TRVs on all radiators - conflicting advice

[David.WE.Roberts]

First, the problem.

The coldest room in the house is the lounge but the radiator without the TRV is in the hall.

We don't always want to heat the lounge, but when we do we can overheat the hall (and consequently the upstairs) especially if the wood burner in the kitchen/dining/living area is chucking heat into the hall as well.

The thermostat is wireless so we can move it with us into the room we are using at the time, and possibly shut off the other radiators.

So ideally we could do with a TRV in the hall.

However this would mean that all radiators have TRVs.

Now some Googling says this is O.K. and some says it is not.

I am thinking that if we have a twin pipe system (where there is a flow and return pipe across which the radiators connect) or if we have a single pipe system with the radiators all teed off for flow and return then there should probably be enough water in the pipework to flow round and through the boiler without overheating.

The house is a 1930s three bedroom semi.

So is it safe to TRV everything?

Or should I take a TRV off one of the bathroom radiators?

Cheers

Dave R

[Tim Watts]

If you TRV everything you need either a limited short bypass - towel rads are ideal for this, or a pressure based bypass valve. I think the principle use is to stop the pump knackering itself with a totally "off" hydraulic system.

[jrwalliker]

Yes, if you do one of two things:

1) Install a bypass valve. This is a spring-loaded valve which allows wate r to flow when hte pressure exceeds a preset threshold. Otherwise when all the TRVs happen to shut off the back pressure will become very high which loads the pump more. Also, hte last valve to closed will make a lot of tur bulence noise as it does so.

2) Install a new pump with electronic motor drive. These often have optio ns to vary the pressure-flow curves in a helpful way. Some will just maint ain constant pressure, while others will reduce the pressure as the flow dr ops. (I can't help feeling that there might be scope for instability in suc h a system, particularly if there are extra pumps for underfloor heating.)

John

[Adam Funk]

Some combination boilers (& possibly other kinds, but I'm not sure) now come with an internal bypass.

[David.WE.Roberts]

Should have said that it is a combi so no chance of tinkering with the pump.

Thanks to you and Tim - but I am still confused about the 'over pressure'.

I am assuming that there is a loop of pipe which can flow and return even if all the radiators are turned off because otherwise if one TRV was shut the whole system would stop.

In which case where does the bypass go?

Or is it that the pipe only loop is not enough volume? I guess this may be the 'back pressure' bit although I am not clear how the back pressure is eased by a radiator full of incompressible water somewhere in the loop.

I've looked at an installation diagram for a bypass valve but that seems to be on a gravity system where if both hot water and central heating control valves are closed then the water has nowhere to go.

I am assuming that with a modern combi boiler this cannot happen.

Boiler is a Worcester Bosch CDi although can't find which power level at the moment.

Cheers

Dave R

[brass monkey]

All I do is fully open the TRVs where I need heat (and take the thermostat there, obviously). The hall doesn't have a TRV but is shut completely off anyway.

[Dave Plowman (News)]

Depends on the boiler installation - if it has a bypass loop. Probably not.

Normal is a towel rail in the bathroom. Which generally will never produce too much heat to the room.

[Colin Stamp]

As the rad valves shut down and the flow deceases, the pressure developed by the pump will increase. Automatic bypass valves use the pressure increase to decide when to open.

No. Unless you have a bypass valve, the heating circuit will have flow and return pipes which effectively go out to the furthest radiator. All the other radiators are then teed off those. The system flow will only stop if all the rad valves are closed.

On a combi it'd go from the flow to the return, just like a radiator. Incidentally, it's not there to protect the pump. It's there to prevent the boiler running with no flow, which could easily be curtains for it.

There isn't a pipe-only loop, just the radiators. if at least one rad valve is open, there will be some flow. Otherwise there won't.

It can if the room stat calls for heat, all of the rads are shut-off and there's no bypass valve.

Cheers,

Colin.

[chris French]

Nope, there are flow and return pipes, rads are each fed off of these. and the end of the pipes they will effectively be joined by a rad.

Closing down a TRV will only stop the flow through it's rad.

If you had a lop of pipe, water would go through there rather than through the rads.

Typically between the CH feed and retrun pipes near the boiler

[David.WE.Roberts]

This is the bit which I am having trouble visualising.

Are you saying that in simple terms there are two pipes, one flow and one return. The two pipes are connected by the radiators (one side to the flow, the other side to the return) so that if any radiator valve is open, water will flow from the flow to the return. However if all radiators are shut there is no direct connection between the flow and return?

[Presumably this could be avoided if there was a bypass link at (or beyond) the furthest radiator which wouldn't seem to add much load to the system and would also ensure that hot water was always available to all radiators instantly.]

Anyway, if this is the standard installation it looks as though I will need a bypass valve, or one radiator with no TRV.

I'll probably swap TRVs between the hall and the lounge to get the non-TRV radiator in the coldest part of the house.

Cheers

Dave R

[David.WE.Roberts]

Gah!!

I have two plinth heaters in the kitchen/dining area.

These don't have TRVs (no point as they are buried underneath units) so they are always flowing and returning. The electric fans are controlled by a wall thermostat. So I do have a flow and return loop through two radiators without TRVs.

So I can put a TRV in the hall.

Phew - that was harder than it should have been.

Cheers

Dave R