Auto bypass/bypass valve required in this situation

[SpyDiy]

Hi,
Recently had a regular Oil boiler with no internal bypass (warmflow B21) and new radiators installed via a funding grant. Grant surveyor specified all radiator to have TRV's. Plumber left one out in Kitchen. When I checked work noticed TRV missing but on specification. He said it was needed to allow flow if all other trv's closed shut. I showed him the specification so the plumber then added a trv on that one also. So all radiators in house have TRV's. Did some research on this and found that a bypass valve (and piping between flow and return) required if all radiator have TRV’s. Checked and there was no bypass on the boiler or pipework for this. Asked the plumber and he said that was the old way and now that modulated pumps are used you do not use bypasses as the pump will shut itself down when it detects all trv shutting down and no flow. Is this correct or is the plumber just trying to not install the bypass. I cannot find anything that supports this online.

 

[Harry Bloomfield]

Is this correct or is the plumber just trying to not install the bypass. I cannot find anything that supports this online.

I don't know what the rule is, but common sense suggests that the boiler still needs to dispose of its heat, after all TRV's close - which means either a bypass, or a trv left off.

 

[Johntheo5]

Oil boilers have a very large HEX compared to a gas boiler, ~ 20L or greater vs a gas boilers say 2 to 3l, the hi limit stat also doesn't operate/lock out the burner unti the temperature reaches 110/113C, the HEX soaks up this excess heat, my 20kw firebird HEX temperature rises to ~ 90C after all zone valves close (no pump overrun), my boiler control stat setting is 70C.

 

[SpyDiy]

John, does that mean a bypass not needed then and will pump still suffer?

 

[Johntheo5]

John, does that mean a bypass not needed then and will pump still suffer?

Yes, (no bypass required), pump overrun should be disabled if the system has been installed with a pump overrun. The circ pumps do not shut down if all TRVs close but will ramp down except in fixed speed mode, mine ran for 20 years like this on a old fixed speed circ pump before I installed a A rated pump but was still in perfect condition.

 

[SpyDiy]

John, does that mean a bypass not needed then and will pump still suffer?

Yes, (no bypass required), pump overrun should be disabled if the system has been installed with a pump overrun. The circ pumps do not shut down if all TRVs close but will ramp down except in fixed speed mode, mine ran for 20 years like this on a old fixed speed circ pump before I installed a A rated pump but was still in perfect condition.

Do not know if pump overrun can be turned off , from looking at warmflow manual only seems to allow you to change it from auto to a fixed delay/duration.

 

[Johntheo5]

I'd be a bit wary of disabling it, if all ready installed, despite what I stated above, if the boiler is still under warranty and sprung a leak then it might invalidate the warranty, so as they say, on mature reflection, maybe best to install a ABV or let a the TRV off one rad, as suggested above.

 

[SpyDiy]

ok

 

[SpyDiy]

Just checked with the water pump manufacturer "Embrass Peerless" and they confirmed what he said. Their pump does shut down when there is no flow and a bypass is not required. Must be a very very new thing as no mention of this anywhere on internet searches I did.

 

[Johntheo5]

That's very interesting, to say the least, I know A rated pumps are pretty smart but didn't think they were clever enough to switch off with no flow, you can do a test of a sort by switching off all zones with boiler firing and see does the pump continue to run for its overrun period or shut down since you have no bypass, maybe that's why the overrun function has a Auto or fixed delay/duration settings.

 

[fixitflav]

Just checked with the water pump manufacturer "Embrass Peerless" and they confirmed what he said. Their pump does shut down when there is no flow and a bypass is not required. Must be a very very new thing as no mention of this anywhere on internet searches I did.

But how does it know to restart when required, eg by a TRV opening? If it's not running there is no flow.
More generally, how is the system controlled? Do you have a roomstat? If so, an option is to remove the head of the TRV in that room, then there is no pump deadheading issue.

 

[SpyDiy]

But how does it know to restart when required, eg by a TRV opening? If it's not running rhere is no flow.
More generally, how is the system controlled? Do you have a roomstat? If so, an option is to remove the head of the TRV in that room, then there is no pump deadheading issue.

Do not know how it would know. maybe sudden loss in pressure as TRV opens (shuts off if pressure to high as blocked). Room stat in in dinning area (connected to kitchen) so could do that, but, pump company saying no bypass required so hopefully ok

 

[fixitflav]

Do not know how it would know. maybe sudden loss in pressure as TRV opens (shuts off if pressure to high as blocked). Room stat in in dinning area (connected to kitchen) so could do that, but, pump company saying no bypass required so hopefully ok

OK but you wouldn't normally have a roomstat and a TRV in the same room, as they'll fight