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Mitsubishi 8kW - strange energy use readings

 
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Air Source Heat Pumps (ASHPs)
Last Post by HarriUp 3 years ago
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 Kev M
(@kev-m)
Noble Member Member
Joined: 5 years ago
Posts: 1276
12/08/2023 8:51 pm  

Posted by: @westkent
↑

@kev-m 

one issue I’m having is that my hot water loses very close to 1 degree an hr throughout the day.

I do have an annual service in a few weeks so I’ll mention it.

Mine loses about 0.6 degrees per hour so a bit less.  Is your HW tank outside the house, e.g in the garage? That would make it lose a bit more than in the house.

 



   
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 Westkent
(@westkent)
Trusted Member Member
Joined: 3 years ago
Posts: 67
12/08/2023 10:03 pm  

170L tank in the airing cupboard where the immersion tank was.

pipes arnt lagged that feed off the tank which I’d imagine they should be although I’d of thought that reason alone wouldn’t amount to 1 degree an hour??



   
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 Kev M
(@kev-m)
Noble Member Member
Joined: 5 years ago
Posts: 1276
13/08/2023 7:27 am  

@westkent 

It will lose heat through the tank insulation as well as the pipes.  The pipes will make a small but significant difference.  Your tank is smaller than my 250l one so the pipes, etc., will have a bigger effect. If you're really interested in HW tank heat loss then have a look at this. 

AIVC.ORGAIVC.ORG "https://www.aivc.org/sites/default/files/airbase_7801.pdf"

Also, what is your HW temperature?  The hotter it is, the faster the tank will lose energy. A 55 deg tank will emit more heat (i.e. lose energy faster) than a 45 deg one. 

Let us know what the engineer says. 

 



   
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 Westkent
(@westkent)
Trusted Member Member
Joined: 3 years ago
Posts: 67
13/08/2023 10:16 am  

Hot water is timed to come on between 5am to 7am at its maximum 60c at 9.5p/kWh.

Thats ample enough h/w for 2 showers and the washing up.

My upcoming service will be carried out by a contractor from the Housing association not Mitubushi unfortunately so I’m unsure how knowledgeable they’ll be with advice on the settings.
My COP over the last 2yrs is only 2.4. 
Iv recently changed my setting from adaptive room to curve although we’ve not really used the heating side since the change of setting.



   
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 Kev M
(@kev-m)
Noble Member Member
Joined: 5 years ago
Posts: 1276
14/08/2023 7:32 am  

Posted by: @westkent
↑

Hot water is timed to come on between 5am to 7am at its maximum 60c at 9.5p/kWh.

Thats ample enough h/w for 2 showers and the washing up.

My upcoming service will be carried out by a contractor from the Housing association not Mitubushi unfortunately so I’m unsure how knowledgeable they’ll be with advice on the settings.
My COP over the last 2yrs is only 2.4. 
Iv recently changed my setting from adaptive room to curve although we’ve not really used the heating side since the change of setting.

Based on mine, I suspect your losses from the HW tank are as expected.  It's not going to cost much; mine costs less than 2kWh per day to maintain temperature even if I don't use any.

Is that your heating COP or overall?  

I moved to auto adapt last winter and found COP went down but overall consumption was the same or a little less.  With AA you need to be careful if you vary your room temp because AA bumps the flow up quite high when heating up to a target room temperature. 

 



   
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 HarriUp
(@harriup)
Estimable Member Member
Joined: 4 years ago
Posts: 108
15/08/2023 12:59 am  

It seems a shame that Mitsubishi can't or won't explain the misreporting of energy consumed, I wonder whether it is a data recording issue rather than measurement issue. It is noticeable that on days when a hot water cycle runs for 1 hour and the remaining 23 hours have no actual heating generated that the system records 2.3kWh consumed – 100Wh each hour. All the reports out of MelCloud are given to 0.1 kWh or 100Wh – there will be a minimum unit of use that is recorded and if the unit is recording in blocks of 100W then it won't distinguish between 0< and 100W. So the 25W or so it draws is inflated from under 0.6 kWh to 2.3kWh. Though as I also find that when there is no hot water cycle either, i.e. just a continuous 24 hrs of not running the pump at all, this figure rises to around 3kWh then it's either a curious anomaly or proves I am spouting nonsense.

All water tanks will lose heat at a rate that Mitsubishi have measured. For a 170l tank they give a standing loss of 1.63kWh over 24hrs. Which I think is measured from a tank filled at 60°. But that will be a best performance and the tank won't have had heat-conducting copper pipes connected to it when that measurement was done. If I grasp this correctly that rate would equate to the water losing around 8° over those 24 hours. My tank pipework was not lagged initially and it was noticeable how warm it made the space it was installed into, and how hot some of those pipes felt. Even the hot water loop that circulates through the heat exchanger would still feel hot several hours after a cycle suggesting to me that the water in the loop was passively circulating from within the tank as it cooled in the pipe. It is damned fiddly trying to insulate the tank pipework as it is all bends and valves and pumps but you are reducing heat loss from the one part that you are trying to keep really warm 24/7/365.


Mitsubishi EcoDan 8.5 kW ASHP - radiators on a single loop
210l Mitsubishi solar tank
Solar thermal
3.94kW of PV


   
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