Central heating bypass valve

The bypass valve is just a means of maintaining constant pressure on the radiators as the TV valves close down.

As they all close down, pressure rises in the system which tends to defeat their operation.

the radiators as the TV valves close down.

defeat their operation.

So Just putting it between the central heating flow and return somewhere would achieve the desired operation??

Or, the easiest, and cheapest option, is to either leave one rad on full, or take off the adjustable head, so the flow will always go through that rad, even if the others are at full heat, with their TRVs shutting off the flow through them. In reality, is your house really that warm that every radiator TRV shuts at the same time? Because that is the only time when the bypass valve will come into play.

Interesting, two replies but no answer.

I assume this is a system with a diverter motorised valve, where the hot water circuit is guaranteed to provide sufficient flow, but where you're worried that the central heating circuit may be closed down if all the TRVs are shut. In this case I can see no problem with placing a bypass valve on just the central heating side.

The other posters are correct in suggesting that it is normal to have one radiator without a TRV, and this radiator is normally in the vicinity of a thermostat.

I would therefore suggest that instead, you don't fit a bypass but remove the TRV body from ideally your hallway radiator and add a thermostat there, if you don't already have one.

I'm not sure that you *do* necessarily need a by-pass valve.

The usual reason for fitting one would be if the boiler requires pump over-run but there is no guaranteed flow path. This is often the case with S-Plan systems which, with all zone valves shut, leave nowhere for the over-run flow to go unless there's a by-pass.

But if you have W-Plan system with a diverter valve (or is it a Y-Plan system with a mid-position valve?) it can never close completely - either the HW or CH ports (or possibly both) will be open. If just the HW port is open, there will be a flow path through the HW cylinder's indirect coil. If just the CH port is open, there *should* be a flow path through one or more radiators.

When you say that all your rads have TRVs, do you mean *literally* all? Is there no room stat? If there is, it is usual for the rad nearest to that *not* to have a TRV.

The generic fitting instructions for by-pass valves are likely to cover the general case (including S-Plan) where they *do* need to be fitted after the pump and before any zone valves, but in your case - if you decide that you *do* need one - I can see no reason why it can't be where you suggest, just by-passing the CH circuit because, except in CH-only mode, there will always be a flow path through the HW circuit.

It should be an automatic diverter valve, it's like a pressure relief valve and the valve opens, directing water straight into the return, as the pump pressure creeps up/flow rate decreases/TRVs close. Honeywell and others make them. I've one in my shed from when I upgraded to something better; see below.

A simple by-pass will reduce the boiler/system efficiency all year round.

A more elegant, though much more expensive, solution is a modulating boiler and weather compensation. Most recent boilers have the optional add-ons to achieve this.

In mild weather, the boiler flow temperature is reduced, so the TRVs don't have to throttle the flow so much to control room temperatures. Flow through the boiler remains relatively high.

Normally the tee in the pipework is between the pump and the heating system. The pump is usually in the flow these days, so just "downstream" of the pump.

Unlike a safety valve it is designed to open progressively. It's important to correctly adjust it so that as the TVs close, it opens, so keeping the pressure constant. So when all the TVs are open, it will be closed. And when all the TVs are closed, it will be open.

It also maintains a constant flow of water through the boiler so making it more efficient and reducing local hot spots in the heat exchanger. Also boiler thermostat works better.

Diverter valves were an earlier idea. Instead of just shutting the flow off, water is diverted straight back to the return. This overcomes the problem of maintaining constant water flow through the boiler but doesn't solve the problem of varying pressures in the radiators/heating system.

In practice, if pressure falls away in the heating system, the more remote radiators go cold and the nearer ones stay hot. ie, the flow pattern in the heating system changes.

So what is needed is constant pressure on the heating system and constant volume going through the boiler.

The bypass valve does both.

A.Lee spake thus:

The bypass valve is also used when the zone valve(s) close if the boiler over-runs in order to prevent the water overheating.

Thanks for all the replies, I should have been a bit more specific, the system is a 'Y Plan' but with no room stat, All the Rads have TRVs, but I suspect in reality the Bathroom, which is quite large with only a heated towel rail never gets close to shutting down.

When I mentioned the diverter valve I was referring to the Motorised valve.

I only started thinking about a bypass valve as I have just had a new (oil) condensing boiler and I believe they are a bit more fragile than my old

I agree that whilst the hot water and heating is on then as long as hot water is required then there will always be a return path, that's why I had come to the conclusion that if the bypass was in the central heating circuit then it would achieve the aim of never 'dead ending' the pump

Cheers

That's right. Some boilers have a minimum length of pipework which must exist through the bypass loop circuit (I presume to provide sufficient volume to allow time for the boiler to respond to the increasing loop temperature without overheating).

OTOH, some have a built-in bypass loop.

Check the installation instructions for yours.

Condensing boilers and by-passes are not a good mix, in that the latter often stops the former condensing. Read the manual & see what the maker's suggest.

You should have an indoor thermostat, or indoor air temperature sensor. I'd get that fitted before anything else.