Question about heat pumps in hot temps

[MillCreek]

We are going into our third day in a row with temps over 100 degrees, which is very unusual for the wet side of the Pacific NW.  Lots of chatter at work about how they cannot get their house temps under the upper 70's or low 80's with the heat pump.  Would a dedicated air conditioner have greater cooling capacity than a heat pump?  Not that you can currently find an air conditioner or fan for love or money in this area right now.

[Nick1911]

We are going into our third day in a row with temps over 100 degrees, which is very unusual for the wet side of the Pacific NW.  Lots of chatter at work about how they cannot get their house temps under the upper 70's or low 80's with the heat pump.  Would a dedicated air conditioner have greater cooling capacity than a heat pump?  Not that you can currently find an air conditioner or fan for love or money in this area right now.

No.  Well, not if it isn't a larger unit, of course.

The seattle-tacoma summer 1% design temperature is only 80F.

[Ron]

Nick knows.

60k BTU's/5 tons of A/C is the same, regardless of how the heat is removed.

Windows, sun exposure, insulation, whether the system was installed under sized, to spec or oversized all play a part.

[K Frame]

"We are going into our third day in a row with temps over 100 degrees, which is very unusual for the wet side of the Pacific NW."

That's the money statement right there. Simply put, heat pump systems are (or should be) sized for historic "normal" conditions in the area where they are installed.

Oversized systems are more expensive. And, in the case of cooling where humidity removal is also a requirement, an oversized system can cool too quickly and leave the air feeling uncomfortably clammy.

My heat pump was originally sized for my house as I bought it. But when I ramped up the insulation in the attic, that had the same effect as oversizing the cooling aspect. The end result is I now have to run dehumidifiers to deal with the ambient humidity in the house.

[Ben]

I'm on the East end of the heat dome Millcreek is under. There haver been a ton of articles locally about cooling, energy use, etc. floating around this week. One of the HVAC companies was suggesting that heat pumps are designed to optimally work within 20 deg of the outside temp - that is, if it's 100, don't expect it to get you below 80 efficiently.

I'm still fairly new to heat pumps, so don't know how accurate that is. It has been around 100 here the last few days, and it doesn't seem like the heat pump is working that much harder to keep me at 74, which is what I'm running during the day while this heat wave is on (ceiling fans are on). My energy use graph from the power company shows some increased KWs for sure, but still not approaching what I see for usage in DEC when it's 30 outside and I have it set for 69.

I've been setting it to 71deg for night, and I've seen it struggle a little to get from 74 to 71, which it doesn't do when I change temp like that when the highs are in the 80s. It's around 85 outside when I set that around 2100.

[Ron]

20 degrees cooler than the upper ambient temperature design temp would be considered pretty normal in a residential A/C system also, "she's doing all she can do cap'tn and she cant do no more". The same complaints are heard here in the midwest during 100+ heatwaves or high 90's with high humidity. More systems are designed for the hotter temps around here just by nature of the fact we normally have temps in the 90's every summer.

[dogmush]

Weird.  We routinely have temps over a 100, and have multiple months with the highs always in the mid-90's and my heat pump keeps my house at a comfy 69F 24/7.  So it's not a limit of the technology.

I suspect Nick hit the nail on the head that the cooling capacity isn't sized to remove that much heat due to regional decision making.

[Jim147]

Mine struggles in the high heat and humidity like everyone's. If I have to put a new unit in I'm thinking to do a oversized multistage unit.

[Brad Johnson]

If I understand correctly, heat pump operation only has an effect issue during heating needs. As an A/C, it operates as any other A/C would and the only things affecting operation would be sizing and efficiency (Jim, is this right?)

Brad

[Jim147]

Pretty much. Btu is Btu until it gets too cold. The heatpump can draw heat out of the air to zero or below but it doesn't do very well much below freezing. Around here I normally set the switch over around 28-30 so people stop calling about cold air out the vents. But most of the back up here is gas so much cheaper than electric.

[Zardozimo Oprah Bannedalas]

If I understand correctly, heat pump operation only has an effect issue during heating needs. As an A/C, it operates as any other A/C would and the only things affecting operation would be sizing and efficiency (Jim, is this right?)

Brad

Yeah, it's just an AC unit with valves and fiddly bits that let it run in reverse for when you want heat.
I suspect that since record temps have rarely gone past 100 in the area that a lot of people have undersized AC that simply can't hold up to Arizona summer levels of heat (or has serious problems with coolant, or nobody's ever changed the filter, etc).
I'm going on the second summer of no AC since I moved north, and... it's not actually that bad, as at worst it gets a little uncomfortable on the south side of the house when it's in the mid-90s, but when the sun goes down you open the windows and turn on the fans and 65 degree or cooler air comes in to make everything comfy and cool for bedtime. Also, AC install is problematic because these idiot crank open windows someone had to have. If I get the go-ahead to continue telework I might get one in the near future anyway, as if/when a heat wave like Portland's seeing comes here, I'm not sure my fans and swamp cooler would be sufficient.

[Bogie]

Where I am, the gas company charges about $30 a month whether you use any or not.
 
I use next to nothing nine months out of the year. So I went all electric.
 
My heating costs me about $100 over minimum usage ($150 vs. last months' bill was $55 with minimal usage). AC during the summer costs me about $100 a month.
 

[K Frame]

If I understand correctly, heat pump operation only has an effect issue during heating needs. As an A/C, it operates as any other A/C would and the only things affecting operation would be sizing and efficiency (Jim, is this right?)

Brad

Effect issue? Huh?

A heat pump is just that... it pumps heat. Home units usually can pump heat in two directions -- into the home, or out of the home.

External ambient temperature affects both parts of that cycle.

[Ron]

Effect issue? Huh?

A heat pump is just that... it pumps heat. Home units usually can pump heat in two directions -- into the home, or out of the home.

External ambient temperature affects both parts of that cycle.

Technically, anything that uses the refrigeration cycle "pumps heat".

That is what refrigeration and air conditioning is, moving heat from one place to a different place using the refrigeration cycle.

Your air conditioner absorbs heat from the air circulated over it's evaporator coil and pumps that heat outside to the condenser coil where it is removed.

Cold/cool is our experience of the absence of heat.  If you want a space to be cool/cold you have to pump the heat out of the space.

Heat pumps just reverse the refrigeration cycle flow through the system depending on the season, flipping which coil they use to absorb heat and then discharge the heat depending on the season.

Refrigeration is just moving heat around.

[K Frame]

"Technically, anything that uses the refrigeration cycle "pumps heat". "

Exactly.

And as I said, ambient air temperature at the exchange point, whether it's outside or in your kitchen, affects the cycle no matter which direction the heat exchange is going.