Hacker News Reader

Comment by signaturefish | Hacker News Reader

In the context of heat pumps, a "backup heating strip" is a resistive heating element - electricity goes in, heat comes out just like an old electric heater. They're generally used during defrost cycles for the external air handler in cold weather, or to provide a temporary thermal boost when the heat pump is having performance trouble e.g. when the exterior temperature is near the bottom of the heat pump's operating range.

More detail at https://carolinacomfortsc.com/hvac/what-are-heat-strips/ (no affiliation, they were the first authoritative return from my web search).

View on HN

Whole-house heating with resistive electric heating is going to get expensive, quickly. If they're serious about installing these in cold weather climates, they should have more information about the cut over temperature and the efficiency of the heat pumps at those temperatures.

Just for context, my heat pumps cut over around 40-45 degrees to gas-based heat.

You have an older system. New systems can provide 100% rated capacity well below freezing. Mine is -5F and not specifically a "cold climate" unit. The unit in OP seems to offer 100% nameplate capacity at 5F.

These are brand new heat pumps. I don’t think it’s possible to have high efficiency at cold temps because of the need for a defrost cycle. I just check and mine are around 2 COP at 8F and 5 COP at 40F.

It is not clear why your heat pump cuts over at 40 degrees if it still has COP of 2 at 8 degrees. Presumably a cost-effectiveness calculation if gas is cheap in your area. If you had heat strips instead of gas, the "cut over" temperature would be much lower. If your unit is capable of operating at full capacity below 8F, it would likely only use resistive heating below 5F. For most climate zones, that is fine and your unit does not sound like one specifically deemed a "cold climate" heat pump, the which now regularly achieve full rated output down to -15F. Of course, it's only rarely that cold even in "cold climate" zones so operating down at a COP of 1 for these short durations is not a real problem. The impetus for systems designed for lower temperature operation is not really aimed at those with cheap gas but at those living where heat pumps were previously only feasible for heating if using much more expensive ground source systems. Achieving year-round heat with only the heat pump eliminates the need for a furnace so you have to include this missing gear and installation cost in your analysis.

Yeah, I'd like to see those figures too, and I think they might surprise you (and me). I don't have figures or references to hand here, but as I understand it modern heat pumps should be good down at least a little way below freezing. They're widely used and popular in Norway, for example, and the Norwegians aren't exactly new to cold weather.

High efficiency heat pumps with built-in defrosters (or "heating strips") can often provide full heating capacity (ie: 48,000btu for a 4 ton unit) down to -12f, with reduced output at even lower temps. The COP efficiency drops quite a bit though and it will eventually get to a point where it is essentially as energy efficient as a resistive heater. Whether it's more efficient than using gas or not is a difficult calculation based on your gas price, heat pump COP temperature curve and electricity price. If you expect to see <-20f temperatures regularly, you'll want/need a backup heat source anyway.

Reply on news.ycombinator.com