central heating pump over run MTVs close

[elmsoft]

My (old) Potterton boiler has an off delay for the pump. When the MTV switch lives click off a permanent live allows the pump to continue for a short while to dissipate excess heat.
But what I don't get is this;
What is the point of running the pump if both MTVs are closed, effectively isolating the boiler? Presumably they stop the water flow anyway?
The boiler has an additionalcut out and I am having to reset it from time to time. I'm guessing that if the timer switches the valves shut while the boiler is firing then the boiler overheats and the cut out operates. Next time the timer calls for heat the boiler doesn't respond and has to be manually reset. Bit of a nuisance on cold mornings.

 

[Madrab]

The system should have a bypass fitted.

 

[elmsoft]

Thanks for reply
Is this the bypass?

 

[Terrywookfit]

It would appear to be ..yes.

Dog rough and unsuitable fitting though.

 

[elmsoft]

OK!!
Anyone got a link to a better way of doing it?
Thanks for your advice.

 

[ianmcd]

https://www.screwfix.com/p/straight...VCbLtCh3jqQHmEAQYASABEgJ4D_D_BwE&gclsrc=aw.ds

replace the existing ballofix valve and replace it with one of those

 

[elmsoft]

Wonderful! I am so embarrassed not to have sorted this myself!!
Thank you so much!!!!

 

[Madrab]

You will need to increase the pipe size where the ISO valve is at the moment from 15mm to 22mm. Either that or you'll need a 15mm valve which aren't easy, nor cheap to get.

Worst case put a proper 15mm gate valve in there. Do remember though bypasses, whether manual or auto need to be setup properly.

 

[EFLImpudence]

My (old) When the MTV switch lives click off a permanent live allows the pump to continue for a short while to dissipate excess heat.

It can't be a permanent live, can it?

 

[Nige F]

You will need to increase the pipe size where the ISO valve is at the moment from 15mm to 22mm. Either that or you'll need a 15mm valve which aren't easy, nor cheap to get.

Worst case put a proper 15mm gate valve in there. Do remember though bypasses, whether manual or auto need to be setup properly.

You might get a pair of reducing sets 22/15 for rhe compression ends of the 22 valve . But it might still be too long

 

[Nige F]

Wonderful! I am so embarrassed not to have sorted this myself!!
Thank you so much!!!!

Don't be embarassed ! just click on thanks - it appears under "quote" on the blue bar with your cursor DiY help is the name of the game.