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300 Liter Tank - Do I have to heat it all?

 
Domestic Hot Water & Cylinders
Last Post by JamesPa 2 days ago
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 martingale
(@martingale)
New Member Member
Joined: 2 days ago
Posts: 1
Topic starter 30/07/2025 9:14 pm  

Hello,

I'm moving into a house soon that has a 300l DHW unvented tank with an ASHP. It's just the two of us (most the time) so this tank is unfortunately a good 100l too big.

Is there any strategy I can use to only heat say half the tank to the specified temperature? Or is it all or nothing and if this is longer term, maybe I should swap it for a smaller tank?

 

Thanks for any advice!

 

 

This topic was modified 2 days ago by martingale

   
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Morgan
 Morgan
(@morgan)
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Joined: 4 years ago
Posts: 619
31/07/2025 7:37 am  

You will only heat the full 300L from Stone Cold. Thereafter, you will be topping up and heating what you use. If the DHW tank is well insulated, I would personally leave it as is.  This, however, is a personal opinion, and I'm no expert.  I do suspect that the cost of swapping the tank will be hard to recover unless there is a value/market in your second-hand tank.

Retrofitted 11.2kw Mitsubishi Ecodan to new radiators commissioned November 2021.
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 JamesPa
(@jamespa)
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Joined: 2 years ago
Posts: 2691
31/07/2025 9:22 am  

I agree with @morgan. 

You can turn down the target tank temperature so you aren't diluting the water when it comes out of the tap/shower.  40C is probably about the lowest you can go and so a good starting point, but you may need to turn it up a few degrees depending on pipe runs and the relationship between when you heat the water and when you use it.  The lower you can get it the lower your running costs (and heat losses) for a given amount of hot water delivered.  Do run a legionella cycle weekly though.

This post was modified 2 days ago 3 times 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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