HVAC for a single level “Pretty Good House”
Hi all
First discourse on the forum–I’ve been reading it incessantly and now it’s time to get involved. Over the next few months, I’ll be soliciting advice and opinions on various aspects of a new house we are building for retirement located in Hood River, Oregon. I thank you all in advance.
Some basics to start: we are building a single level, 1500 SF home based on PGH principles. The foundation will be slab on grade, we’re planning a double wall construction with 12″ of cellulose, and a shed-style roof with a ceiling filled with blown in cellulose. We’re shooting for R-values of 10-20-40-60 and an ACH50 of around 2. We have good southern exposure and are taking advantage of that using some passive solar principles (e.g., adequate glazing, appropriate eaves, thermal mass via concrete flooring). My questions today focus on heating (and perhaps cooling) what I hope will be a relatively tight, well insulated single level home. The single level aspect is important–the house, though smallish–is a bit spread out. I have read a lot about heating a tight, well insulated home, but I wanted to solicit advice specifically for single level homes that may have more spread than a multi-story home. Specifically:
1. Knowing that I have access to natural gas, and our electric rates here in the PNW are relatively cheap, how would you propose this house be heated? Basically, the house has a great room (ca. 700 sf) with kitchen, dining, living; master on one side; and two smallish bedrooms on the opposite side. It’s not just a simple rectangle, but more like two rectangles joined about 1/3 of the way along their long axes. I’ll post a pic here later on.
2. Based on what I’ve read, I’ve decided that in-floor radiant heat isn’t the way to go–although we have decided to have it in the bathrooms only. I’d be curious to hear your opinions on this decision.
3. The current idea is a single minisplit combined with a gas fireplace insert heater in the great room. I’m not sure where the minisplit will be located, but I assume somewhere central in the house–probably near the entry and hallway as you walk in. Our contractor has an idea to put a return air duct high on the wall near the fireplace, connect that to an air handler, and then duct the warm air to the bedrooms, mostly. What do you think of this idea? Moving warm air from a wood or other heating device is a topic that has received a fair bit of attention, but it seems vague to me and I don’t know how efficient it will be. We wanted a fireplace for ambiance anyway, thus we thought why not use it for heating also? Perhaps the air handler and duct work will be superfluous in this PGH?
4. Cooling, we don’t think, will be important for us. We’re not big AC users and really there would be only a rare need for it. We plan on having a nice (Big Ass brand) ceiling fan in the great room and the minisplit–if we install it–will provide cooling if we ever need it.
I guess that’s it for now. I’m looking forward to hearing your expert opinions on this and other things over the ensuing months and hope I’m not taking too much advantage of the resources here. I too will try to contribute when I can!
Cheers
Matt
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Replies
1 mini split
Electric floor heat master bath
Electric floor heat under any work desk area if you do daily (hours of) desk work.
That's it... Keep all simple
Panasonic bath fans for vent needs.
And yes to NG fireplace. Heat choices are good and ambiance to boot.
Dana says the Daikin units will do dehumidification. Tight ACH homes have higher moisture concerns.
Matt,
As long as your heating system is properly designed and installed, almost any system can work well: a natural gas furnace, a couple of ductless minisplits, a few ducted minisplits, or a conventional air-source heat pump.
The key is to create a very well insulated envelope with low levels of air leakage... and it sounds like you are doing that. So continue to focus on the envelope, and don't worry too much about your heating system.
Thanks guys, I appreciate the feedback, and will do. Finances do come into play here, so I don't want to "overdo" it re: HVAC. We'll be doing some energy modeling soon and I suspect that will be enlightening. Thanks again--it's reassuring to get advice and opinions from experts in the field.
Cheers
Matt
Simulate both heating and cooling loads- your cooling loads are likely to dwarf your heating loads. At those R-values at your 99% outside design temp of ~+20F your heat load of a 1500' house is likely to be within the output of a single 3/4 ton mini-split (unless you make your house geometry much more complex than the 2 intersecting rectangles) and your heating/cooling balance point is likely to be in the mid50s F. Your cooling loads are likely to exceed your heating loads by more than 2:1 unless you manage your gains (particularly late-day gains from west-facing windows) very carefully.
Not only should you forget about ducting the heat from the gas-burning insert to the rest of the it's probably better to forget about the gas-burning insert entirely, and focus on reducing the heat & cooling loads of rooms doored off from the area served by the point source heat/cooling & major solar gain rooms.
Your heat load is truly tiny. An R60 attic has a U-factor of 0.017 BTU/sq.ft. per degree-F, an R40 wall has a U-factor of about 0.025. Assuming an interior design temp of 70F and outside of 20F that's a 50F delta-T. Your roof losses are then:
1500' x 50F x U0.017= 1275 BTU/hr
Assuming a 15% window/floor fraction that's 0.15 x 1500= 225' of window. Figure 150' of the windows will likely be a high-solar gain U0.30 window for the passive solar, the other U0.20 for daylighting & egress, and figure a couple of R4/U0.25 doors for another square feet and you're talking window & door losses of:
150' x 50F x U0.30= 2250 BTU/hr
75' x 50F x U0.20= 750 BTU/hr
40' x 50F x U0.25- 500 BTU/hr
Total windows & doors: 3500 BTU/hr
Since it's not a 30x50 rectangle ( 160 perimeter) you're probably looking at a perimeter of about 250', and even with 10' walls that's 2500' of gross wall area. Less the 265' of window & door that's 2235' of U0.025 wall for a heat loss of:
2235' x 50F x U0.025= ~2800 BTU/hr
Add it all up and you're at 7575 BTU/hr. Even if you posit a hefty 3000BTU/hr for air leaks ventilation and slab losses your only at about 11KBTU/hr, and that's BEFORE subtracting off plug loads & the mammalian body heat inputs, let alone adjustments for thermal mass effects.
If you limit the doored off rooms to about 20' of U0.20 glazing they'll be pretty much self-heated by the thermal output of the occupant when occupied. Rather than ducting air in to keep it at temp 100% of the time, the lossiest of those rooms can be heated at high comfort with electric cove heaters under occupancy sensor control, limiting the room temp via thermostat can keep comfort high while being stingy on power use. Unlike electric baseboards, cove heaters heat the objects in the room directly rather than heating the air, which makes it comfortable even during the recovery ramps. And unlike electric panel radiators, cove heaters have no thermal mass to speak of, so the radiant output comes on in a few 10s of seconds rather than minutes.
Consider NOT bringing natural gas to the home.
You may save:
-at least several hundred for plumbing, lateral, and tap fees for the hook up.
-$8-$20 monthly service fee
If you install PV, that savings cannot be used to offset your gas bill. You shouldn't cook indoors with gas. A gas fireplace requires another large hole in your thermal envelope. You won't be able to use the gas fireplace for long before that room gets too hot.
I agree - use two minisplits and forget about the gas FP. If you insist on buying it, don't count on it for heat because A: it'll be more expensive to use than the MS and B: it will provide waaay too much heat.
Plan on 1 ACH@50, not 2. It's really not that hard to achieve if you pay attention to the details.
A way to have the gas fireplace, without the monthly costs which is some areas are ridiculous, is to go propane. And get a sealed unit to deal with what some say may be a hole in your home.
I like fires myself... if not inside you could do a nice set up outside your home to enjoy. Life should be enjoyed.
Thanks all for the advice and comments--good food for thought. Like Martin alluded to, we will pay attention to the building envelope and I'm confident however we choose to heat/cool our home will work effectively. A few specifics:
1. I will try and post a floor plan--it'll give you a better idea of these "intersecting rectangles".
2. While I'm still learning about U-values and SHGC's, etc., i do understand what you're saying Dana. As I mentioned, we will be doing an energy modeling analysis for this house and it should steer us in a clear direction for an energy efficient heating/cooling system. I am not familiar with cove heaters, so I'll have to check them out. Not even sure, after reading these posts, that we'll need them at all.
3. Regarding NG, to be honest, I was kind of excited to have it already at the property line. I like to cook with gas--I do so now with propane. I'm an aspiring home chef who plans on doing some serious cooking during retirement, so I really like the idea of having gas--at least to cook with.
4. Now, having said that, do I need, or should I use gas, for other appliances or for heating? We were warming (no pun intended) to the idea of a NG fireplace--yes, for ambiance--but I also though it could serve as a functional heater. Perhaps I need to re-think this. Adding ducting seems superfluous. Minisplits I like because I can get some cooling if I need it.
5. If we do go with a gas fireplace, I've read the debates between sealed and direct vented units. It's a bit confusing, but my take is direct vent is the way to go.
Again, you guys have given me lots to think about, and there will be more. Thanks, thanks a lot. And yes, AJ, life should be enjoyed. We want an energy efficient home, but we do want to enjoy life a bit! Hence, the warm floors in the bathrooms!
Direct Vent is a synonym for sealed combustion, so I can't imagine what the debate is about.
I don't think anyone should deliberately introduce carbon monoxide into a tight house:
https://www.greenbuildingadvisor.com/blogs/dept/green-building-news/hazards-cooking-gas
Big range hoods sufficient for a gas range are controversial to say the least:
https://www.greenbuildingadvisor.com/blogs/dept/musings/makeup-air-range-hoods
Lots of folks like this alternative:
https://www.greenbuildingadvisor.com/blogs/dept/guest-blogs/induction-cooktop-our-kitchen
Geez Kevin, that's a lot to assimilate--and lots of stuff I didn't know. As I suspected would happen, there's going to be lots to think about. I'll be honest--I like cooking with gas, but that's only compared to a basic electric element cooktop. I have never used an induction top. But I will research them. I was glad to read the article about range hoods--I always figured they were oversized and glamorized. Thanks for the reply!
I read the induction cookbook posts cited by Kevin. It mentions what seem like very high phantom loads.
Does anyone know if one could install a switch for an induction cookbook to shut it off when not in use?
Would the heavy power needs, and big wires make such a switch impractical?