Central Heating Problem

[Ian Waddell]

Hi everyone,

I had a plumber round a few weeks ago to fix an overflowing central heating tank in the loft. I was in a hurry, so he just fitted an isolating valve to the inlet and turned off the cold water supply. He also removed some of the water from the tank, but said that the water in the tank was very hot which shouldn't happen.

I've left the isolating valve switched off, but the pump has begun making a lot of noise, so I went up in the loft, and the central heating tank had run dry... I have now let more water into it. However, before I fork out loads on a plumber, I have a couple of questions:

1. Why is the water in the central heating tank getting so hot (there was steam coming off it)
2. Why is the tank running dry so quickly... I would have thought water loss from the system should be relatively low - but half the tank has been emptied by it in 3 weeks.

What could the problem be and how much will it cost a professional to fix it?

Any answers much appreciated.

Ian

[Alan]

In message , Ian Waddell wrote

One reason could be is that the pump speed is too high and is pumping hot water up the expansion pipe back into the tank. Hot water in a cold loft will evaporate fast.

[sammoseley]

Rodney, where is the isolating valve, on the cold feed into the header tank? if so this should NOT BE TURNED OFF, no wonder the tank's running dry, not to mention dangerous. It is designed so the system can expand into this tank through the vent pipe which hooks over the top of the tank. Are all the radiators hot? If not there could be a blockage causing the pump to pump up the vent pipe into the tank or up the feed into the tank. How hot are the radiators getting, ie is it boiling up. The cold feed into your tank should NOT be isolated. This tank should normally be about 1/3rd full with the ball valve adjusted to that level for precisely the above reason, so hot water can expand into it without causing it to overflow. If switching the isolator on causes it to overfill your ball valve needs adjusting or has gone. I also assume you are talking about your heating header tank which is the smaller of the 2 tanks in your loft in most systems.

[Ian Waddell]

Yes, I'm talking about the heating header tank. I have turned the isolating valve back on, but as I understand it, the water in the header tank should not be getting hot. All the radiators get hot, so I don't think there's a blockage anywhere. The boiler makes a knocking noise - so could there be air trapped in the system that means the expansion makes the vent pipe overflow back into the header tank?

[sammoseley]

so is the tank still overflowing or is it sitting at about 1/3rd full is the hot water coming into the tank through the vent pipe over the top or the feed pipe at the bottom of the tank? how hot are the radiators getting, hotter than usual, is it boiling up = boiler thermostat gone? try draining some out of a drain c*ck and make sure the header tank starts filling up, this will check for a blocked feed from the tank has anything changed with the pump, could be overpumping and needs pump speed turning down

[John]

Goes to show that plumbers might be able to bend and fix pipes - but they don't all know how a heating system needs to operate.

Good luck.

[Ian Waddell]

The tank isn't still overflowing.... I think that was caused by a sticky float valve. I guess I'll need to hang around in the loft and check whether the hot water is coming through the vent pipe or through the feed pipe... or both! I don't think the boiler is over-heating as it does shut off when it reaches temperature. The rads aren't any hotter than you'd expect.... worried about the knocking noise from the boiler though... it's overdue a service I think so I probably need to get a Corgi round to have a look at that.

[sammoseley]

yes get it checked out, my money would be its over pumping or badly positioned pump if the feed isnt blocked which you can check yourself as above, anyway good luck with that

[Ian Waddell]

How would I adjust the speed of the pump if it's over pumping? It may be a coincidence but the pump got replaced a couple of years ago by another plumber that fitted my bathroom incl. new radiator. I'm wondering if the pump he put in was too powerful for the system?

[Clot]

I'm not an expert in this area but the first thing I would do is ensure that the header can fill with cold. The pump might have various speed settings that you could adjust i.e. reduce. You will need to remove the air from your system and I would suggest flush to remove the crud which may now block the system. Google flushing and also inhibitor in CH systems.

There are many helpful posters here who will be able to give you more detailed advice.

[Alan]

There may/will be a 3 position speed adjustment knob/control on the pump

- probably on the front/side of the plastic housing where the mains supply wire enters.

[sammoseley]

not all pumps have adjustable speed but if so normally a switch on the pastic part I wouldn't mess with it too much but get it checked out as the whole system should be balanced correctly with the boiler stat temp, pump speed, radiator valves If you do manage to turn the pump speed down make sure you bleed the whole system afterwards as there is likely to be air in there.

[TheScullster]

"Ian Waddell" wrote

The header tank can get warm-to-hot in normal operation due to expansion of the contents of the system. The loft temperature would have to be seriously cold though for this to result in "steam" being visible IMHO. Important to discover whether water is being "pumped over" the vent as others have said. This will introduce air into the system and increase the chance of sludge forming. No issues with trying the pump speed lower, as long as the system performs adequately afterwards (you can always turn it back up). Best to carefully note any changes you make. Particularly if you close or open any valves, note how many turns you change them by.

Phil