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DHW heat loss through pumps.

 
Domestic Hot Water & Cylinders
Last Post by JamesPa 2 minutes ago
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 ngillam
(@ngillam)
Estimable Member Member
Joined: 2 years ago
Posts: 62
Topic starter 18/11/2025 2:33 pm  

Hi all,

I have an Ecodan 5kw ASHP with 210litre pre-plumbed DHW tank, we live in a very well insulated new build on a very exposed hill but our COP is 4.9 with heating on 24hrs so all good on that front.  The next challenge now we have a cold spell is the best way to set up the DHW supply to taps and shower, we heat to 47degs with a drop of 5....if I leave the Heatmiser timers off the water stays hot all day/night, if I leave them on (for constant HW demand) then the heat dissipates over a few hours meaning low temp for showers when we need them.  We've started switching the timers on as soon as we want showers but this drops the temp by 4 degrees or so as I assume it "fills the system" and with the colder weather it dropped to 38 this morning with combined timer switch on and ASHP defrost.

We both love hot showers twice a day but we feel we're compromised and would like to get the best set up even though it'll cost us a little on COP.

Does anyone have an idea on how to get around this?  The other half doesn't like having to set the timer to 30 mins for shower....she often forgets in the morning....I can hear her tutting as I write this.

Thanks

Neil.



   
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 JamesPa
(@jamespa)
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Joined: 3 years ago
Posts: 3437
18/11/2025 2:52 pm  

Welcome to the forum.  feel free to ask any questions.

I think you may need to explain your set up a bit more.  What are the 'heatmisers' doing exactly and what is their purpose; what have they got to do with DHW?  How is DHW set up on the ecodan controller?

At any given time you can either be doing space heating or dhw heating but not both.   Normally DHW 'takes priority' in the controller over space heating and you can either set it up on continuous reheat or on timed.  In continuous reheat mode it will reheat at any time of day when it drops a certain amount below the set temp, often 5-10C.  In timed mode it will only reheat at the set times and only if, at the start of those times, it has dropped by a certain amount below the set temp.  All this would normally be set up on the ecodan controller.

Assuming you take showers in the morning and evening then why not set the DHW to timed with a reheat period overnight and a reheat period say mid afternoon?  Depending on how much water you use you might not need the second reheat period.

If you are losing more than 0.5-1 degree per hour then your tank isnt 'well insulated'.  Here is mine, also 210l albeit the Vaillant one, on, it seems, a day when we used hardly any DHW (perhaps we were out).

 

image

4kW peak of solar PV since 2011; EV and a 1930s house which has been partially renovated to improve its efficiency. 7kW Vaillant heat pump.


   
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 ngillam
(@ngillam)
Estimable Member Member
Joined: 2 years ago
Posts: 62
Topic starter 18/11/2025 3:28 pm  

@jamespa Thanks for prompt reply, the tank stays hot all day, all night and is well insulated.  The reheat on the Ecodan controller is set to reheat during the times we want, twice a a day, the heatmisers seem to control the pumps to send hot water to taps and shower.  The issue is when they're on they seemingly pump water around the house continuously  losing temp in a few hours......I could set the heatmiser for a few hours in the morning for us but when a child decides to shower outside that window no hot water is pumped to taps even if the tank has hot water.....does that make sense?

Having heatmiser timer on 24hrs just seems wasteful and acts like another form of underfloor heating!  But losing up to 10 degrees on switching pump on and defrost also seems wasteful.  Or is it just a feature of heatpump set-up?

Thanks,

Neil.



   
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 JamesPa
(@jamespa)
Illustrious Member Moderator
Joined: 3 years ago
Posts: 3437
18/11/2025 3:41 pm  

Posted by: @ngillam
↑

the heatmisers seem to control the pumps to send hot water to taps and shower.  The issue is when they're on they seemingly pump water around the house continuously  losing temp in a few hours..

Posted by: @ngillam
↑

Having heatmiser timer on 24hrs just seems wasteful and acts like another form of underfloor heating!  But losing up to 10 degrees on switching pump on and defrost also seems wasteful.  Or is it just a feature of heatpump set-up?

Ah, you have a water circulation loop, (like in hotels) which does exactly that, keeps water circulating round the house continuously so you can enjoy the benefit of instant how water 24*7 rather than having to wait a minute while it works its way through the pipes from the cylinder.  As you say its additional, inefficient, UFH in winter and unwanted heating in summer.  This is absolutely nothing to do with heat pump set up or indeed heat pumps, its a part of you plumbing design.  I didn't realise anyone fitted these in domestic dwellings other than vast mansions!  

Is the distance from DHW cylinder so large that you actually need this feature, or would it be better simply to bypass/remove the pump and let the water pressure do its job (assuming you have an unvented cylinder)?  It might be necessary to do some capping off also if you go down this route.  

I have heard of a control system for circulator loops where you measure the recirculating water temperature and only turn it on when it cools, and/or at certain hours of the day.  Therre may be other schemes - Im not an expert in circulating loop control systems though and there maybe nobody here who is.  The Buildhub thread on general plumbing or Hot Water cylinders might be your best bet, - it really has got nothing to do with ASHPs.

 

 


This post was modified 1 minute ago by JamesPa

4kW peak of solar PV since 2011; EV and a 1930s house which has been partially renovated to improve its efficiency. 7kW Vaillant heat pump.


   
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