Just to wrap this up, the VR40 has been installed and seems to work perfectly. Shame it required another add-on board, but at least it all works now. I've left a rad valve slightly open until my heating guy can come and install an ABV on the secondary circuit. Happy days.
How to control secondary pump from Evohome
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Sorry for reviving this thread, but did you fit an ABV on your secondary side?
I'm having a LLH installed and all of the installers have said I can do away with the ABV, but I'm thinking ahead to installing HR92s on all my rads, in which case is it possible for all of the TRVs to be closed, yet the boiler still firing? The difference in my case will be that the secondary pump will be driven from the zone valve on the secondary side. Not knowing how evohome works, is there any scenario where all of the HR92s are closed, but the zone valve is still open? Or is that impossible?
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Originally posted by dty View PostYes there is. It seems odd to me, but when closing down, Evohome shuts the HR92 valves before it shuts off the zone valves, boiler, etc.
P.
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Originally posted by dty View PostYes there is. It seems odd to me, but when closing down, Evohome shuts the HR92 valves before it shuts off the zone valves, boiler, etc. Plus there’s always the scenario of missing radio messages. You need a bypass of some sort. Period.
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Originally posted by fezster View PostI thought this may be the case. Obviously, with a LLH, the boiler has a pathway to expel it's heat, but I was thinking about the pump on the secondary side not having anywhere to pump to, perhaps leading to premature failure. I'm loathed to install an ABV as they often seem to pass water when not intended, so am thinking perhaps a modulating pump is the answer.
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Originally posted by dty View PostIf you can find a modulating pump that is happy to be deadheaded, please let me know. If you do go for a modulating pump, be aware that you can't then use an ABV at all as they end up fighting each other.
I wanted to try and understand what happens on the primary side of an LLH when the boiler modulates down? As the flow rate is fixed, the delta T at the boiler must surely reduce. Is this what you see in reality, though?
The secondary side flow rate will vary according to heat demand (assuming a variable speed pump), but the primary side is fixed. Trying to get my head around why a LLH is often cited as the best way to separate primary and secondary flows - is it more about convenience than efficiency?
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Checking the installer manual, it specifically says to set the pump to one of the fixed speed modes if using the boiler with a LLH. And, indeed, when I look at my modulation power and my dT, they are directly proportional to each other, as would be expected with a fixed speed pump. The secondary side, however, is running on variable speed.
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