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Old 20.12.2017, 16:06
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Central heating - what does this valve do?

Hey all, our oil powered central heating is running a little sluggish in that it takes several hours to heat the radiators and even then they are barely warm to the touch. The downstairs ones are definitely warmer than upstairs, which indicates some sort of imbalance. The earliest I can get an engineer over is 5th Jan, so I thought I would try a bit of DIY.

The two gauges from what I can gather are water temperature leaving the boiler and temperature returning (after completing the circuit of radiators). My question is the grey cross-over valve and why there is even one there?? Shouldn't the returning water pass back into the boiler and not short-circuit. Its got about 30 turns so seems to be quite a fine control valve but I'll be damned if I can find any information on central heating websites as to its purpose.. perhaps it might be the key to why the system is not working optimally.

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Old 20.12.2017, 16:30
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Re: Central heating - what does this valve do?

Differenzdruck-Überströmventil / Differential pressure bypass valve

Reason you need it: Think about all thermostats closed because the room temperature is above the set temp and the pump is still running. Where should the water go?

By turning the knob you can adjust the pressure when it will open.
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Old 20.12.2017, 16:34
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Re: Central heating - what does this valve do?

I take it you did do the obvious already: Getting the air out of the radiators? (that's usually done on the upstairs ones).
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Old 20.12.2017, 16:40
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Re: Central heating - what does this valve do?

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I take it you did do the obvious already: Getting the air out of the radiators? (that's usually done on the upstairs ones).
Yes, I've bled all the radiators or at least all those that have bleed valves. Suffice to say its a very old house.
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Old 20.12.2017, 16:41
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Re: Central heating - what does this valve do?

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Differenzdruck-Überströmventil / Differential pressure bypass valve

Reason you need it: Think about all thermostats closed because the room temperature is above the set temp and the pump is still running. Where should the water go?

By turning the knob you can adjust the pressure when it will open.
Many thanks for the excellent explanation. Is there a guide or a tool to know when the valve should be adjusted and by how much?
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Old 20.12.2017, 16:41
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Re: Central heating - what does this valve do?

It turns on the LHC.
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Old 20.12.2017, 16:53
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Re: Central heating - what does this valve do?

There is another valve underneath that is automatic and goes to the Hoval boiler. In the photo its off, but when the heating is on it goes to about 3, however if I put it to 'manual-mode' and turn the dial to 10 then the temperature leaving the boiler shoots up to about 50 degrees (from about 40). It might be that its out of whack (it was replaced about 2 years ago at the last service).

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Old 20.12.2017, 16:55
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Re: Central heating - what does this valve do?

When did you take the picture ? very late at night when everything is shut off ? The temperatures of 20 degrees (centigrade) flow and 10 degrees return look very miserable to me.
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Old 20.12.2017, 17:02
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Re: Central heating - what does this valve do?

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When did you take the picture ? very late at night when everything is shut off ? The temperatures of 20 degrees (centigrade) flow and 10 degrees return look very miserable to me.
Yes it was taken when the system was switched off, usually its 40 and 30 under auto control which I don't think is that warm.
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Old 20.12.2017, 17:03
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Re: Central heating - what does this valve do?

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Many thanks for the excellent explanation. Is there a guide or a tool to know when the valve should be adjusted and by how much?
It is a mix valve. It allows you to run a mad hot boiler (which is more efficient). It mixes a portion of return (cooler) water so your feed temp isn't so hot that the paint peels, or you burn yourself, or the rad mounted tstat can't handle the overshoot.

Sounds like your pump might be failing, or the boiler... ain't.

Write down how many turns you turn the valve. Also the two temperatures before you start. Close the valve 3 turns then check after 30 min if both the feed and return temps go UP. If the feed goes up, but the return stays the same then keep turning the same direction until the return starts to go up.

If feed or return go DOWN you're turning it the wrpng way.
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Old 20.12.2017, 19:06
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Re: Central heating - what does this valve do?

Your water presure is a bit low! Gauge on the RH below. Fresh water feed.
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Old 20.12.2017, 21:17
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Re: Central heating - what does this valve do?

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Your water presure is a bit low! Gauge on the RH below. Fresh water feed.
Around 1.2bar with a cold system is not so bad that it should take hours to heat up the radiators when turning on.
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Old 21.12.2017, 07:11
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Re: Central heating - what does this valve do?

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Your water presure is a bit low! Gauge on the RH below. Fresh water feed.
I think we have the same pressure gauge, heating performance in the attic drops off when the pressure drops below half way on the operating scale. If you have bled the air and have one cold radiator it is worth taking the thermostat off (either a clamp or rotating ring) to flush the thermostat valve, while it is off there is a pin which sticks out, push this in a few times to free it up, if it is stuck in tap the whole valve gently, it should move in and out fairly freely.
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Old 21.12.2017, 14:56
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Re: Central heating - what does this valve do?

Our setup is almost identical - what speed is your pump set to?

The previous owner had it set on the lowest, he probably didn't heat all the rooms, so the furthest points of the house were cold and the return water was also cold.

I turned it up a notch, and now the return water is only a couple of degrees colder than the outbound, and even the most distant floors are warm. The only difference is you can hear the water flowing.
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