C/H expansion tank is hot!

[David J]

I am pretty sure that it is, as Cynic suggested, a gravity hot water system with a pumped C/H system. There is also an enclosed gas fire in the lounge on the same unit.

It is all very primitive. I checked in the airing cupboard to see what is hanging off the cylinder, and there is no 3-way motorised valve, just pipe connections. No immersion heater back-up either!

Why I initially dismisseed the idea that the pump was in backwards, was that it was installed by a regular plumber - not a general handyman (no offence TMH!)

The C/H header tank has now returned to its normal levels after my pump switch-off, plus the additional rad bleeding done this morning. But there are signs on the end of the overflow pipe sitting over the tank that there have been recent flows.

If I get nowhere with the plumber I will do a sketch of the overall system, as you suggested.

Thinking about it, surely the pump is currently in the 'return leg', but facing the wrong way?

David

[Tim W]

Harry Bloomfield wibbled on Sunday 06 December 2009 09:52

That would be hard as the pipe to the tank is full even when cold. To do what you propose would require a magic ballcock that cut off when the water was several feet down the pipe below the base of the tank!

[cynic]

Your comment about an overflow pipe sitting over the tank concerns me. I have occasionally found such a set up where the overflow from the higher tank discharges into the lower tank but it is incorrect. However if this is the case and the higher tank serves the domestic hot water then if the ball-float valve is worn it can let a significant amount of water by overnight when demand on the mains is low and the pressure is high. Any watert hus passing into the heating feed and expansion tank will contain dissolved air and could create some of your problem. It will certainly lead to corosion and sludge creation! As others have said older systems sometimes had the pump in the return leg from the radiators thus creating a slight suction on the rad system, leading to air being drawn in through old or weak joints. If the pump is in the flow pipe and pushing water away from the boiler it will tend to pressurise the system and the air entrainment will not occur. We must wait for your diagram with interest

[Roger Mills]

Most likely a Baxi Bermuda.

If it's gravity HW, there won't usually be any motorised valves - the HW circuit works by convection whenever the boiler is firing, and the CH only works when the pump is turned on. But I *still* want to know how the fill and vent pipes are connected. I have drawn a very crude diagram of typical setup, which you can find at

That shows what I believe to be the usual way of connecting the fill and vent pipes - into the gravity circuit in the airing cupboard. Is the system in question like that? If not, what *is* it like?

Why? If it's in the return leg, it *has* to face the boiler - otherwise it

*ain't* the return leg!

[Harry Bloomfield]

It happens that Tim W formulated :

No magic involved...

The boiler, rads, pipe and tank will of course all be full of cold water initially. As the system heats up there will be point at which the water in the pipe will hot below a certain level and cold above it. As the water warms and expands, the interface between the two will gradually rise up that pipe.

Thermo mixing will be a minimum in a narrow pipe, unless and until the hot water expands up to the tank, where it should quickly mix with the cold water in there and the tank will then cool. A warm header tank means something is wrong.

[Tim W]

Harry Bloomfield wibbled on Sunday 06 December 2009 22:01

OK - I see what you are saying - my bad. I though you were saying there would be no appreciable increase in the water level in the tank. You were actually saying the tank shouldn't get hot.

My apologies...

[David J]

Dave, many thanks for this very comprehensive reply, which I have printed out. My thoughts now are that your Item 2 covers this problem.

#Whether the pump is in backwards we will establish later with the plumber. Now, after reading all the replies when folks realised the it was a gravity/pumped system, I'm not so sure again.

What I can do this morning is to restart the system, with the pump on the lowest speed setting, and see how it all performs. I can check what is going on in the header tank as well.

David

[John Rumm]

That does not really stand up to scrutiny. Much will depend on the layout of the pipework, and how much head the tank has got. In many cases where the tank is only a couple of feet above the normal flow pipes, a couple of litres of expansion can easily be enough to drive some hot water to the header by expansion alone. The starting and stopping of the pump etc will also help mix the water as it causes surges in the level.

So while a header tank steaming away full of 80 degree water is a good indication of substantial pumping over, one that is "warm" may be quite normal for a given installation.

[Roger Mills]

May I suggest that when you first turn the system on, you set it to HW-only mode and note whether or not the pump starts. If it's a fully pumped system, the pump *will* start on HW-only. If it's gravity HW and pumped CH, the pump won't run until you also turn the CH on. That should help to determine what sort of system it is.

[Harry Bloomfield]

Tim W was thinking very hard :

No problem!

[David J]

You are quite right there , of course. I checked the C/H piping layout against your sketch here:

and it is identical. I can put a line under this story now, I hope. The original plumber came this afternoon as promised, and heard the tale from me.

He confirmed that he has installed the new pump correctly, but conceded that his leaving the pump on the max speed (70) was an error and could have caused the overpumping which he could see had been happening recently. So he apologised for that, and agreed that the pump should be left on the minimum speed (36) for now. If cold weather later shows that the rads cannot cope, the pump speed can go to the middle (58) range. There are only 6 rads in the s/det. house.

We then noticed that one rad upstairs that was OK with the max speed, was now only luke-warm and possibly sludged. He said that this could also have contributed to the earlier overpumping.

This C/H system has had no attention for at least a decade, so it was agreed that he would return in the Spring to possibly replace one rad that has been weeping from a rusting bleed thread, flush the entire system and add some corrosion inhibitors. This was a no-charge visit, but he has the promise of another job next year.

David

[Dave Baker]

The blocked rad can't have been affecting anything provided it, and all the others of course, are on the pressure side of the pump and the vent is on the return. If blocked rads caused overpumping then it would happen every time a TRV shut the flow off. Remember also that rads in a circuit are fitted in parallel not in series otherwise if one rad's TRV shut down it would block the flow to all the rest. There's always a bypass pipe.

Doesn't sound like he's the sharpest tool in the box to be honest.

[Dave Baker]

I should just add that a pump circuit should always be arranged thus. Pump --> Boiler --> Rads/Immersion --> Vent --> Pump.

That minimises the temperature of the water the pump has to deal with which is good for pump life. If there's anything with a resistance to flow between the vent and the pump then it will contribute to overpumping so the vent should be as close to the pump as possible and upstream of it.

[David J]

Many thanks for all the advice given here. I've had C/H myself since the 1960's - I even put one in an early house with the help of a mate, before it was common-place. He was handy with a blowtorch and showed me how to deal with York Fittings! But I have never before experienced the phenomena of 'Pumping over', and it was quite a shock to see this in another person's house last week.

That house has quite a mixed bag of different aged radiators. The one in the downstairs lounge is actually on the upstairs circuit. Solid floors have prevented a tidy pipe run for that one, I suppose.

Ref your: "Doesn't sound like he's the sharpest tool in the box to be honest". Have you ever heard of the expression NFN? Normal for Norfolk, which is where we are!

Ref your: Pump --> Boiler --> Rads/Immersion --> Vent --> Pump.

The circuit in this house is similar to that given by Roger for a gravity/pumped system, ie, the c/h vent is a 1/2" continuation of the gravity 1" return pipe from the cylinder. The gravity h/w and the rad water are mixed inside the boiler, so are all the same water, wrt expansion are they not?

Finally he mentioned putting 2 inhibitors into the system after it is drained down next year for a flush. I just had Fernox anti-corrosion put into my own system last year. What else could he be referring to there?

David

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