Automatic bypass Valve plan, your thoughts

[DorrisMancer]

I just checked the W-B installation instructions (32/50 Danesmoor utility boiler): "An automatic bypass valve must be connected between the heating flow and return where TRVs are used on all radiators, fitted to give at least a 3 metre circuit when activated." I couldn't find anything about a minimum flow rate or how to set the auto bypass valve, but the (ahem!) "professional" installation has a bypass loop of just less than 2m.
I don't want to jeopardise the warranty (the boiler is about 3 years old) so I'll ask W-B about using bypass valves with modulating pumps and Evohome - I'll report back if I get anything that might be of general interest.

[bruce_miranda]

Good luck with getting the boiler manufacturer to give you any advice wrt Evohome and their boiler. Look for radiators without a TRV in your system, like a towel radiator or similar. They all serve the same purpose. All boilers will have a minimum flow rate required.

[DorrisMancer]

Yes, I should have put "if" in bold typeface ;-)

All 20+ rads and towel rails have HR92s.
AFAICS there's no sign of min flow rate in the boiler spec

[DorrisMancer]

FYI they replied with this https://www.worcester-bosch.co.uk/pr...culation-pumps - I'm struggling to see how this provides significant cooling flow through the boiler because the pump and boiler are (presumably) wired in parallel and both the pump and call for heat will stop at the same time.

[bruce_miranda]

I can see how this is meant to work. Basically the 3 port valve actually act as your auto by pass. But this only works if the heat demand is binary and you have a method to switch the 3 port valve flow to performing it's by pass function. Hence why its considered a method to deal with Pump over run bypass. Pump over run only happens when there is NO heat demand. You need a solution that performs a by pass then there is partial heat demand.

[DorrisMancer]

It's a binary boiler. The scheme would work if there was a means to keep the pump running after the call for heat had ended and the 3-port valve had switched to "bypass" mode, but they don't mention this so I've asked for clarification.

In practice, the boiler has never locked-out so I assume it's got enough thermal mass to cope with the burn stopping coincident with the loss of water flow.

[bruce_miranda]

The boiler must have a pump output that has a variable pump overrun. Most modern boilers do.

[DorrisMancer]

This boiler (W-B 32/50 Danesmoor) doesn't have any electrical outputs, only inputs (L, N and switched L)

[bruce_miranda]

So then what controls the pump over run?

[bruce_miranda]

Infact that boiler doesn't even need a pump over run.