Thermal Siphoning

[David492]

I have an old Baxi Solo boiler, which has been converted to a pressurised CH system but gravity HW with a standard storage tank. There appears to be 3 2 port valves as the conservatory used to have a separate central heating system (I think), see picture attached.

We also have a solar panels with an Immersun unit which diverts unused electricity to the immersion unit in the tank.

All has been working fine for last 4 years but now when the HW comes on, on timer, the boiler fires up and the radiators start to get hot. CH is off and mostly likely the water in the tank will be hot as Immersun unit has been heating water through the day and reports that "Water Hot".

I'm told that the radiators getting warm could be thermal siphoning and if it is, what can I do to rectify the problem. I tried to reduce the temperature on the thermostat on the immersion tank but that made no difference.

Any suggestions on what is going on and how to fix please

 

[Hot&Cold]

If the system is installed correctly,then it will be a zone valve letting by.

Do the rads in the conservatory warm ?

 

[David492]

Hi Old & Bold,
The radiators in the conservatory have been removed and decommissioned about 4 years ago.

Can you clarify what you mean by "a zone valve letting by" please

 

[Hot&Cold]

letting by in this instance is when the internal valve has returned to an off position using a spring.The spring will hold the internal valve shut onto the valve seat to create a water seal and prevent circulation. So if the valve,valve seat has not sealed or the valve has jammed open,then the heated hot water to your cylinder will "let by" and warm some radiators.

Test by firing the boiler up in hot water mode only,watch & listen to which zone valve opens (it could be marked hw) AND test the other 2 zone valves by placing your hand as far left as possible under the insulation on to these 2 pipes,you are feeling for a gradual increase in temperature. if it increases then the attached zone valve is letting by.

if everything is already very hot,allow the system to cool first.

good luck,

 

[David492]

Thanks Old&Bold, I will give it a try, alas I will need to remove the cupboards that are in front of it first!

If one of the other valves is letting by, is it a case of replacing?

 

[Hot&Cold]

If one of the other valves is letting by, is it a case of replacing?

Yes

 

[David492]

Thanks Old and Bold, replaced CH port valve which was stuck open and all working. I appreciate your advice