Starting out on my heat pump journey

[AE-NMidlands]

when I looked at the link the spec said "Temperature hot water (max. — with auxiliary heating) °C 55 - 75"
Looks like immersion (or other) top-up is expected. Is that always the case with heat pumps doing DHW?

[Ronski]

No, although it is an optional extra to fit an immersion heater, both in the uniTower and other HWC.

[Moxi]

From what I have read about DHW from heat pumps you can get all the DHW you want at very good CoP's as long as you try where practicable to only heat water to the lower end of the hot water scale, that seems to be 40 to 55 degrees in most literature, you can push the heat pump to get you up to 65 degrees but that will adversely impact the CoP and is harder on the heat pump in general. To me the running for DHW is just like it is for heating - low n slow if you need lots of high grade hot water quickly then the HP is the best option - but then you lose out on the 2 or 3 to one HP advantage for heating the water.

I think my solution would be hot water tank sized for the needs of the family and filled by an A2W HP to around 45 degrees then in the solar months boost with the immersion if a quick reheat is required otherwise let the A2W HP do the recovery - then theirs just the need to posturise now and again for legionella if you feel there's a risk - that can be done when excess solar is about or on off peak electric if you have a suitable tariff.

Thats as far as I got so far

Moxi

[Joeboy]

I tend to agree Moxi but two caveats. SWMBO who owns a dishwasher demands the ability to have hot water on tap capable for handwashing dishes. I tend to agree. The end result for us is 65 deg water. The cop does take a bit of a kicking but tbh honest I'm ok with that. I'm still on a huge multiplier over 1:1 as was.

We also set the ashp to run in it's biggest window after 11am. With hindsight and considering 'air' 2pm would probably be peak for ground level air temp, hence ashp fuel?

[Moxi]

I will fess up to being a kettle boiler for washing water - it’s not warm enough for pots if you can bear to dip your hands all the way under - for me it’s cheaper than firing up the combi boiler and the kettle is next to the sink so that means for us at least DHW is for ablutions and that kind of means that a tank at 40 degrees should be plenty.

I am not ready yet to scrap off a serviceable boiler and ever mindful that changes need to be minimal upheaval because of the kids and pets for SWMBO that my journey to full electric will be A2A first for fast heating and cooling, a direct hot water cylinder for cheap hot water off peak and in solar months then remove the combi boiler and maybe retro fit a A2W heat pump when they’re more common/ cheaper to provide base load DHW and heating via existing rads and use A2A as a temp top up on the ten days a year where a 35 degree flow temp doesn’t warm the house enough - maintain log burners for those times when we are without power for a few days.

Minimises disturbance to bite size chunks spreads out costs and gives me a degree of variance that appeals though I appreciate others will have their own specific preferences.

Moxi

[MikeNovack]

........ temp doesn’t warm the house enough - maintain log burners for those times when we are without power for a few days......
Moxi

Not JUST for providing all the heat when no electric power.

The "efficiency" (COP) of an ASHP heat pump decreases as the air temperature outside decreases, so uses a lot more electricity during an unusual cold snap. During such times you could take much of the load with your log burners. Not going to be that many total days during the heating season, so not going to be a total of a lot of wood. IF you are in a climate where many days it is cold enough that your ASHP's defrosters are working (maybe not common in your climate but necessary in mine) could maybe also be wanting to burn some wood to keep the heat collector form getting so cold a defrost cycle turns on.

[Stinsy]

........ temp doesn’t warm the house enough - maintain log burners for those times when we are without power for a few days......
Moxi

Not JUST for providing all the heat when no electric power.

The "efficiency" (COP) of an ASHP heat pump decreases as the air temperature outside decreases, so uses a lot more electricity during an unusual cold snap. During such times you could take much of the load with your log burners. Not going to be that many total days during the heating season, so not going to be a total of a lot of wood. IF you are in a climate where many days it is cold enough that your ASHP's defrosters are working (maybe not common in your climate but necessary in mine) could maybe also be wanting to burn some wood to keep the heat collector form getting so cold a defrost cycle turns on.

Interesting how often I hear “HPs are not effective in cold weather” however the exact same models we use are popular in Scandinavia.

[chris_n]

........ temp doesn’t warm the house enough - maintain log burners for those times when we are without power for a few days......
Moxi

Not JUST for providing all the heat when no electric power.

The "efficiency" (COP) of an ASHP heat pump decreases as the air temperature outside decreases, so uses a lot more electricity during an unusual cold snap. During such times you could take much of the load with your log burners. Not going to be that many total days during the heating season, so not going to be a total of a lot of wood. IF you are in a climate where many days it is cold enough that your ASHP's defrosters are working (maybe not common in your climate but necessary in mine) could maybe also be wanting to burn some wood to keep the heat collector form getting so cold a defrost cycle turns on.

Interesting how often I hear “HPs are not effective in cold weather” however the exact same models we use are popular in Scandinavia.

And here in the Austrian Alps, although in Scandinavia and here when it is really cold it is usually very dry too so not so much moisture in the air to cause icing in the first place.

[MikeNovack]

Interesting how often I hear “HPs are not effective in cold weather” however the exact same models we use are popular in Scandinavia.

And here in the Austrian Alps, although in Scandinavia and here when it is really cold it is usually very dry too so not so much moisture in the air to cause icing in the first place.

Less effective is not the same as not effective. Please do not twist what I said.

The maximum theoretical performance of an ideal heat pump OR ideal heat engine (reversible Carnot cycle) depends on the temperatures at which heat comes in and heat goes out measured in absolute degrees. If the temperature at which heat comes in is the higher, it is a heat engine (produces power) . If the temperature at which heat comes in is lower, a heat pump (consumes power).

efficiency of the heat engine (T_h - T_c) / T_h
coefficient of performance of the heat pump T_h / (T_h - T_c)
This is actually simply a different way to state the Second Law of Thermodynamics. Carnot was early 18th Century (I think he died circa 1830)

PLEASE -- I am NOT saying that heat pumps do not work when the T_c is low compared to the T_h. I am simply saying that they work less well (require more energy input to pump a given amount of heat). In other words, an ASHP will require more energy to pump a given amount of heat heat up to 27C (300K) if the air outside were -20C (253K) than if it were 0C (273K) because T_c would be lower. The denominator greater, so lower COP.

Do not imagine technology could invent either a heat engine that outperformed a Carnot or a heat pump that did so. Carnot is by definition reversible, so if such a better heat pump (or heat engine) existed, it could be run against a Carnot going the opposite direction and produce energy from nothing (just the heat moving back and forth between those temperatures).

But in a practical sense, you also need to take into account the humidity of the air if the T_c is below 0C. Water vapor in the air will frost onto the collector. This is what happens in your freezer. Your freezer is a heat pump, pumping heat from the inside of the box into the room (as heat leaks back into the box through its walls). Because the inside (if a freezer) is below 0C, frost forms and you need to do defrost cycles. Because the inside (if a fridge) is above 0C, defrosting not necessary. But defrosting is an extra energy cost. IF the winter air is VERY dry, that is less of a problem. But remeber, the amount of water air can hold is about halved each 8C lower. So if humidity 25% at 0C it would frost on a collector operating at -16C. In MY climate not a terribly low temperature..

I'd expect to be teaching at least this much "thermo") in high school physics. Those of you who had A level Physics covered this, yes?

[Joeboy]

As to A level physics retention, which I did indeed pass. I was too busy, shagging, fighting and drinking for it to imprint too deeply. Ah the days!

Try not to appear CONDESCENDING Mike, it won't fit here.

Best of fortune to ye Ronski, glad to hear you have started your journey.