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Central Heating

Boiler heating radiators when heating water

We recently had Nest Learning Thermostat installed as our old analog programmer on the boiler wasn't easy to change (moving pins around in the dial) as my wife works shifts. We are happy with the way this controls our temperature in our house however our current boiler (28 years old) does not allow you to just set up a timer for the hot water separately, so we are having problems with heating our water.

This means the thermostat on the hot water cylinder is set to 60 degrees and the boiler is kicking in all day to keep it at this temperature, and this is also heating the radiators at the same time and making the house really hot and wasting money.

At the moment we have turned the cylinder thermostat down to stop it telling the boiler to kick in and are turning the immersion heater on when we need the hot water, but this is only a short term solution.

I really need some advice and I think we have the following options but please let me know if you know another ways around this:

1. Fit a new programmer so that we can schedule the hot water to heat up a set times in the day (however I am worried this will still heat up the radiators).

2. Fit a timer to the immersion heater and use this to heat the water tank instead as this shouldn't heat up the radiators. We have PV Solar panels so this may cover a bit of the cost in the summer but will this cost more in the long run because of the price of electric?

3. Replace our boiler with a a new combi boiler that just heats the water as and when we use it as our boiler is going to need replacing in a few years anyway. Also will a new boiler save money on our bills as it is more energy efficient?

Thank you in advance for any help or advice, we are really confused at what is the best solution for us.




We currently have a Y plan heating system.

12 Answers from MyBuilder Heating Engineers

Best Answer

Please state what type of heating system you have i.e. Y or S plan or gravity fed hot water?

See links below if unsure:


Answered 27th Apr 2014

I think you need a heating engineer to have a look at you valve to see that it's not letting-by.


Answered 5th May 2014

Sounds to me either the wiring to the new nest thermostat is incorrect or heating valve is letting by, either way a heating engineer is required to correctly diagnose the fault


Answered 3rd Sep 2016

the nest is a roomstat so whatever boiler it is needs a timer for hot water


Answered 21st May 2014

hi it sound like you might have the nest wired incorrectly ? you heating on the programer should be on all the time and your hot water should be used as normal ive fitted several


Answered 10th Aug 2014

Diverter valve has stuck


Answered 10th Nov 2014

Good advise
Update your system and controls.
Running a Boiler 28 years means it not efficient,bet the Gas supplier are happy with you as your throwing money away.
Secondly Hot water & heating controls work and should present no problem no matter the Boiler and as long as it is wired correctly.
AJ Gas & Water Services.


Answered 12th Apr 2015

your 3 port valve is stuck open which is causing radiators to heat up when hot waters on change valve


Answered 17th Jun 2016

Hi Jon, with a Y-plan system you should be able to have independent HW and CH, your 3-port valve will need looked at. The nest stat shouldn't affect it if it has been installed correctly. As for getting a new boiler, it would probably be a good investment and pay itself back as a 28 year old boiler efficiency will be poor (even when well maintained), however if it ain't broke. . .


Answered 11th Nov 2017

Your three port valve is stuck open.


Answered 10th Jun 2018

It sounds like Either your diverted valve is not in working order, Or your 3 Port Valve is letting by... Hope you get it sorted!


Answered 20th Mar 2019

99.9% of the time it’s the 3 way valve passing when hot water is on and yes it’s definitely a external control issue .


Answered 22nd Apr 2019

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