ASHP defrosting question

[BoulderDash]

Given that it’s likely to be predominantly 0-5C and moist during the winters where I live in SE England, does anyone have any suggestions as to how I can get a flow temp >25C in these conditions? I had thought that the impact of defrosting on ASHP performance had largely been overcome, but if this is the issue with my system, then it seems significantly inferior to a conventional oil-fired boiler. At the moment I seem to be spending about twice as much on energy for less heat..

[SteamyTea]

1 minute ago, BoulderDash said:

Given that it’s likely to be predominantly 0-5C and moist during the winters where I live in SE England, does anyone have any suggestions as to how I can get a flow temp >25C in these conditions? I had thought that the impact of defrosting on ASHP performance had largely been overcome, but if this is the issue with my system, then it seems significantly inferior to a conventional oil-fired boiler. At the moment I seem to be spending about twice as much on energy for less heat..

Oversize the heat pump.  why sizing is so important and cannot be directly compared to a thermal boiler.  They work different ways.

But you should be able to get a flow temperature above 25°C easily.

[Gone West]

2 hours ago, joe90 said:

bring on the beast from the east No2

For Christ's sake don't say that. We've been snow free here up to now but it's all going to change for the next few days . West is best.

Edited February 6, 2021 by PeterStarck

[dpmiller]

Been playing a bit with the defrost settings on mine- the start of the week was socked-in-cloud wet and around +2c for a few days. Probably about as bad as it gets...

 

But i was able to reduce the duration and end temp a fair bit and still get a good defrost and clearance of water. The steamy puff is much reduced.

[ProDave]

2 hours ago, joe90 said:

Been on fir 4 hours and no sign of defrosting ?, house up to 22’ so it’s off now, bring on the beast from the east No2 ?.

Beast From East No 2 been here since Christmas.  Cold bitter wind and snow coming from the E right now.  Not seen the ASHP defrost today (but then I have not been watching it particularly)  but no sign of ice forming on the heat exchanger, even when it briefly got up to 2 degrees today. Now back down below 0.

[BoulderDash]

3 hours ago, SteamyTea said:

Oversize the heat pump.  why sizing is so important and cannot be directly compared to a thermal boiler.  They work different ways.

But you should be able to get a flow temperature above 25°C easily.

Do you think that a larger pump would help with this issue? The installer dismissed this possibility by saying that a bigger pump would do exactly the same. Is an 8kW pump for a house with 176m2 floor area in the right ball park? How do you go about calculating the right pump size?

 

Sorry, lots of questions... 

[PeterW]

20 minutes ago, BoulderDash said:

Is an 8kW pump for a house with 176m2 floor area in the right ball park? How do you go about calculating the right pump size?

Depends !! What are the heat loss calculations for the house and what is the heating mechanism ..?? UFH..? Rads..? Both ..? All impacts on ASHP sizing. 
 

8kW I would say was borderline and if it’s also doing DHW is possibly slightly under sized. Next step up would probably be 11/12kW so would want to see a proper set of calculations. 

[SteamyTea]

@joe90

Quick check before bedtime 23:30PM

Your dewpoint temperature is now at 1.4°C, your outdoor conditions are 1.9°C and 95% RH, So looks like you should not freeze up.  But only by the skin of its teeth.

 

[dpmiller]

how do you work that out @SteamyTea? the coil will be much cooler than ambient, it's how it works...

[SteamyTea]

1 minute ago, dpmiller said:

how do you work that out @SteamyTea? the coil will be much cooler than ambient, it's how it works...

Bit of education and guess work.

The airflow though the heat exchanger will be fairly rapid, so the air will not cool too much as the volume will be quite high.

Though at 95% RH, it may well happen, but on a very small scale.  Could well happen if the fan stops.

And it may have frosted up, Joe90 has not reported back.

[dpmiller]

some numbers from mine- which has a variable speed fan, tracking compressor load. At Tamb 4c (measured at the rear of the unit) the coil can get down past-2c. I've set defrost to start at -2c, a little colder than the factory setting. Just before defrost started it looked like this:

 

a thin hoare frost over the whole coil, no obstructive solid ice.  By the time the unit went through it's processes- fan off, ramp down compressor, change direction, ramp up, repeat, about seven minutes had passed but the coil had de-iced in half that.

[joe90]

44 minutes ago, SteamyTea said:

Joe90 has not reported back.

After it went off yesterday afternoon, it’s not run again, house warm enough.

[SteamyTea]

1 hour ago, dpmiller said:

a thin hoare frost over the whole coil, no obstructive solid ice

Seems your system is working right.  Do you collect data all the time on the air temperatures?

36 minutes ago, joe90 said:

After it went off yesterday afternoon, it’s not run again, house warm enough.

Phew, got away with it.

And you have a nice cosy house.

[joe90]

Just come on again, (SWMBO left the front door open after walking the dog ?).

[troggy02]

3 hours ago, dpmiller said:

some numbers from mine- which has a variable speed fan, tracking compressor load. At Tamb 4c (measured at the rear of the unit) the coil can get down past-2c. I've set defrost to start at -2c, a little colder than the factory setting. Just before defrost started it looked like this:

 

a thin hoare frost over the whole coil, no obstructive solid ice.  By the time the unit went through it's processes- fan off, ramp down compressor, change direction, ramp up, repeat, about seven minutes had passed but the coil had de-iced in half that.

 

Curious, ours continues to extract heat until the unit is solid white at the back. In your picture it looks pretty unobstructed, so maybe defrosting a little early?

[epsilonGreedy]

17 hours ago, BoulderDash said:

How do you go about calculating the right pump size?

 

 

A good question. For two years I have been wondering what the rated output of an asph actually means. Is it:

1. Maximum heat output at cop 4.
2. Maximum heat output at an ideal output water temp.
3. Maximum heat output in ideal environmental conditions.
4. Maximum electrical input power draw.
5. Maximum electrical input power draw at worst case cop 1.

 

 

[SteamyTea]

8 minutes ago, epsilonGreedy said:

For two years I have been wondering what the rated output of an asph actually means

RTFM

 

Here is, as an example, an Ecodan one.

 

[dpmiller]

4 hours ago, SteamyTea said:

Seems your system is working right.  Do you collect data all the time on the air temperatures?

Not really,  it's only in a week and I'm trying to get some baseline data. But it's only a few button presses to scroll through the current position any time I'm walking past the plant room...

[Eileen]

I have the same issue with defrosting. If it’s below +6C outside we reluctantly turn the ufh off because it still burns power but provides no heating (water never gets to temp before next defrost cycle). We are a passivhaus so we can survive comfortably without heating for a day or so.  I’ll keep reading this thread for suggestions, but I have a general question. Very occasionally, when starting up, I notice the power consumption running at 5kwh for about 30 mins. it’s normal run rate is about 2.5kwh. It’s related to low temperatures, I assume a defrost,  but has happened when first turned on, often at noon when I’ve waited for the outside temp to get above 5. Is there some process that explains a 5kwh usage. It’s very rare and not part of its general heating or defrosting pattern of power consumption.

[SteamyTea]

1 minute ago, Eileen said:

I notice the power consumption running at 5kwh for about 30 mins. it’s normal run rate is about 2.5kwh. It’s related to low temperatures, I assume a defrost,  but has happened when first turned on, often at noon when I’ve waited for the outside temp to get above 5. Is there some process that explains a 5kwh usage.

I think you mean kW, the power the ASHP is drawing, not kWh, which is the energy it has used.

If you are getting frosting at 6°C, some sound wrong especially if you are not getting enough time to heat the slab up a bit.

[Eileen]

Thanks Steamy Tea, yes sorry, kW, forgive my schoolgirl errors. Thanks for confirming that something is not right, we’re finding it very hard to get anyone to advise us, no-one seems to know what to suggest,  We don’t know if putting in a more powerful unit would guarantee to resolve the problem so are reluctant to try it and find out it that ot makes no difference. At the moment it’s not fit for purpose, but we are obviously very committed to this type of heating. Do you know if the rare 5kW spike is a normal thing, or if that in itself is also an indication of a problem?

 

[dpmiller]

Does your heat pump also control- or have an internal- immersion heater?

[SteamyTea]

16 minutes ago, Eileen said:

We don’t know if putting in a more powerful unit would guarantee to resolve the problem so are reluctant to try it and find out it that ot makes no difference

Without knowing the heat loss of the house, and the DHW needs, and the size of the heat pump, it is just speculation.

[Eileen]

Steamy Tea, it’s a certified passivhaus so low heat loss. It’s a Daikin Altherma LT Split EHBH08CB3V wall-hung indoors, ERLQ006CAV3 outdoors . It uses a Sunamp for DHW but that doesn’t use the ASHP (that’s another story, we initially hoped it would. But for 9 months the sun is enough to fill it and then direct from the grid for the other 3.) I think we suffered a bit from being early adopters 3 years ago, different tradesmen for the different techs, and no local experts in marrying all the technologies. It’s taken us this long to know that it’s a real issue and not just user error or teething troubles, because it’s only on for 3 months of the year and needs a run of frosty nights.  Finding an expert is the hard bit. No-one we called can tell us if a bigger unit will make any difference. Any tweaks an engineer has done can take a year to reproduce the conditions that cause it, and every experiment to see if it works differently with different usage patterns needs the same conditions for days/weeks.  Apparently the next size up would fit our current pipe work, but there’s no guarantee it would make a difference, but we’d need whole new pipe work if we went a size above that. So do we go one size up and risk failure  or rip out more and possibly over-engineer it in the hope of fixing it. If anyone knows any Daiken  heat pump experts in Devon, and ideally know Sunamp too, who can say ‘X is the problem and Y will fix it’ and is willing to take on maintenance of an installation they didn’t put in, I’d love to hear.

 

DPmiller, I’ll check on that, I know when it was first installed Daiken sent a man I believe he disabled an heater within the internal unit because being in the plant room it would never need it. if the external unit has one would it explain this occasional  power usage surge? I see lots of defrosts but this spike is very rare, only saw it once or twice, each time for about 20-30 mins, and it sticks out like a sore thumb on the solaredge app household power usage.

 

Thank you both for your help, I am going to contact Daiken direct now that I have the confidence to say it really is an issue that needs to be fixed.

 

[SteamyTea]

49 minutes ago, Eileen said:

it’s a certified passivhaus so low heat loss. It’s a Daikin Altherma LT Split EHBH08CB3V wall-hung indoors, ERLQ006CAV3 outdoors

That tells us nothing.

What is the expected heat load for the house. This is often expressed as kW per ⁰C temperature differences, or as a maximum heat load i.e 4 kW.

 

Without looking up the details if your heat pump, do you know the maximum thermal output?

 

52 minutes ago, Eileen said:

I know when it was first installed Daiken sent a man I believe he disabled an heater within the internal unit 

Sounds like he disabled the resistance heater that is used for the legionella cycle, which would not have been needed as your DHW is taken care of by the Sunamp.