Do I need an External Bypass Valve

[hendersonm66]

Hi

I'm hoping someone can help me out.

I have a POTTERTON PROMAX COMBI 28 HE PLUS A boiler which the manual states that the boiler utilises the primary side of the DHW plate heat exchanger as an automatic integral bypass.

I'm about to fit a Honeywell system which will place a TRV on all the radiators including the one in the living room which has the fixed position valves.

What I cant find information on is do I still need to fit an Automatic External Bypass Valve even though this Internal Bypass is mentioned. With the new Smart TRV's the condition will arise that all the TRV's close when no zone requires heating, turning off the boiler, but there is still the 3-minute pump Overrun to worry about.

Does anyone have experience with this?

Mike

 

[oz-plumber]

If there is enough flow for the boiler to operate when producing hot water then there will be enough flow for the heating system by-pass through the primary side of the DHW heat exchanger

 

[hendersonm66]

If there is enough flow for the boiler to operate when producing hot water then there will be enough flow for the heating system by-pass through the primary side of the DHW heat exchanger

Hi

Thanks for the answer, It's been interesting starting to understand the internals of the boiler.

I can see how this could act as a bypass during heating operating when TRV's start closing in and flow through the boiler reduces below the boiler required minimum flow.

I'm looking at fitting a Honeywell Evohome System, and for a fact at some point whether due to a TRV fault, a TRV requiring recalibration, or some other scenario, the Honeywell main controller is going to call for heat with no TRV's open. I was concerned that the internal bypass wouldn't be suitable to cover this scenario and that it might end up overheating or cycling too often or some other damage. There will be TRV's on all radiators.

There is also the 3 minute overrun after the boiler is called to shutdown. From what I can find on the Internet it's to allow the boiler to dissipate heat before stopping completely. Of course, half of what I read on the internet could be written by people with less of a clue than I have ;-)

Would this function still be carried out by the internal bypass?

Cheers

Mike

 

[Last Plumber]

If it uses the method you stated, then in theory, it should be fine.
You can only try it and see.

Let us know how you go on?