Do I need a bypass valve _and_ an expansion vessel, with TRVs on all radiators?

jp123

I have a Tado setup, and I'd like to put TRVs on all my radiators.

I've read many posts around here mentioning that when you have TRVs on all radiators, a bypass valve is needed, to avoid overloading your boiler pump (when all TRVs are closed, yet the boiler is still pumping).

However, I've consulted my boiler's manual (it's a Remeha Calenta 40C), and the manual says the following (emphasis mine):

"4.6.4. Connecting the expansion vessel

Install the expansion vessel on the heating return pipe.

If using a heating and domestic hot water production type boiler on an installation in which the flow can be fully disconnected from the return (e.g. by using thermostatic valves), you should either fit a bypass or fit an expansion vessel to the heating flow conduit."

I have an expansion valve installed on the heating return pipe as they describe. I don't have a bypass valve.

Given that the manual is clear that either an expansion vessel or a bypass is needed (not both), and I have an expansion vessel, it seems like I should be good to go. Is this accurate? Am I missing anything stupid?

(I did hear that some boilers have an internal bypass valve, but looking at the diagrams for mine, I don't think that that's the case).

Thanks in advance for any information!

GrilledCheese2

I think you've misunderstood the wording. You must always have an expansion vessel on the return circuit. In addition you must have an expansion vessel on the flow circuit if you install TRVs on all radiators. So, two expansion vessels in total.

I assume the reason for this is to protect the flow circuit when the boiler is in DHW mode. When drawing hot water from a tap a diverter valve isolates the flow circuit. If all radiators are closed then there is a section of pipe without protection for expansion. I'd guess the expansion effect is minimal, but the manufacturer's instructions must be followed.