Radiators warm when only Hot Water is on

[Igordon]

My mums radiators are getting warm when just the hot water is on.
I have checked the 3-way valve and this seems to be functioning correctly as the hot water side of the valve gets really hot while the central heating side is cold. When the central heating is on then the valve opens OK and I can feel the pipe getting hot as you would expect. The valve was also only changed about 6 months ago.

We have had some central heating engineers out who have flushed the system out (cost £220) which hasn't sorted it out. They are now suggesting changing the 3-way valve again as they said they don't know what else it could be. They now want another £150 + Vat to do this.

Could the valve be faulty still ? What else could be the problem ?

 

[45yearsagasman]

The test is to allow the hot water only to run and then feel the CH port on the 3 way valve which should remain cool or cold.
If it warms then there are 2 possibilities
1, The valve is passing
2, You have reverse circulation.
Have there been any pipework alterations on your system recently, eg new rads?

 

[Igordon]

The only alterations I believe she has had done is new radiator thermostats added to all the radiators when she had the 3-way valve changed

 

[45yearsagasman]

Suspect the 3 port valve. Although recently replaced, system debris can allow the valve not to seat 100%. They can be cleaned if thats the case.