Central heating question - By pass pipework

[David]

I have been going over our heating pipework and I have found something that puzzles me. Our systm systme is fairly standard fully pumped system with two motorised valves feeding central heating and an indirect cylinder. The pump and valves are installed in the airing cupboard next to the cylinder on the upstairs landing all in 22mm pipework. The pump is fitted on the feed side of the boiler. The system has cylinder and room stats.

There is a 15mm pipe fitted just after the pump that bypasses the valves and joins to the return to the boiler. This pipe has a 15mm gate valve which is currently open (never really been touched). I am puzzled for the purpose of this pipe. The fact that the valve is open means that some of the feed water will be bypassing the cylinder and/or heating, returning to the boiler. I can only guess that it is there to make filling the sytem easier so that water can fill around the valves. This would mean also that in normal operation the valve should be closed otherwise a proportion of feed water will simply be fed back to the boiler fooling it into shutting off before anything has reached really reached temperature.

I would appreciate some advice if anyone has seen this before and/or knows why it is there. Heating was installed by the Gas Board by the previous owner. Thanks David.

[Andy Hall]

This is a bypass as you say. The purpose is to cover the case where the boiler is running and the CH and HW thermostats are both satisfied. The motorised valve(s) can be closed and then the water from the boiler could have nowhere to go, even though the pump is kept running by a thermostat in the boiler. This flow needs to happen in order to keep a flow through the boiler for a short while until the heat exchanger cools enough such that the water doesn't boil.

Older types of boiler using cast iron heat exchangers had much more thermal mass (heat storage capacity) than modern ones which tend to have both smaller water content and less thermal mass. It was common also to have a convection (aka "gravity") circuit to heat the HW cylinder and that would automatically allow heat to dissipate anyway.

The gate valve should be slightly open to allow a small flow through and the main flow will then happen around the 22mm circuits. It shouldn't be closed or you will risk boiling and bumping in the boiler which won't do it much good.

It is possible to replace this arrangement with an automatic bypass valve. These are set to open at a certain pressure. In normal system operation, the valve will be closed, but when the motorised valves close, thepump pressure will force the ABV open.

[manatbandq]

It's similar to having one radiator (often the bathroom) with no TRV if all the others have TRVs. It provides a path for the water flow when all the TRVs are closed but the pump hasn't stopped.

In your case it sounds like both valves could close when the cylinder and room stats stop calling for heat so you have a bypass on the pump side of the valves.

It needs to be partially open, how much I don't know.

MBQ

[David]

Andy, many thanks for explanation, it is now clear.

An idea occurs. I was thinking of installing a heated towel rail in the bathroom using the domestic hot water feed so that it will work all year round. What about using this bypass circuit to drive this by simply extending it. As long as the circuit cannot be closed it will still provide safety to the boiler at the same time warming up some towels. The supply would be pumped and it can still be partially restricted to allow the main heating and hot water system to get priority. Thanks David.

Andy Hall wrote:

[Andy Hall]

That's completely reasonable and sometimes done. You would fit a lockshield valve (i.e. one needing a screwdriver to remove the cap to adjust) at both ends to avoid the risk of closing it off, or you could use a thermostatic valve and then fit an ABV across this radiator so that if the radiator is off completely you still have a bypass.

One thing to watch is that most towel rails don't have much heat output especially when covered with towels. You would probably need additional heat or to choose a type of rail which is more like a conventional radiator.

[David]

[David]

Many thanks

David.

David wrote: