TRV question (poss. duplicate)

(I can't find any evidence that this post made it to the newsgroup - apologies if it's a duplication ...)
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.
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On Sat, 8 May 2004 19:51:01 +0100, "Mike Faithfull"

Hard water isn't an issue because the water in the system is fixed. However, when you release trhe valves there's a fair chance of leaking around the spindles.

I wouldn't take "British Gas says so" as a recommendation to do anything apart from open one's wallet and touch one's toes.

One radiator is left without TRV in the area where the room thermostat is installed because otherwisethe TRV will "fight" for control with the thermostat causing the heating to shut down too soon.
Any need for a bypass is normally taken care of using a short pipe near the motorised valve(s) with a lockshield valve on it - or the gravity HW as you say.
If you are going to change valves, there are two things you might as well do also.
1) Change the lockshield valves as well. I would recommend Pegler Terrier types with a drain cock built into the tail. They are quite inexpensive and good quality.
2) Take the opportunity to take the radiators off the walls and outside and give them a pressure wash through. Take care because heating water stains. I've posted a complete method on doing this a few times.
Don't forget inhibitor when you have finished.

.andy
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In an earlier contribution to this discussion,

It certainly has! See the reply which I posted to the original message at 7:52pm.
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wrote:

To find evidence, one's News reader must be set to automatically download new messages. D'oh !
Anyway, thanks for all the replies on this - the Honeywell reference is additional useful information. Given that my system is very simple, (no zone valves, no tank stat) and I can't face the upheaval of moving furniture, pulling up carpets, lifting floorboards and drilling holes in walls for additional wiring, I think I'll have to continue living with the boiler keeping itself warm. The room thermostat is in the downstairs hallway, about half way along, with the nearest rad up by the front door, so if I make that one the TRV-free zone, it should all work out very well.
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