Boiler cold feed/expansion tank is HOT! (2023 Update)

[RossG]

Hi

After returning from Christmas with the inlaws my neighbour informed me that one of the loft overflow pipes had been dripping whilst we were away. I decided to have a root around tonight when I put the decorations away for another year and was suprised to find the water in the expansion tank was hot!

We have a Baxi Solo 3 PF with a Honeywell Y Plan control system with more than the minimum head stated in the installation guide. I then remembered that the plumbed who last serviced the boiler muttered something about the pipe layout not being particularly good and causing turbulence in the flow pipe.

I'm guessing that the dripping overflow is simply condensation from the hot water, however, why is the tank getting hot?

Regards

Ross

[MatSav]

Could be that your boiler thermostat is kaputt, and the "flow" is boiling, passing a mix of steam and water up the expansion pipe into the header tank. A tell-tale sign is "hammering" noises in the pipework. Chances are that the over-pressure may also have damaged the impeller of your pump - the sign of this is very hot radiators upstairs, but cold downstairs.

[Aidan]

Perhaps you have the answer in your question.

"....the plumbed who last serviced the boiler muttered something about the pipe layout not being particularly good"

Probably pumping over, water going down the cold feed and back up the open vent. Check the layout of the pipes, if there's a lot of pipes and fittings between the CF and OV connections, or if the CF connects to the boiler return and the OV to the boiler flow, then this is probably the cause. The plumber may have noticed and didn't mention it because he didn't want to get involved in sorting it out. It needs to be sorted. The oxygen being absorbed by the water will cause internal rusting of the rads. Had any rads leaking?

[Set Square]

Sounds like it's pumping over. You need to go into the attic when the heating is running and observe the F&E tank. If there's a constant stream of water flowing out of the vent pipe back into the tank, this is the problem. A contributory factor to overflowing could be too high a level in the F&E tank. When the system is cold, there should only be about 3" of water in the bottom of the tank - just enough to cover the feed pipe comfortably.

This is thoroughly bad, and will introduce oxygen inti the system - causing corrosion. If you pump has a speed control and is currently on anything other than the minimum setting, turn it down a bit. If this cures the problem whilst still allowing the heating and HW systems to work ok, then fine. Otherwise you need to sort the pipework out - making sure that the F&E pipes are both connected into the main flow pipe *before* the pump, and within a few inches of each other.

[Aidan]

Or similarly, CF to pump suction side, OV to pump delivery side.

Watch the OV going into the F&E tank with the 3-port valve in all it's possible positions (heating on HWS off, HWS on HTG off, HTG & HWS on).

[Dave Plowman (News)]

It's pumping over and should be seen to ASAP.

Has the system ever been properly balanced? (see FAQ) Is the pump speed setting on anything other than low? All the pump is required to do is circulate the water - speeding it up more than the minium needed to do this is likely to cause pumping over. If reducing the pump speed to minimum causes some rads not to work, balancing the system should sort it. This is a fairly simple but rather time consuming task - but pretty essential. A plumber could charge quite a bit for this because of the time involved.

Only if correct balancing results in a rad or rads still not working should the speed be increased.

Pumping over introduces air to the system resulting in much accelerated corrosion. If you can't stop it after this, make sure the water level in the header tank is only a couple of inches above the bottom, and consider extending the vent pipe as high as physically possible - although of course the opening must still be above the tank. Doesn't matter if it has to go off at an angle to clear the roof joists.

[Christian McArdle]

By the time you've diagnosed a pumping over/venting/feed issue in an F&E system and attempted fixing the thing for the xth time, you might as well have just chucked the lot out at the start and replace the tank with a sealed system kit that just doesn't have these issues. The conversion kits cost around 40 quid and are a simple fit with only basic plumbing required.

all boilers support this course of action, though, so you need to check the actual make and model. Baxi still make a Solo 3 PFL model, which is compatible with sealed operation. I don't know if your model is also compatible, but strongly suspect so.

Christian.

[Dave Plowman (News)]

Hmm. Have you counted the number of recent threads where people are having problems with pressurised systems? Of course if the OP's system is so badly installed it has fundamental problems then it might be the answer. But I'd make a start at balancing it and turning down the pump before spending any money.

[Ross Galvin]

Depends on what constitutes a lot of pipe(s), the layout is roughly like this

OV. |___________ CF | _____|_____| | | | --------- | | | | | | | |Boiler | | | | Pump

The flow comes out of the top of the boiler enters a T-joint, continues straight up for 50mm before turning horizontally. The pipe then runs

600mm horizontally before turning upward again. This is the overflow pipe and it goes through the ceiling to the cistern. This vertical run must be over the 1m min height specified in the installation manual.

The other leg of the T-joint proceeds about 300mm before another T which is the 15mm cold feed. This run is obviously longer than the

150mm max specified in the manual. It sounds like this is the problem?

Not yet, and most of them are pretty long in the tooth.

Thanks for the info

Kind regards

Ross

[Ross Galvin]

Will do

Blimey, not at all, OK the tank is small, roughly a foot in height but the water level is very high, probably about an inch or two at most below the top. I mean the ballcock is nearly horizontal

Will do

Ah no they are at least a foot apart :-(

Thanks for replying

Kind regards

Ross

[Ross Galvin]

Hard to tell, we have only been here for a year and a half, probably not though.

Medium at the moment, but I shall reduce it ASAP!

Right, well, someone else mentioned the water level and it is indeed relatively high so they looks like another reason.

Thanks for the information I shall return to the loft

Kind regards

Ross

[Ross Galvin]

Hmm there are no hammering noises although the upstairs rads could be a hotter than those downstairs but they are not stone cold. Couldn't it just mean the system needs balancing?

Thanks for replying

Kind regards

Ross

[Christian McArdle]

The problems with the pressurised systems have been nothing to do with being pressurised, though. They are mainly leaks. The reason you don't see quite so many issues with non-pressurised systems is that the water just dribbles away for years (causing timber rot) and gets replenished without anyone noticing.

Christian.

[Dave Plowman (News)]

Could be, although doesn't a pressurized system run at a higher pressure than the average open one?

Also, from reading here, most just seem to top up a failed pressurized system rather than looking for leaks.

[Ross Galvin]

Gents,

After taking all the useful advice on board I turned the pump to it's lowest setting and bailed out the expansion tank, to the tune of 26 pints in fact. I left the tank with approximately 5" of water in, which was the point at which the ballcock kicked in.

I've been popping back up, on and off, over the last couple of days and the water has been consistently cold. However, the water level has crept back up, in fact it has risen by 3 or

4 inches! I then noticed that the rust coloured high water mark was ABOVE the level of the open vent exit. I bailed the tank out again, then help a cup of water over the end of the OV. I could then see Brownian motion at the entrance to the cold feed pipe with my torch, and after poking my index finger in the cold feed I could feel hot water emerging. I took the cup away and the flow quickly stopped.

My gut reaction is that the ballcock is gently leaking, the tank is filling up, submerging the OV, and the system is then seesawing. I suspect a new ball c*ck and some inhibitor are called for, what do you think?

Regards

Ross

[Set Square]

The ballvalve *could* be faulty - but there's another, perhaps more likely - and expensive(!) - explanation.

Do you have a tap on the supply side of the F&E tank? If so, turn it off so that no water can enter through the ballvalve, and see whether the level still rises. If not, just put a piece of wood across the top of the tank, and tie the arm of the ballvalve to it.

If the level still rises (other than just by expansion when the system gets hot) you could have a leak in the indirect coil inside your hot water cylinder, which is allowing domestic hot water to enter your primary heating circuit.

Do you have a large header tank for the DHW in the attic? Is the water level in this *higher* than that in the F&E tank? If so, and if there *is* a leak in the coil, the higher static pressure in the DHW system will cause water to flow into the primary circuit.

If this *is* the problem, you'll need a new hot water cylinder. If you *do* need to replace the HW cylinder, make sure you get a fast recover one (with a large heat exchanger inside). Unless the existing cylinder is fairly new, the new one will almost certainly be better insulated - so you'll end up getting faster hot water, and wasting less heat.

[Ross Galvin]

Eek

No

Yes

Ah, no, in fact the top of the tank is lower than the bottom of the cold feed tank

So it looks like the ball valve is the culprit then?

Kind regards

Ross

[seo]

spot on full marks

[rde42]

I am sure your comment will be appreciated, even if it is 18 years late.

[Brian Gaff]

Maybe you need to quote the original so those not on home owners club have some idea what you are talking about. Brian