As we mention on our house signage, we have no furnace… But we still do have a little bit of heating need, and have been considering a couple of ideas of how to meet that need. Its looking like the system we’re going to use are a couple of simple in-wall Cadet heaters. This was the original plan which we abandoned for a while in favor of a mini split heat pump system, and now that we’re needing to actually specify a system in order to get electrical in place for it we are going back to the Cadet heaters.
A heat pump was something that we had been considering early on but never looked at it very seriously until we had a meeting with our LEED for homes representative who very emphatically encouraged us to go with the heat pump rather than electrical resistance heating for efficiency reasons. In general heat pumps are a great way to go, the Cadet heaters are basically considered to be 100% efficient, but any heat pump we would install would be more along the lines of 250% to 300% efficient and in addition to increased efficiency, earns us quite a few LEED points thanks to higher performance on their energy model. A decision to go with electrical resistance heating loses those points (possibly enough to keep us out of Platinum) and would cost more to operate because of the increased energy usage.
Why It is still the better choice
The simple answer is payback time. We have done a pretty good job of reducing our need for consumption by reducing the size of our house. Assuming about 400W of internal heat sources (people plus appliances), we are looking at a house that will stay toasty on its own down to about 55 degrees, below that we may need some supplemental help.
We expect in total that our total cost for heating with electrical resistance to be around $100 for a year and with a heat pump operating efficiently to be around $35. So while the heat pump is significantly more efficient, it only costs us a fraction of an already small number. The price difference then of a mini split heat pump system installed and Cadet heaters is roughly $3,000, divide that by the $65 annual savings and you get a payback time of 46 years. Then consider that we are paying for this with a construction loan that will be rolled into a mortgage so over the next three decades we’ll be paying interest on that $3,000 causing the amount spent on the heat pump in our lifetime to nearly double, bringing the payback time to upwards of 90 years.
I’m generally very pro efficient systems, and this is where small houses really shine… By saving money and building a smaller house, we are now saving more money by not needing to buy the fancier heating setup. While electric resistance is less efficient, our annual $100 expense is less than half of what most of our neighbors spend on heat each month. And our bet is that some time in the next 90 years, we will be able to afford a new and better system, whether it is something that hasn’t been invented yet, or an evacuated tube array for a solar hot water radiant floor system, or simply offsetting our usage with photo-voltaics.
The problems with HERS and Passive House
This really cuts to the center of why small homes make sense and why they are rarely built. The building industry as a whole has embraced quantifying homes on a per square foot basis, and most of the standards that people are used to using break down with small homes. The HERS index that LEED uses is no exception, according to their model our house is efficient, but not outstanding. Thats because they look in terms of energy used per square foot rather than energy used as a whole. Even though they have an adjustment for house size for your overall score, it is not sufficient to overcome the serious deficit that we are given with their model. Where this method of scoring becomes irresponsible, is when you consider that heat loss is mostly dependent on surface surface area, so for the best score you want to maximize the amount of floor area you have while adding as little to your envelope as possible. Logically then, assuming you build your entire house to the same construction standards and don’t add complexity, building bigger will always win you better efficiency even while it increases your total energy usage.