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Water tank location – where can I put it?

 
Air Source Heat Pumps (ASHPs)
Last Post by JamesPa 1 year ago
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 PowerADE
(@powerade)
New Member Member
Joined: 1 year ago
Posts: 3
Topic starter 11/02/2025 2:48 pm   [#2098]

Hello

My gas boiler is near end of life and I am looking at my options. I live in a 3 story 4 bed detached house with fairly new radiators that are about 2x the BTU that are needed in each room and the heating uses 15mm plastic pipes embedded in the studded walls. I have 3 bathrooms.

I am looking into if a heat pump will be suitable, however I have some questions.
I have no spare space in the house for a water tank other than if i use one of my existing kitchen cupboards to have it in OR under my stairs space which is not much higher than a washing machine, is that possible?

Also as my homes garden is north facing, the only outside space I have for the pump itself is fully shaded and never gets direct sun. Is this also and issue? (I will not be able to install the pump at the front of my property due to my parking)

Best Regards
Adrian



   
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JamesPa
 JamesPa
(@jamespa)
Illustrious Member Moderator
Joined: 3 years ago
Posts: 4717
11/02/2025 4:23 pm  

Posted by: @powerade
↑

I have no spare space in the house for a water tank other than if i use one of my existing kitchen cupboards to have it in OR under my stairs space which is not much higher than a washing machine, is that possible?

Typically DHW tanks are cylindrical and ~1.4m high (or more)

Is the loft a possibility?

If not then with the relatively small space available you may have to make some compromises on the amount of water you can run off without a reheat, or on the running cost by heating to a higher temperature than you would otherwise do.  There are solutions to this problem but they do involve some compromise (as does almost everything in life!).  How much DHW do you use?

Posted by: @powerade
↑

Also as my homes garden is north facing, the only outside space I have for the pump itself is fully shaded and never gets direct sun. Is this also and issue? (I will not be able to install the pump at the front of my property due to my parking)

No it doesn't make a material difference. Heat pumps draw air in from the surroundings they dont rely on solar radiation falling on them.  All other things being equal a sunny place has some advantages over somewhere North facing but not enough to exclude the latter.


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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downfield
 downfield
(@downfield)
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Joined: 1 year ago
Posts: 131
11/02/2025 4:29 pm  

I assume you don't have any useable loft space for the DHW tank?  We installed a Mixergy 210l in our loft when the Ecodan was installed 18 months ago and it has worked well.

Failing that, have a look at the Heat Geek mini heat store - this video explains all.  It should fit under the stairs but has limitations as described in the video.

 


Mitsubishi Zubadan 14kW with Mixergy 210l DHW in 220m2 barn property. 24 solar panels = 9kWp with GivEnergy 5.0kW Hybrid inverter and 19kWh GivE batteries. Jaga Strada fan-assisted rads throughout. Landvac vacuum glazing/triple glazed windows.


   
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 PowerADE
(@powerade)
New Member Member
Joined: 1 year ago
Posts: 3
Topic starter 11/02/2025 5:35 pm  

@jamespa thanks. Unfortunatly my loft is converted and there is no space.

We currently have a 35kw gas boiler but even that's not enough to run any hot water tap at same time as a shower without the shower going cold. Not sure that means a low capacity tank is OK.



   
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bontwoody
 bontwoody
(@bontwoody)
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Joined: 4 years ago
Posts: 1009
11/02/2025 5:38 pm  

Also a thermino heat store is a possible solution. 
https://sunamp.com/en-gb/hot-water-solutions-thermino-range/

I think @toodles has one?


House-3 bed partial stone bungalow, 5kW Samsung Gen 6 ASHP (Self install)
6.9 kWp of PV
5kWh DC coupled battery
Blog: https://thegreeningofrosecottage.weebly.com/
Heatpump Stats: http://heatpumpmonitor.org/system/view?id=60


   
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JamesPa
 JamesPa
(@jamespa)
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Posts: 4717
11/02/2025 5:54 pm  

Posted by: @powerade
↑

We currently have a 35kw gas boiler but even that's not enough to run any hot water tap at same time as a shower without the shower going cold. Not sure that means a low capacity tank is OK.

Thats not directly relevant because you are comparing real time heating with stored hot water.  Real time heating is dominated by the raw power you can deliver to the tank (which will likely be less than 35kW).  With My 7kW heat pump and stored hot water I can run all the hot taps and a shower simultaneously, but of course eventually I run out of stored water.

For stored hot water what matters more is how many showers (or other things using lots of DHW) you have in quick succession.  This determines the size of tank you need.  If you can give some indication of that we can point you in the right direction

Posted by: @bontwoody
↑

Also a thermino heat store is a possible solution

Good point, I had forgotton those; this maybe a good use case.


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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 PowerADE
(@powerade)
New Member Member
Joined: 1 year ago
Posts: 3
Topic starter 11/02/2025 7:23 pm  

@jamespa usually 2x 10 mins showers in morning, separated by 30mins. And same mid evening. That's on average.



   
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JamesPa
 JamesPa
(@jamespa)
Illustrious Member Moderator
Joined: 3 years ago
Posts: 4717
12/02/2025 2:22 pm  

Posted by: @powerade
↑

@jamespa usually 2x 10 mins showers in morning, separated by 30mins. And same mid evening. That's on average.

OK

Morning and evening are far enough apart you can reheat in between, so we can consider one alone.

2 * 10 min showers consume the following

@20l/min 400l

@10l/min 200l

@6l/min 120l

If you store water at 48 and shower at 40 you get about 25% more yield than the tank size, if you store water at 55 and shower at 40 you get 50% more yield than the tank size.  

You can get a 125l cylinder which is just over 1m high (you will need some additional clearance for the exit pipework.  If you store at 55 will give a yield of 180l, something like 8l/min for your showers which you may find acceptable with a water saving head.  There are also these  which are specifically designed for small spaces.  Im not sure they really offer much advantage over a small cylinder though unless access for pipework is difficult.

You could squeeze a bit more out of a small tank by turning on the immersion heater and heat pump simultaneously during draw off, so you have a 'fast' reheat.  In the 30 mins between showers you can hear 114l from 10C to 40C with 8kW - 3kW from the immersion and (say) 5kW from the heat pump.  This might give you ~280l in total from a 125l tank.

Otherwise its Sunamp and their Thermio heat batteries, which use a phase-change material to get greater storage density than water can achieve.  The 210l model will supply 300l water at 40C, which should suffice for your needs.  Its 870mm high so would fit in the space of a kitchen cabinet or under the stairs.  They are rather expensive and very heavy, but if compact and high yield is what you require, then it is probably what you need.  Its also the (relatively) straightforward option.

In summary its do-able, the best solution depends on your requirements.

 


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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