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Heat pump using loads of power overnight

 
Air Source Heat Pumps (ASHPs)
Last Post by Kev M 3 years ago
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 HJBHome
(@hjbhome)
Active Member Member
Joined: 4 years ago
Posts: 3
Topic starter 13/12/2022 7:03 pm  

Hi - we have a Ecodan and it’s doing really well though the cold snap. I’m noticing some nights, but not every night, it seems to be using loads of energy (like 40 kWh) between 12:30 - 4:30, which coincidentally (and luckily) is our cheap charging time for octopus go. 

 

I thought at first we were charging the EV those nights, but we aren’t. Nothing else changes between the nights this happens and those it doesn’t. Any ideas what might be causing this?

I’m wondering if the hot water heating, which will be scheduled to happen during the cheap tariff, is just taking the four full hours because it’s so cold out? But then it’s no warmer night before last than last night, so not sure how that would work. Perhaps we used less hot water so it didn’t need to heat much? 

Other new thing is a deep hum coming from the heat pump at times - much noisier than usual - but the Internet tells me that’s normal for defrost cycles. Perhaps we just haven’t been cold enough for this to kick in before. 



   
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 mjr
(@mjr)
Prominent Member Member
Joined: 4 years ago
Posts: 301
14/12/2022 1:53 pm  

Posted by: @hjbhome
↑

Any ideas what might be causing this?

It's blooming cold and, in most places, wet air, which results in lower performance and lots of defrost cycles.

Do you have the melcloud app? The Reports: Hourly Temperature Report might show you whether it's hot water (what temperature do you heat it to? Is there a legionella cycle on the very expensive night?) but you only get a day's history in there. Connecting MELCloud to an external monitoring system will give more history but is fiddly and completely unofficial and not guaranteed to work forever.

Just as comparison to show how things have changed this week, here's a recent heating cycle on my openEnergyMonitor:

image

Note that the outdoor temperature sensor reading (bottom line) spikes when it defrosts and it doesn't even get the flow up to the target temperature before the first defrost. So it's working harder to extract heat, using some of that heat to defrost the unit and the target flow temperature is higher because it's colder outside (weather compensation). Here's the same graph from when it was warmer/drier a few weeks ago:

image

(I don't have a more recent clean comparison because hot water often gets reheated in that time window which makes the graph dip as it switches between that and heating.)

And yes, the external unit is noisier when it's working harder, either due to cold or defrost. Fortunately, this isn't a time when the neighbours are sitting outside in their gardens and at least two of the nearby oil boilers are actually noisier, even stood near my pump!

(Edit to add: the pound cost values on the graphs are wrong because that app does not cope with time-of-use rates.)



   
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 Kev M
(@kev-m)
Famed Member Member
Joined: 5 years ago
Posts: 1276
14/12/2022 3:53 pm  

Posted by: @hjbhome
↑

Hi - we have a Ecodan and it’s doing really well though the cold snap. I’m noticing some nights, but not every night, it seems to be using loads of energy (like 40 kWh) between 12:30 - 4:30, which coincidentally (and luckily) is our cheap charging time for octopus go. 

 

I thought at first we were charging the EV those nights, but we aren’t. No

 

thing else changes between the nights this happens and those it doesn’t. Any ideas what might be causing this?

I’m wondering if the hot water heating, which will be scheduled to happen during the cheap tariff, is just taking the four full hours because it’s so cold out? But then it’s no warmer night before last than last night, so not sure how that would work. Perhaps we used less hot water so it didn’t need to heat much? 

Other new thing is a deep hum coming from the heat pump at times - much noisier than usual - but the Internet tells me that’s normal for defrost cycles. Perhaps we just haven’t been cold enough for this to kick in before. 

 

I'm not sure your Ecodan could consume 40kWh in 4 hours on its own.  That's a run rate of 10kW.  The most power I've ever seen my 14kW Ecodan using is about 6kW and it was running flat out because it was producing nearly 15kW at the time. The maximum current for my ASHP is 35A and that's only 8.4kW.  

Is there any chance a back up heater (if you have one) or immersion was being used? If your HW is set very hot and you've used a lot the day before this is possible.   

What flow rates are you running your heating on?

It will be doing frequent defrosts but that just means some of the energy produced never makes it inside your house. 

 


This post was modified 3 years ago 2 times by Kev M

   
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