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Old 10-18-13, 01:51 PM   #9
MN Renovator
Less usage=Cheaper bills
 
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"Right now we are on an electric budget of $281 per month year round and they are suggesting we bump that up!! ouch!"

With this house, you could use two standard space heaters(1500 watts each) and it would maintain a difference of 75 degrees between the inside and outside. So if keep the house at 70 degrees, it could be 5 degrees outside. This would be if the house was very air tight and for the duration of the coldest period you had the ventilation system off. If we include ventilation and use just one space heater, it could be 70 inside and 40 outside. Of course keep one thing in mind, this is factoring no added sun and doesn't factor in the thermal mass effects of a non-carpeted concrete slab, so you could have slightly less heating and the temperature won't drop that quickly with the level of insulation you have in the house. Randen's post above regarding his solar heated slab that lasted through the night shows the potential for thermal mass heat storage. My heat load calculations don't factor that in but in general that heat has to come from somewhere and heat will then need to be replenished by solar again or supplemented by another source.

I'm not sure what your electric rates are but if you were stuck with no sun for a 24 hour period and the temperature outside was stuck at a temperature 75 degrees colder outside versus inside and your electric cost was 12 cents per kwh, it would cost $8.64 for the day. I don't suggest electric resistance heating as a primary heat source though because electric resistance heating, propane, and oil heat are the most expensive ways to heat a house!

Central air is a bit out of the question with such a low heat load, even the smallest 95+% efficient furnace(40,000 BTUhr) that runs on propane or natural gas has a heat output that is 4 times as much as what you need on your coldest day. It would fire up for only 25% of the time, 15 minutes per hour on the coldest night. ..which isn't really all that bad of a thing except you need the ductwork for about 600 CFM installed and you'd probably also end up connecting a 1.5 ton air conditioner or heat pump to it which would be oversized for your houses load and less efficient than a mini-split inverter heat pump setup that would be both cheaper to install in new construction and far more efficient.

Hydronic heating using a heat pump with solar assist or mini-split inverter heat pumps are the choices I'd personally think are the most appropriate.

Hydronic heating using an 'electric boiler' that isn't sourced by a heat pump would give you warm floors but wouldn't be cheaper to heat the house than electric baseboard or space heaters.

Hydronic heating using geothermal and assisting with solar would give the lowest electrical cost but the initial installation cost would be very expensive unless you are able to DIY a large part of it as randen suggests. I personally can't do that sort of work with geothermal design and install but if you can the high labor parts and work with a geothermal person who can help you, it would make this much cheaper than if you didn't DIY a good part of it.

Mini-split heat pumps are my personal choice. I take a different approach when I've modeled my house, which would be a retrofit of my current 2100 sq ft multi-level house. I'm looking to get my heat load from 25830 at a Minneapolis design indoor outdoor 70f/-12f 82 degree difference down to 9730. The fun thing is I've actually monitored my usage and am within 500 BTUhr between my heat load calc and actual furnace output during the coldest day in Minnesota this winter. To get down to 9730 I'd need to replace all of my windows with U 0.2 or better(R5), R60 in the attic, add an extra 4" of XPS to the exterior, and be sure that I've covered as many air leaks and thermal bridges as I possibly can get to. For good measure I'd also replace my natural gas equipment with PVC vented 90+ equipment to get rid of the sieve that is the stack effect. I'm also planning to reduce my glass area with the house, I have large picture window and two side windows that will lose 1 foot of height and be replaced with insulation because the winter sun doesn't wrap around the house enough to bring heat through them so they are terrible year round, when replaced they will be heat rejecting triple-pane for sure. I have a serious disadvantage because I don't have a single window in my house that benefits well in the winter to heat my house. The only reason why my projected retrofit uses less energy than yours is because of less window area and less thermal transfer through the glass. My current cooling load is higher than my projected heat load, with glass reduction and heat rejecting glass I should be able to shore them up and use a single 12k BTUhr inverter heat pump to cover nearly all of my heating and cooling needs. Unfortunately we drop to -20f here in MN usually once or twice a winter with -10f or lower about 5 days each winter. Below 0f, a mini-split heat pump struggles to produce the heat I'd need. Since my house was originally designed with forced air HVAC and the era of house has it, I'll keep that system installed for the sake of selling the home(with features expected from being built in the 80's) in the future and for heat when it is cheaper to heat with natural gas.

Here's a different look at solar heating in a net zero sense. If you are planning for PV, size your PV to include additional capacity to cover water heating and heating/cooling loads. With the low cost of PV and the fact that you get additional electricity fed back through solar in the summer when you don't need the solar heating, the house you could pay back the winter usage and water heating, as well as power the house. I'm actually thinking if you were looking for net zero energy use for heating and cooling the house and heating water that it would be cheaper with PV versus water. ...you don't get the benefit of warm floors with that idea though, but if you don't end up heating the slab with hydronic, its another option.

If you are looking to put down tile floors, have exposed concrete or another similar surface I think you really need to use hydronic floors to keep them comfortable. Same with if you are looking to heat the house with solar water heating, this would be the best way to do it.

Since I don't have thermal mass and retrofitting without a real hydronic option, I'm part of the super insulation camp with minimal windows. I am planning to build a solar air collector or two and duct that into the house eventually. With that I can have the glass benefit me by heating the house but when the sun isn't shining, the fan shuts off and the damper closes and I don't lose heat through that same glass at night. I also don't need to worry about summer overheating or overhang planning.
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