Author Topic: Central Heating bypass valve.  (Read 376 times)

Snakehips

  • Twixt London and leafy Surrey
Central Heating bypass valve.
« on: February 21, 2021, 06:56:40 pm »
Does anybody have any experience of such things?
I have a 15mm manual valve on my system which seems to be stuck in the wide open position. There is no perceptible temp difference between the pipework on either side of the valve. Attempting to close it down a bit achieves nothing.
I am looking to replace this with an  automatic valve.
You can get cheapos from about £10 from e.g. Screwfix, or you can pay about £50 for a Honeywell.
Can anybody recommend a product/supplier? Is it worth paying the extra for a well known name?
Mustn't grumble in the circumstances .

Feanor

  • It's mostly downhill from here.
Re: Central Heating bypass valve.
« Reply #1 on: February 21, 2021, 07:19:45 pm »
I'd advise to close the bypass totally, and use one of the bathroom towel rails as the 'always-on' path to avoid dead-heading the pump.

I've never found bypass valves to work well.
They always result in freshly-heated water returning directly to the boiler, which then shuts down as 'satisfied', leaving the radiators cold!

Snakehips

  • Twixt London and leafy Surrey
Re: Central Heating bypass valve.
« Reply #2 on: February 22, 2021, 09:24:38 am »
They always result in freshly-heated water returning directly to the boiler, which then shuts down as 'satisfied', leaving the radiators cold!
That's what happens on my system. My more than adequately specced boiler sits there doing next to nothing or turning itself on and off while the rads heat up very slowly.
Without a bypass I wouldn't have a problem on the CH side because of the bathroom towel rail, but I have a three way valve CH/Hot Water Tank system and I'm worried about pump 'dead-heading' in Hot Water only operation.
Mustn't grumble in the circumstances .

tonycollinet

  • No Longer a western province of Númenor
Re: Central Heating bypass valve.
« Reply #3 on: February 22, 2021, 09:30:16 am »
If you don't have a thermostatic valve on the hot water, then surely there isn't a problem in hot water only - it just circulates via the cylinder.

Re: Central Heating bypass valve.
« Reply #4 on: February 22, 2021, 09:47:26 am »
Some suggestions here (and elsewhere) worth a try. If going for a replacement don't do cheap, its just not worth the aggro later.

https://www.diynot.com/diy/threads/auto-bypass-valve-stuck-open.494187/
Get a bicycle. You will never regret it, if you live- Mark Twain

Snakehips

  • Twixt London and leafy Surrey
Re: Central Heating bypass valve.
« Reply #5 on: February 22, 2021, 12:47:53 pm »
If you don't have a thermostatic valve on the hot water, then surely there isn't a problem in hot water only - it just circulates via the cylinder.
The three way valve gets a signal from a sensor on the side of the hot water tank, so when that detects that the desired temp has been reached it closes the valve. Meanwhile the pump continues for a minute or two under the control of the condensing boiler in order , I believe, to protect the delicate bits in the boiler.
Mustn't grumble in the circumstances .

Snakehips

  • Twixt London and leafy Surrey
Re: Central Heating bypass valve.
« Reply #6 on: February 22, 2021, 12:52:35 pm »
Some suggestions here (and elsewhere) worth a try. If going for a replacement don't do cheap, its just not worth the aggro later.

https://www.diynot.com/diy/threads/auto-bypass-valve-stuck-open.494187/
Interesting thread. Sadly a couple of contributors seem to have problems with the Honeywell DU145 Bypass valve which is what I consider to be the relatively expensive option!
Mustn't grumble in the circumstances .

Feanor

  • It's mostly downhill from here.
Re: Central Heating bypass valve.
« Reply #7 on: February 22, 2021, 01:30:00 pm »
If you don't have a thermostatic valve on the hot water, then surely there isn't a problem in hot water only - it just circulates via the cylinder.
The three way valve gets a signal from a sensor on the side of the hot water tank, so when that detects that the desired temp has been reached it closes the valve. Meanwhile the pump continues for a minute or two under the control of the condensing boiler in order , I believe, to protect the delicate bits in the boiler.

Not quite.
In a Y-plan system with a 3-port mid-position valve, the valve does not have a 'closed' position.
It's either fully open to the CH side, fully open to the HW side, or mid-position open to both.

In HW-only mode, once the tank stat is satisfied, the boiler and pump shut off ( optionally with some pump over-run ).
The pump will not dead-head in that situation.

See for example here:
https://boilerboffin.com/how-does-a-y-plan-heating-system-work/

(The video on there is actually very instructive. Mid-position valves really are the work of Stan, and are a bit brain-hurty.)


Snakehips

  • Twixt London and leafy Surrey
Re: Central Heating bypass valve.
« Reply #8 on: February 23, 2021, 11:24:40 am »
Not quite.
In a Y-plan system with a 3-port mid-position valve, the valve does not have a 'closed' position.
It's either fully open to the CH side, fully open to the HW side, or mid-position open to both.

In HW-only mode, once the tank stat is satisfied, the boiler and pump shut off ( optionally with some pump over-run ).
The pump will not dead-head in that situation.

See for example here:
https://boilerboffin.com/how-does-a-y-plan-heating-system-work/

(The video on there is actually very instructive. Mid-position valves really are the work of Stan, and are a bit brain-hurty.)
Thanks, clearly my notions about the 3-port mid-position valve were something of an oversimplification.
I think I will have to watch that video a few more times. I already understand a boiler cut off quirk that has been bugging me lately.
Mustn't grumble in the circumstances .