Automatic bypass valve

[ValleyBoy1958]

I am trying to get my head around how the automatic by-pass valve on my oil fired boiler works. I understand the principle: on a C/H system where all the rads have TRV valves then the situation could arise whereby all the TRV's close and the pump would be struggling against a closed system. This is almost the case with our system but there is one rad (a bathroom towel rail) which has no TRV and therefore is always open. Nevertheless the boiler has an automatic by-pass valve fitted - schematic attached. My question is this: the adjuster on the automatic by-pass valve is calibrated from 0.1 to 0.5 bar in 0.1 bar increments - what does this mean in reality? For example: if the valve is set to 0.3 bar does this mean that the valve remains closed until the pressure in the C/H system exceeds 0.3 bar and it then opens to allow the by-pass loop to come into operation?

Thanks.

 

[ValleyBoy1958]

Have I answered my own question here or is it such a difficult one that no-one is able to answer? ?    I'd be grateful if anyone has an opinion on that!! Thanks.

[TonyT]

Fit it as far away from the boiler as possible.

[paul evenden]

I have one of those valves, its about two feet from the boiler on the 22mm feed and return pipework. I think the point of it is that your single rad without trv’s probably won’t be able to flow enough water to stop overloading the pump, in our case that will be because all our radiator piping is only 10mm. Also in balancing the system the lockshield valves on the by pass rad might need to be shut down a bit making it less efficient as a by pass solution.

paul

[Radian]

I have a poor opinion of the sort of bypass valves stocked by both SFix and TStat

Seem to operate pretty much at random in my experience.

[ValleyBoy1958]

Thanks to the three of you for your replies. Perhaps I should rephrase my question: what pressure should I set the by-pass valve at? I'm thinking I could set the by-pass valve to the maximum and then close all the TRV's. Then gradually reduce the by-pass setting until the by-pass pipework gets hot?

[Marvin]

Hi @ValleyBoy1958

 

Firstly, ignore the type of boiler you have, bypass valves are about the water pressure. As you say, as the pump circulates the water around the system the TRVs cause the pressure to build up as they close thereby reducing the flow around the system. The theory is, set right, that the bypass valve will open when the pressure increases to ensure there is enough flow for the pump.

 

On 20/12/2021 at 23:29, ValleyBoy1958 said:

what does this mean in reality? For example: if the valve is set to 0.3 bar does this mean that the valve remains closed until the pressure in the C/H system exceeds 0.3 bar and it then opens to allow the by-pass loop to come into operation?

 Yes.

 

The setting of the valve is an individual thing:

Set the bypass at 0.1 bar and the water would probably circulate through the valve and may not reach all your radiators depending on the resistance to travel around the system. Set the valve at 0.5 bar and the water may start to make noises when few radiators are open.

 

So the challenge is to find the right level.

 

I would shut all the radiators except the one furthest away form the boiler and run the system with the bypass valve at 0.5 bar. Does the water passing through the one radiator make a lot of noise?  If yes reduce the number on the bypass valve until the noise reduces. If no, open the bathroom radiator and shut the first radiator and do the same again: Noise? No? ok, Yes? reduce the bypass valve further.

 

Its really trial and error. Your pump may have 3 speeds or be modulating so lots of things come into the mix.

 

You cannot get into too much trouble adjusting this if you follow the above.  Many people will have other ways to set it.....

 

Good luck 

 

Marvin  

Edited December 29, 2021 by Marvin

[ValleyBoy1958]

Thank you Marvin for your thoughts and confirming that my understanding is correct.

 

I have a an oil boiler, brand new, which has replaced my wood pellet boiler. This does not modulate (unlike the pellet boiler) and yes, the pump has 3 speeds, and is currently set on speed 3 as we have a large C/H system (15 radiators all bar one with TRV's) with a manifold system terminating in longish runs of 10mm microbore pipework. I intend to play around with the pump setting as I'm not convinced that it is necessary to have it running at top speed, not least because it is not one of those newer low-energy pumps and consumes about 60W on speed 3.

 

I guess I should look at pump speed first before tinkering with the by-pass setting?

[Marvin]

It would be good to lower the pump speed, if possible, if the system still works at the lower pump speed!!!  If the flow is too slow, when all radiators are fully open some may not heat up. (however this could also be a fault/ or the change caused by the slower water flow, with the balancing of the radiators)

 

Radiator balancing:

Water is lazy. It will take the path of least resistance. If all the radiators are fully open on the lockshield valve and TRV sides you may find that the majority or all the water flow will go through the nearest radiators only.

 

When the lockshield valves are adjusted correctly the flow to the nearest radiators has as much resistance as the other radiators and so they all heat up. However, be aware, changing the pump flow speed can upset this balance. Generally speaking it is an on site / individual system assessment. You could try the "turn the pump down one and see what happens" routine. 

 

Good luck

 

M

 

 

 

 

 

 

 

 

[ValleyBoy1958]

Pump speed now reduced from 3 to 1 and has had no impact on heating up time of all rads, so will leave it on that setting. By-pass valve set to 0.3Bar on a try-it-and-see basis and am currently monitoring the temperature of the by-pass pipework using my datalogger to check its efficacy.

[Fallingditch]

On 30/12/2021 at 23:19, ValleyBoy1958 said:

Pump speed now reduced from 3 to 1 and has had no impact on heating up time of all rads, so will leave it on that setting. By-pass valve set to 0.3Bar on a try-it-and-see basis and am currently monitoring the temperature of the by-pass pipework using my datalogger to check its efficacy.

What was the outcome please @ValleyBoy1958?  Did you get it working OK?