn't worry about the cold bedroom issue if the house is built super insulated and tight, especially if you have a pretty open plan and can keep doors open part of the day. I bet the smallest size (9k btu/hr) Mitsubishi or Fujitsu would be fine for you.

DHW - Keep it simple with the mini split. Not sure of your hot water needs, but we went with a short 30 gallon standard Rheem (electric) that fits under our stair landing. Tankless is attractive because it's small, but that seemed to be only benefit.

Gas vs electric stove - how about stick to all electric so you don't have to deal with any combustion?

So, I'd skip the air mixing idea and just do a small mini split and simple tank or tankless DHW. For ventilation, seems like you have a couple good options.

Thanks for the tips Brian.

First floor is totally open, 2nd floor has 3 rooms and a hallway, but doors would likely be open all day.

Without ventilation, how could air would circulate upstairs?

I haven't done a room-by-room manualJ yet, but the other heat load calcs I've done come in around 9kbthu, and doesn't account for passive solar heat gain.

I was planning on gas water heater, for cost of operation, but that would be another source of combustion or a fancy unit that is sealed. Will have to think on it.
Also, would love to put water heater closer to bathroom/kitchen, which stairs are not.

I'll have to think about electric stove- I do like the induction ranges, but they are pricey.

I haven't done a room-by-room manualJ yet, but the other heat load calcs I've done come in around 9kbthu, and doesn't account for passive solar heat gain.

What have you done in the way of investigating heat loss? It's my understanding that "super insulated" homes come in at 5 btu/hr/sf or less, even in much colder climates and you are up at over 10 in a mild climate.

Alternatives that can combine DHW and space heating include radiant ceiling

Don't even think about radiant in a well-insulated home in a mild climate. Electric resistance radiant in some small well-defined areas for warm floors might be an exception.

Some folks say that the HRV will move the heat/cold around,

That is correct. You need a small HRV with supply to the bedrooms and exhaust from kitchen and bathrooms.

The other issue I've been reading about is the 'cold (or hot) bedroom' problem

Pay attention to your room-by-room heat load. Don't make the windows any bigger than you have to for egress and aesthetics, then get good quality casement or awning windows to keep air infiltration down. Then, make sure that your envelope plan really does seal everything well. You should be fine.

Especially if I get a gas stove?

Don't complicate your nice simple design unnecessarily. Go electric on the range.
1) Well insulated
2) Well Sealed.
3) Ductless mini 1 head gives you heating and air conditioning.
4) HRV with recirculate and bypass functions.
5) Heat pump water heater
6) Electric range
7) Buy yourself some PV panels with all the money you're saving.

Some folks say that the HRV will move the heat/cold around

No, a HRV will not move enough air/btu to make a significant difference. But leaving the door open will.

JonR is correct if you leave the doors open, have the fresh air enter in the bedrooms and evacuate the air from the bathrooms, kitchen and laundry rooms it will even out the air temp extremely well! My recent build of an ICF home and the HRV running 24/7 20 minutes on and 40 off has evened out the air temps in the 1900 sq ft bungalow extremely well and i'm very happy with the outcome! John

Thanks for all the feedback everyone.

@ICFHybrid:
I used Mark Rosenbaum's heat loss calculator spreadsheet to get the total heat loss for the building- http://blog.heatspring.com/tag/myson-heat-loss-calculator/

Heating design temp difference of 47F.
R20 slab, R40 walls, R60 roof, R3 windows (12% glazing, and yes, I'm hoping to bump up to an R5 window, but not sure if I can afford it)
30CFM of ventilation @ .76 effectiveness. .05CFM50/ssf infiltration
That came to 8439BTU/hr.

I need to get back to the library today to see Table 4A and some others in the ManualJ book before I cam play around with ManualJAE. That free spreadsheet requires info from tables that are only in the book.
I'm also considering PHPP to do even more modelling, but am dragging my feet on spending money on that package- it's really going down the rabbit hole!

On to how HRV affects air temp mixing room to room- With doors open it sounds like most people think it'll be fine. At least it makes sense in winter, when hot air from downstairs rises. But what about in summer with cooling on first floor? Not sure that would work out as well.

I hadn't heard about recirculate/bypass function on HRVs, that would certainly make things simpler than a redundant air mixing system, though I would think it would need to be oversized to provide enough flow for air mixing, vs the amount of fresh air I need (30CFM). I'm wondering how that could be calculated- if I know the volume of the air in each room, and the max CFM of the HRV, could I use math to figure how fast it would mix the air in the house? I'd imagine temps would also be affected by conduction through the walls/floors, air moving up the stairs, and infiltrating between floors. Then there's stack effect that would help in winter and harm in summer...

@ JohnyH- glad your system is working so well. Is that for heating or cooling or both? Is that 1 or 2 story?

@ICFHybrid- I also hadn't heard of a heat pump water heater- I'm assuming it's both more efficient, and more expensive?

Bungalow, unfinished basement with walkout. I was extravagant and put 4" of foam under the basement slab. Since it is built on bedrock first the gravel, foam sheets, 6 mil poly and then the mesh. the floor is completely isolated with foam even from the footings. I don't heat the basement but with the HRV and the forced air (Heating and Cooling) the house stays a very uniform temperature. I did have a passive solar super insulated (double wall) two storey with basement that I had built in 1987. I will be the first to admit it was a great house but I did have problems with over heating in the summer (even with the wider eaves) AC ran longer. Heating it was easy but the mornings were a tad cool in the winter! Now in the new ICF home the thermostat, a smart one, is set for 70 degrees farenheit 24/7 and heat and cool. MORE than happy with the low costs of heating and cooling. I really did not want to worry about pumps tubing leaking and the upkeep of all things expensive. Forced air gas and electric AC was the way to go here in Ottawa, Canada. The only time it is switched off is when we open the windows! John

What I did forget to mention!! The HRV is a completely seperate duct system from the heating cooling ductwork system, that is really important! The basement has a 112 " height so it wasn't an issue to do this! John

Ah, thanks JohnyH. In my case I don't have a basement, nor forced air heat/ac, so my situation with recirculation/air mixing is completely different than yours. Glad yours is working out so well though.

I've been looking up HRVs that have recirculation capability, looks like Fantech, Goodman, and Venmar all have models that do that for $700-$1400 depending on features, efficiency, etc.

simpler than a redundant air mixing system

You've mentioned 'air mixing' a number of times and I'm not sure what you mean by it.
There isn't that much use for the recirculate function on an HRV. The whole point is to exhaust air, recover the energy and bring fresh air inside.
The bypass function bypasses the heat recovery, which is primarily used to bring cool night air in without putting the heat from the house into it.

I believe there are a number of threads about heat pump water heaters in the forum having to do with 'appliances'.

What I mean by air mixing, is moving the air from the first floor, which has been heated or cooled by the minisplit to the 2nd floor which is hotter or colder depending on the season, thus getting more even temperatures in the whole house.

I was under the impression that was what a recirculate function on an HRV would be for, maybe that was wrong.

Bypassing the heat recovery seems like a different function?

The purpose of the HRV is to provide needed fresh air, the obvious removal of bad air and some energy recovery at the same time.
Warm air and cool air is going to move on it's own. If you want it to, then let it, if you don't want it to then stop it. You could do this with a door between the two floors. For example, in the Summer, you may want the warmer air to go upstairs during the day, so you would open the door. That would help the lower level stay at a more comfortable temperature. You could also vent the upper story, creating a natural "stack effect", drawing cooler outside air in at the bottom.
In winter, you would close the door, keeping the warmer air downstairs in the living areas. At night, open the door to allow it to go upstairs to the sleeping areas.


If you need more control than this, the obvious next step would be to buy a ductless mini with two heads and put one upstairs and one down stairs. The extra cost is very small.