Heating will only work when Hot water is switched on

[MrsDizzy]

Have searched for this but can't find an answer.

We have a Heatrae Sadia Megaflo system. Turned the heating on for the first time in ages this weekend and quickly noticed that it will only work when the timer clicks in for the Hot water. Never had this problem before. Is there anything we can do/test, or is it likely that there is some sort of fault with the system?

Thanks!

 

[twgas]

it is likely to be a fault on the system but with so little info its a job to know, my first instinct is MV.

 

[weseeyou]

Welcome to the forum

MV as in motorised valve (MV)
It is an electrically operated 3 way valve which diverts hot water from your boier to the hot water storage tank or your heating . If it fails it fails in the default position of hot water only. If you have hot water but no heating it is diagnostic .
You'll probably find it with the hot water storage cylinder. There should be a manual over ride lever on the valve body. If you move it to the central position you should get heat.It has sprung resistance so is slow and hard to move

make sure your room stat is set high and calling for heat

 

[reneel]

i can help here - i have had this problem before and had the MV changed. more than likely the micro switch isn't putting on power to the pump and boiler when heating is calling for heat.

so you will need a new actuator (MV head more than likely) or if the MV is an older type then a whole new valve. but moving the manual overide lever will not put the heating on (in 95% of MV's) it will only allow heating to be on WHEN hot water is calling for heat.

 

[twgas]

MV as in motorised valve (MV)
It is an electrically operated 3 way valve

should be 2 port(s) as unvented cylinder (megaflow)

 

[MrsDizzy]

Thank you for the speedy replies!!!

Will get my husband to have a look and probably try to call someone out to have a look at it.

 

[reneel]

if you have 2 x 2 port valves and push the manual lever of the faulty MV on the heating circuit over, then when the hot water is on then the heating circuit will get warm too. sounds like a faulty micro/auxiliary switch. i have had a faulty MV before and learned something from the situation!

 

[weseeyou]

MV as in motorised valve (MV)
It is an electrically operated 3 way valve

should be 2 port(s) as unvented cylinder (megaflow)

thanks for that very helpful lets argue about how many ports the MV has rather than welcome and help the OP with practical advice

 

[weseeyou]

i can help here - i have had this problem before and had the MV changed. more than likely the micro switch isn't putting on power to the pump and boiler when heating is calling for heat.

so you will need a new actuator (MV head more than likely) or if the MV is an older type then a whole new valve. but moving the manual overide lever will not put the heating on (in 95% of MV's) it will only allow heating to be on WHEN hot water is calling for heat.

this isn't terribly helpful really and a bit misleading.

It is the room thermostat that calls for heat provided the boiler is set to CH and the boiler stat is high enough. The boiler controller activates the MV to divert flow from the cylinder to the CH system providing the boiler stat and room stat are high
Manually operating the MV is a diagnostic. If the CH works then you eliminate everything else and know for certain the fault lies with either the valve itself or the associated MV actuation mechanism ie the room stat or boiler controller module etc

 

[twgas]

MV as in motorised valve (MV)
It is an electrically operated 3 way valve

should be 2 port(s) as unvented cylinder (megaflow)

thanks for that very helpful lets argue about how many ports the MV has rather than welcome and help the OP with practical advice

maybe I should explain that not all motorised valves are 3port, some are 2port, if OP has 2*2ports then telling her its the 3port valve is a bit misleading. although I agree with most of your diagnostics a 3port can in fact fail in any position.

 

[weseeyou]

I was trying to make the explanation as simple as possible .Technical detail often only confuses the issue, especially with non technical enquirers. As the OP 's ID is Mrs I not unreasonably assumed that She ? probably has very little understanding of the operation of a HW & CH system.
Hopefully we have given the OP some appreciation of the likely source of the problem.Hopefully they will call in a service engineer.