Posted by: @alastair

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Hi all

We have a Daikin Altherma Monobloc 11kW unit that's going through its first winter - and it's our first winter with an ASHP.

We've not seen any evidence of a defrost cycle at work and during the cold periods (below 0C) we've been getting thick frost deposits on the coils. It is able to generate enough heat to keep the house warm, but it's regularly pulling 8-9 kW and the fan is going full tilt and is reaching 70dB. It ended up costing us nearly £200 in December and we are just shy of £120 for January. Which wasn't in the plan.

We have a Daikin engineer booked to look at it but I just want to make sure from others on the forum that this isn't expected behaviour. I want to be informed before the engineer turns up, because it looks unlikely to be cold when they do and I guess it'll work just dandy in warmer temperatures.

Thanks in advance for any comments you have

Al

I can't speak for anything other than my ASHP but my Mitsubishi Ecodan defrosted every 1-2 hours in the December and January cold snaps.  I never saw much if any ice on it.  Are you sure it's pulling up to 9kW?  That's a lot. 

And £200 isn't a lot for December; we're way more than that.  What tariff are you on? 

@kev-m It was defo iced up which was what concerned me initially.  I took screenshots of the energy usage to show Daikin - we were under the impression it might pull 5-6 kW in very cold temp but not 8 - 9 kW. I guess the £200 isn't huge, but we also have a battery so we had budgeted on the basis that the battery would cover a large chunk of the heat pump usage (which it sort of does when the air temperature is >6C or so) and for a monthly outgoing this was a bit of a shock. If we had four cold months in a winter (increasingly unlikely I know) it would be a bank breaker! We are on Octopus Go so are using the night tariff period as much as possible.

@alastair 

Ah, I hadn't realised you had batteries.  It shouldn't be iced up for any length of time  and it won't be doing much for your COP if it is.  See what the engineer says and let us know.

I have a high temperature Daikin split unit and it can pull 8-9kw when it does a defrost cycle, I think it uses the backup heater to do some of that. We are on Intelligent Octopus and December cost us over £270 including car charging, cooking and heating plus a poor solar generation month. We are at around £150 so far this month. Consider how cold it has been and what it would cost to fill a petrol car I’m pretty happy with the cost. 

Posted by: @alastair

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I want to be informed before the engineer turns up, because it looks unlikely to be cold when they do and I guess it'll work just dandy in warmer temperatures.

You should be able to video it frosted up.  And if you monitor your flow temp, you should see a defrost cycle. My heat pump takes 3-4 mins to defrost.  I have not yet worked out how long it is frosted before the defrost cycle kicks in though. [dips in temperature in below chart]

1. He noted that the heat pump is a 14 kW unit (as we had been told on installation but for some reason I have been thinking that it was an 11 kW unit!). He also noted that ~6 kW was required by the house based on the heat loss calcs done at install.
2. He watched our videos of the iced/frosted unit and looked at the energy usage plots that I had created and wasn't concerned by the frosting and was confident that the defrost mode is working as designed. He did however think that the energy draw from the unit was high.
3. He ran the unit for just over an hour and noted that the compressor was working hard on start up but that the once it dropped down it just hummed along - there was no cycling at all. Outside temperature at the time was 5C.
4. Given the size of the unit compared to the heat loss, he lowered the weather comp curve from 48C @ -2C to 42C @0C
5. As we discussed the energy usage and as he gathered data he thought that the backup heater could have been the cause of the high kW draw during cold weather. Given the size of the unit he decided the backup heater is likely not needed and so it has been set to operate at -10C outside air temperatures (or if the water temp falls to 18C as designed)
6. Noise from the compressor and fan was measured at just under 60dB at 1m with the compressor exposed, which he deemed was within suitable tolerance. I still think that's quite noisy, but there we are.
7. He checked the magnetic trap and it was clear.
8. He updated the controller firmware and showed how to move to fully weather comp (leaving water temperature controlled) and we discussed the best way for us t use the system. Decided to leave 'as is' with a weather compensation curve working on a timer system.
9. He was impressed with the flow rate and efficiencies obtained and commented that he thought it was a really good set up, installation and commissioning.

He was a really good engineer - he knew his stuff and was willing to discuss and explain the system and how it worked 

Hope that's of interest

Cheers

Al