Heat pump efficiency

[TimS]

I've seen several posts stating that a solar heater is operating at peak efficiency when the temperature difference is only 1 or 2 degrees. Is this also true of heat pumps? (I would assume that the physics remain the same, but...) There's about 8' - 10' of pipe between the heater outlet and the return, if that makes any difference. The way it is plumbed, I can raise and lower the flow rate through the heater easily enough by simply partially opening and closing valves if it makes sense to do so. I don't have a flow meter, but it's easy enough to guesstimate. BTW, I have an electric air-source unit.

Also, why does my pool heat pump shut down when the air temp gets around 55 degrees or below? I also have an electric air-source heat pump for the house, and it continues to run until the air temp gets below zero. (Well, actually it continues to run even below zero, but it then turns on the auxiliary heating elements.) It's very unlikely that I'd be swimming if the air temp was below 50 anyway, but I'm curious why the difference. Is it because heating water requires more energy than heating air?

Thanks,
Tim.

 

[Bama Rambler]

You are correct, the flow needs to be slow enough that heat transfer takes place but not so slow that saturation occurs.

The unit shuts down because it becomes very inefficient at low temps and the mfg's figure you're not going to be using it at those temps so they don't try to design a unit that works below 55º of so.

 

[JasonLion]

There are pool heat pumps that run at colder temperatures. In order to work correctly at lower temperatures they need to have a deicing mode. Adding deicing costs money, so many of the pool heat pumps leave it off. I'm sure your residential heat pump has deicing, so it can work at lower temperatures.

Unless you have a really large pump, you want the full flow rate going through the heater for the highest heat transfer efficiency. On the other hand, at high flow rates the pump efficiency goes down. In practice 40 to 80 GPM works very well with almost any heater. Most heat pumps and some of the smaller gas heaters are happy with flow rates below 40 GPM, but you may lose a tiny bit of efficiency.

 

[TimS]

Yes, my residential HP has deicing. I'd never thought about it until New Year's Day when it quit working. We'd had a string of cold days, but it suddenly dropped below zero overnight after the humidity had been really high. For whatever reason, the deicing mode stopped working that night, and I had a solid block of ice in the HP. At that time I discovered that the auxiliary heating coils are not big enough to heat the house, and the blower had failed in the electric fireplace. It got a might nippy in the house until I managed to get the HP thawed back out again. Fortunately I was able to locate an HVAC guy open on New Year's Day. He came out and inspected it, and discovered that the control module had flaked out. He managed to reset it, and it's worked fine ever since. (He also didn't charge me a dime!!!!!!! Guess who gets my HVAC business from now on! )

I guess I'm good with the plumbing. With all of the water running through the HP and the pool water at 70, I can feel slightly warmer water coming out of the return. I haven't tried to tell how much warmer it is, but it can't be too much. It's enough to tell that there is a difference, but it certainly isn't a huge difference.

Thanks, guys.