CH piping question.

Hi.
Silly question time. Enjoy your laughs! :)
I am adding a 22 mm pipe run along the back wall which will feed 2 rads and a towel rail. Should I join the feed and return pipes at the end of the run (making it into the proverbial loop) or make going through a rad the only way back for the water?
If I join the ends, I assume the path of least resistance will mean the water won't be keen to run through the rads at all, just keep going round the loop. But if there is no loop what happens if all the TRV's are closed and there is no return route? Is this accounted for in boiler design so it just stops?
I think that not joining is the correct answer, but I want to be sure before I finalise the pipework, so answers on a postcard please.
Thanks.
--
Regards from Mike Barnard
South Coast, UK.
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I am by far the least expert on this here (!) but here goes:

Don't join them - the ends of each pipe end at the rad. So there is no way anywhere that water can get back without going thru a rad (or the HW cylinder) - but see below

If you have all TRVs you either need to fit a bypass which will allow water back to the boiler if all the TRVs and HW cylinder are off, or make sure one rad has no TRV and is never shut off - maybe the bathroom one but check it has sufficient flow for the boiler spec. Or some boilers will shut down where there is no flow but you need to check!
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There are several ways of doing this. Just connecting flow to return will entirely short out the radiators and isn't one of the ways.
1. Use a radiator without a TRV and with lockshields both ends to prevent being turned off by misinformed users. This radiator would normally be placed in the room with a room thermostat, which turns the boiler off when the room is up to temperature.
2. Use a fixed bypass. This is a dead short, but with a valve set to only pass a little water.
3. Use an automatic bypass. This senses the pressure difference. This will open when the pressure gets too much. Some boilers have an internal automatic bypass valve.
Note that if you have an all-TRV system rather than a room stat system, then you are not compliant with building regulations for energy efficiency unless you have a system for detecting that the TRVs have closed so that the boiler can be turned off. This is achievable using a flow switch that measures water actually going through the radiators rather than through the bypass (which is positioned as the first "radiator" on the circuit, before the flow switch).
Christian.
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Well, this isn't electricity. A radiator will present lower resistance than going all the way to the end of the pipe run and returning. This is used in some industrial installations. However, as you and others point out, this might not match the design of the rest of the system, particularly for a modern domestic installation.
--
Andrew Gabriel

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An Automatic Bypass Valve (ABV) fitted after the pump but before the zone valves is a good solution - it is adjustable, with all trv's open you increase the pressure setting on the valve until all radiators heat normally - then as the trv's close, it gradually automatically opens to balance the flow and cuts-down the noise of water that would otherwise rush through the trv's. It is only a mechanical spring loaded valve - but can be fairly expensive (15-20) Alternatively/additionally it is possible to get a pump that senses the increases in pressure and automatically compensates by reducing the pump speed (Grundfos Alpha).
Dave
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In an earlier contribution to this discussion,

Is it fair to asume that these are additional radiators being added to an existing system? If so, this provides alternative flow paths on top of the existing ones - so if they are all closed off, it's no different from the current situation.
Either way, *do not* connect the flow and return pipes directly together. If your system is such that, with all TRVs closed, you could have insufficient flow to stop the boiler from overheating, you need a bi-pass valve - preferably an automatic one. You should have at least one rad with no TRV - and a room stat to turn the whole lot off - anyway.
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Cheers,
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On Thu, 14 Oct 2004 14:43:56 +0100, Mike Barnard

etc...
Thanks for your answers folks. I've done a bit of research and now I understand what the building regs require. Will have a roomstat in the (open plan) lounge / hall / stairs so will have no TRV here, just lock valves. All others TRV and NO loops on the ends of the feeds!
Thanks again. I know more now than I did so thats good aint it? :)
--
Regards from Mike Barnard
South Coast, UK.
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