TRV question ...

[Mike Faithfull]

Sorry if it's a dim question - I've done some Googling but can't find an answer ...

Discussing draining, flushing and 'servicing' the heating system "when the Summer comes" (!) with a friend today, I said I intended to replace all the radiator valves (old system, hard water, valves not operated for years, most of them stuck) and to use TRVs. Friend said when using TRVs there must be one radiator in the system NOT fitted with one, but he was unable to explain or justify this assertion other than by claiming "British Gas says so" (he has a maintenance contract with BG for his system).

Is he right? Shirley it can't be to ensure there's still a flow when everything's up to temperature because the 'non-TRV' rad could still be turned off manually ... and if this were to happen, there will still be flow through the boiler by convection in a 'gravity' hot water system anyway ...

"Confused" of Hatfield.

[Rick Dipper]

If all the TVR are closed but the pump is still running then you have an issue. I have always left my bathroom rad with no TVR.

The exact dynamics will depend upon where your pump is.

Rick

[Set Square]

In order to meet current energy efficiency regs, you need to have a boiler interlock. That is, once all the demands are satisfied, the boiler must shut down rather than continuing to cycle on its own stat to keep itself warm unnecessarily.

The usual way to do this, when using TRVs, is to have one room with a room stat and no TRV. The system should be designed and the room stat set such that the stat only usually switches off when all the other rooms are up to temperature. When it *does* switch off, it switches the boiler and pump off - thus providing the necessary interlock.

[There are alternative ways of doing it, using by-pass valves with flow switches, but they start to get a bit complicated!]

The interlock principle only works where *both* the central heating and the domestic hot water are controlled by thermostats - as in either a Y-plan or S-plan setup [See

]

It will *not* work if you have a gravity hot water system with no controls on it - since this doesn't provide independent control of the DHW and CH circuits. If this is the case, you can still achieve the desired effect by converting it to a C-plan [see the Honeywell reference again]. This involves installing a zone valve in the gravity circuit and a cylinder stat on the hot water cylinder, and wiring it all up as per the Honeywell diagram.

HTH.