How a heat pump works

Post by rovolo » Tue, Feb 14, 2023, 8:06 PM UTC

kenmacd wrote: post #217

pkulak wrote: post #136

Earlier quoted context omitted.

I just found this guy too. He answered a lot of questions I had about efficiency and use below freezing. https://youtu.be/fxEqVuiHhM0

I like his videos, but I don't trust his numbers in this one. He shows a COP of ~1 when it's 10 degrees out. These are the easiest possible conditions for the HP, and where all the specs show the highest COP. His graph showing the HP is running equal to a baseboard heater seems very suspect.

The issue with the COP graph (9:05) is that it's comparing the mini-split system to the prior system. The mini-split doesn't have radiators in the same place as the prior system, so he runs it 1-2˚ hotter (7:50) to get the rest of the house to the same temperature. Also, the system isn't using much electricity on warm days (8:48), so the inefficient placement of the minisplit is more dramatic.

The more interesting graph is the total kWh saved per day (8:07). The colder it gets, the more energy he saves. That's because more heat needs to be pumped into the house, so a worse COP multiplied by more total heat equals more electricity savings (at least for the temperatures he's measured).

Post by Casteil » Wed, Feb 22, 2023, 1:46 AM UTC

snozolli wrote: post #272

Casteil wrote: post #222

Earlier quoted context omitted.

He does, but he omits a lot of important considerations that make heat pumps impractical for normal Americans. If he touches on them, it's briefly - if at all, e.g.: - More expensive units, and more expensive to run power wise. It's always difficult to get adoption of new technology that's "better" but more expensive to buy & repair - to the point where RoI may never happen. - Power grid implications during very low/…

With a heat pump, you -need- an alternative source of heat - since some days when it's particularly cold Modern heat pumps redirect some heat to prevent the coils from freezing. What you wrote is common FUD pushed by the oil & gas companies. That said, you'd be a fool to live in an area with life-threateningly low temperatures and not have a backup heat source.

>Modern heat pumps redirect some heat to prevent the coils from freezing. What you wrote is common FUD pushed by the oil & gas companies.

It's not FUD, and you COMPLETELY ignored & omitted the rest of my sentence from your quote. I'm well versed on how heat pumps work. I was not saying heat pumps don't work below freezing. Rather than simply repeat myself, let me rephrase:

1. Heat pump efficiency gets worse the closer you are to the minimum (outside) temperature they're rated down to.

2. Even with the best (and most expensive) technology, that means efficiency of these units approaches that of a $15 space heater.

3. When it gets TOO cold, they can stop working entirely if they don't have a resistive backup heat system (which, again, is effectively a $15 space heater you paid a lot more for).

Both Canada & the northern US experience these temperatures occasionally (-10f to -30f and worse on some occasions).

Hence the need for backup heat - e.g., a pellet or wood-burning stove, backup furnace running propane or natural gas, or even just something cheap and simple like a $150 portable diesel heater. You don't want to be left without heat in a multi-day power outage after a half inch of ice and high winds.

For backup heat, it's more efficient to burn whatever fuel for heat than run a generator outside to power space heaters or a heat pump at those brutally low temps that often follow those hard ice storms.

Re: How a heat pump works

Re: How a heat pump works