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GSHP COP with and without backup boiler

 
Ground Source Heat Pump (GSHPs)
Last Post by Cowshed_Rich 3 years ago
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 Cowshed_Rich
(@cowshed_rich)
New Member Member
Joined: 3 years ago
Posts: 3
Topic starter 31/01/2023 10:34 am   [#797]

Good morning, 

I have recently had a GSHP installed (Vaillant Flexotherm) including separate electric meters for the GSHP and GSHP Backup boiler. I also have a Sontex heat meter installed. 

I have been monitoring the COP of the system daily to try and improve efficiency. The problem is that the efficiency increase greatly on the days the backup boiler is turned on once per week (for legionella) which I'm struggling to understand?? As far as I'm aware the backup boiler roughly produces 1kw of heat per 1kw electric input. 

I'm calculating COP as follows. Heat Kw/ (GSHP Electric + Backup boiler Electic)

For example on the days the backup boiler is not on I'm averaging ~3.8 COP

The days the backup boiler is on I'm averaging ~4.2 COP

Any ideas will be greatly appreciated. 

Happy to provide more info if needed? 

 

 



   
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 robl
(@robl)
Reputable Member Member
Joined: 4 years ago
Posts: 215
31/01/2023 1:02 pm  

That does sound surprising! 

I expect that you're right - the backup heater is likely to be an inline immersion somewhere.

How do you measure the electrical energy?  How well are the Sontex temperature probes attached (perhaps a photo?). 

Which Flexotherm unit is it - I think they come in 5/8/11kW.  So far as I can see, the units are simple on/off gshp ones with impressive COP, not inverter units(correct me if I'm wrong!).

If you run the unit continuously for (eg) an hour (make sure your thermostat is high) without backup heater, what heat energy do you get, and what Electrical does it take?

Ditto, but now with the heater on - what heat and electrical energy?

 



   
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 Cowshed_Rich
(@cowshed_rich)
New Member Member
Joined: 3 years ago
Posts: 3
Topic starter 31/01/2023 2:38 pm  

Thanks for the reply.

The electrical energy is measured via separate electrical meters, one for renewable (i.e. GSHP) and one for non renewable (i.e. backup boiler). I have attached a photo of this and the heat meters. Also the photo of where the sensors are connected. 

It is a Flexotherm 11kw. It is sold as having a COP of 4.73 @ 35c flow temp. I'm unsure about whether it has an inverter. 

I've run the pump with and without the backup boiler on. 

26/01 - Backup boiler off - Electric 44.38kw; Heat 170kw; COP 3.83

30/01 - Backup boiler on - Electric  46.1kw (of which 5.24kw backup boiler); Heat 219kw; COP 4.75

One thought I had was that the DHW when being run by the GSHP was killing the efficiency/COP somehow? Maybe flow temps too high?? 

flexo heat and elec meters
flexo sensors

 

 



   
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 robl
(@robl)
Reputable Member Member
Joined: 4 years ago
Posts: 215
31/01/2023 6:39 pm  

@cowshed_rich 

I don't think it's an inverter model - this says not:

https://www.toptenuk.org/consumer/product/view/VaillantGmbHflexoTHERMexclusiveVWF874

 

Perhaps you're right, and it's a "marginal COP" thing due to the DHW temperature.  It seems very surprising.  How hot does it run to for DHW?

https://www.linkedin.com/posts/colin-m-3aa3051a3_marginal-cop-for-heat-pumps-activity-6984936104051228672-fqv9/

 

Can you separate out hot water and heating?  I assume that you have a hot water tank - does it have it's own immersion heater that you could use for a while, and stop the GSHP trying to heat it.  Then run the experiment again - measure Heat and Elec both with and without the backup heater ?

 



   
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 Cowshed_Rich
(@cowshed_rich)
New Member Member
Joined: 3 years ago
Posts: 3
Topic starter 02/02/2023 9:28 am  

Yes we have a hot water tank so I will try your suggestion of using the immersion to isolate the Heating and DHW. 

The flow temps get up to 60c when the DHW is on. Looking at the 'Marginal COP idea' that could make sense if the flow temps are so high its actually less efficient. 

I'll let you know how I get on.



   
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