Hello,
As you may have read, I'm having a new boiler. The manufacturer says no pump overrun is required. The instructions say to fit a bypass valve. Why? I thought the valve was required only if there was pump overrun?
If I do fit a valve, better safe than sorry, where does it go? Obviously between the boiler feed and return pipes but as close to the boiler as possible? After the returns have combined?
Thanks, Stephen.
Nope, if you were to manually turn all your radiators off, or had thermostatic valves on them all and the all closed, the bypass allows the water to still move through the boiler.
My new boiler has this bypass inside the boiler, so I would assume it would be fine to put it close to the boiler, although I don't see any reason it has to be there!
Toby...
Well it certainly needs to be after the pump or it won't work so if your pump is seperate from your boiler you can't put the bypass too close. As Toby said it is to provide a return path when all others are closed. The cheap and nasty way (read botch) of doing it is to fit a 15mm circuit with an almost completely closed gate valve so that only a small flow occurs all of the time the pump is running. The proper way to do it is to fit an automatic bypass valve (sometimes called a differential bypass valve) which will remain closed normally but will open when the other return paths are closed due to TRVs closing etc. Of course it may well be that in any given installation the situation of all valves being closed at once never actually occurs in practice but that's fine the automatic bypass will just sit there, closed.
Thanks. I am puzzled by this. Is it to protect the pump or the boiler? After all, if it is to remove very hot water from the boiler, it won't cool down much in the short bypass loop will it?
Do I just fit the valve "out of the box" or does it require any adjustment? I see that they seem to have dial to set the bypass threshold. Do I need to change that?
Thanks.
On Thu, 23 Oct 2008 09:41:11 GMT someone who may be Stephen wrote this:-
Mostly the boiler, by keeping a minimum flow through the boiler even if the thermostatic valves are all shut.
However, it also protects against the undesirable features of the pump pumping against a closed system. As heating pumps are not positive displacement they will not be instantly wrecked by a closed system, but it is not an ideal situation.
If a boiler becomes "very hot" then it will stop firing on the return water temperature thermostat. The bypass valve should ensure this happens. It may then take the pump over-run feature some time to cool down the boiler, but that is less important than stoping it firing.
Bypasses are not ideal as they can lead to short-cycling of the boiler, assuming the control system does not detect this and stop the boiler firing.
There are other ways of providing for a minimum flow in the boiler. In many ways the best option is a thermal store, which decouples the boiler from the heating system and allows each bit to work in a more ideal way. However, that involves more pipes and controls than many are prepared to have installed.
You probably do. How you need to change it depends on your individual system.
Short cycling isn't always a problem. I fitted a system with very little water in it (the heat exchanger in the boiler is a length of 22 mm finned pipe as are some of the rads). It fires for short periods (depending on which zones want heat) but it always runs at its most efficient level (full on) Its been running for 28 years and only worn out one gas valve. It needs a bypass to remove heat from the boiler as it is controlled by five two port valves.
A bit of both but you really need to keep the water moving through the boiler to avoid hot spots within it where the water will otherwise locally boil even though the water at the thermostat is cooler. The idea isn't really to cool the water. The bypass will require adjustment for it's differential pressure. You can follow the instructions or simply set it to max then turn all the other vavles off and check that it opens.
Thanks for the explanation. Wouldn't the thermostat be triggered by the convection or conduction or some such of the very hot water in the boiler without a bypass anyway?
Since I do not have pump overrun and since the rad next to the room stat will always be without a TRV, I can't see what I need one for. I'll fit one to be safe, but I can't see the point.
Thanks, Stephen.
What boiler does you he have? Does it have an integral pump? Is the system a sealed system? Or open vented?
If boiler is a system boiler (integral pump), then a high efficiency plate heat exchanger can be fitted just after the boiler to isolate it from the CH/DHW system. The flow & return pipes from the boiler run to this plate only. The other side goes to the CH/DHW flow and return of the system. A Smart pump on the CH/DHW side. A flow switch on this side just after the pump to give the required "control interlock" required by law.
The room stat circuit runs through the flow switch. When the flow switch is switched off, as no flow through rads, the boiler is switched off and it switches off the integral pump too. The Smart pump winds itself down automatically. If the boiler has a pump-overrun then no problem, the boiler pump pumps around the plate heat exchanger and dissipates heat.
Using this plate heat exchanger ensures full flow through the boiler at all times, so no problems with boiler heat exchanger failing prematurely as being starved of flow. This full flow is not available with a system with TRVs and one rad open with room stat. Poor flow through some boiler's heat exchangers causes premature failure and noise. The boiler is isolated from the rads so no magentite (sludge) enters the boiler's heat exchanger. Then the plate can be screwed out and cleaned if problems of sludge on the CH system side. This plate protects the boiler no end.
The boiler pump can then be on the lowest most economical speed as it is only pumping through the plate heat exchanger. The Smart pump will run down, saving energy, when the TRVs close up or partially close up, so overall you most probably will use less electricity than having the one boiler pump on full speed.
If the boiler is a system boiler with integral pressure vessel, then a 15mm pipe can link the return pipes either side of the plate with anti heat circulation loops. This will conect the CH system to the pressure relief valve and presure vessel - no water will circulate either side of the plate.
You don't need one. If you want TRVs all around see my other post.
No
No
Yes
The heat exchanger sounds interesting but is it (i) a lot of work, or (ii) expensive? It isn't a system boiler, so it doesn't apply to me but I was just curious.
Thanks, that's what I thought. Since the rad(s) near the thermostat(s) have to be without TRVs surely there is always a path for flow. Perhaps the requirement for a bypass is just a belts and braces approach in case these ever get turned off?
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