Expansion vessel

[Department71]

Where do you put the expansion vessel on the central heating system, in the instruction book for our glow-worm system in the UK it shows the vessel on the flow for a sealed system, but on our french central heating system the vessel is fitted on the return circuit back to the boiler (coal/wood) Does it make any difference? as I assume its only there to take up the expansion of hot water in the pipes. The pump is on the flow, that I believe is ok. It has the usual safety valve and pressure gauge fitted.

Any comments appreciated

Steve

[Gluestick]

Might I suggest, Steve, that you post this question to the first thread in this section, on heating etc?

If you are really lucky, Opal Fruit will answer and he is the de facto mine of info on all matters relating to heating and plumbing![:D]

Also, very much worth a look through the Heating threads starting at the beginning.

 

[Gyn_Paul]

I've always put the pump in the return, 'pulling' the water back to the boiler. Apart from anything else it's working on the coolest water. 

I put the expansion vessel on the hot side a) because that's where it was in the 'Boy's Bumper Book of Plumbing'; and b) There was space on that side with the pump on the other.

p

[Michael]

  Steve The expansion vessel is installed on the suction side of the heating circulating pump,preferably on the return to the boiler,where it will be at its lowest temp.This can allso result in the system and rads etc being under positive pressure,eliminating the risk of air being drawn into the system via rad valve packing glands and compression fittings etc.Make sure you have the right size vessel for the system.With a coal or wood system which you mention, there is not much control,unlike positive stats which are on gas/oil boilers to control/shut  them down.michael

[Department71]

Thanks for the replies all.

Glue stick, yes I went to the funnies and had a look there, there was only one reference and that was a picture, the pump and the expension vessel were on different circuits, on was on flow and other was on return, but I don't know which was which.

Think there could be a difference of opinion on this one, the Glow-worm stuff says "A diaphragm type expansion vessel conforming with the constructional requirements of BS4814 shall be connected to a point close to the inlet side of the circulating pump" I can't see it makes much difference where its put, as everyone knows things are done differently in France, maybe this is another one of them.

Steve

[Michael]

When it comes to safety and installing correctly its the same in france as in the uk. The tee for installing the pressure valve must be on the inlet or suction side of the pump,the meanings are the same.[There are other safety reasons for this but its to much hassle to go into here] as it touches on positions of heating control valves etc etc.Between the boiler and pressure vessel there should be no potential restriction.Same as the safety valve[which.can be fitted direct on the boiler or flow pipework].From the tee you run the pipe to the pressure vessel that can be sited anywhere convenient within reason to the boiler.,uk its normally 22 mm pipe, france 20mm.The connection to the vessel is norm bspt three quarter inch.In uk solid fuel boilers should not be used on sealed systems for many safety reasons .In france diyers do there own thing.michael. 

[Gyn_Paul]

[quote user="Michael"]When it comes to safety and installing correctly its the same in france as in the uk. The tee for installing the pressure valve must be on the inlet or suction side of the pump,the meanings are the same.[There are other safety reasons for this but its to much hassle to go into here] as it touches on positions of heating control valves etc etc.Between the boiler and pressure vessel there should be no potential restriction.Same as the safety valve[which.can be fitted direct on the boiler or flow pipework].From the tee you run the pipe to the pressure vessel that can be sited anywhere convenient within reason to the boiler.,uk its normally 22 mm pipe, france 20mm.The connection to the vessel is norm bspt three quarter inch.In uk solid fuel boilers should not be used on sealed systems for many safety reasons .In france diyers do there own thing.michael. [/quote]

I'm fascinated by this as (setting aside my earlier, slightly flippant reply) my reasonimg for siting the expansion vessel/pressure relief valve on the flow is as follows.

For an oil boiler (ditto gas) the greatest danger would come in a situation where the pump failed and the boiler water temperature thermostat failed to shut the burner off. The water would then super-heat and attempt to move up the flow. To my mind the pressure relief valve should be sited as near to the top (flow) boiler connection as is practicable with as few obstructions as possible. if this arrangement is less than ideal, with the pump on the return side I'd be interested to know why before I get out the spanners and the blowtorch.

paul

[Gluestick]

Just had a look at one of my (French) reference booklets. This clearly shows the expansion vessel sitting on the return circuit.

GP: I sort of follow your thinking, however since one is dealing with water, then the first law of hydraulics governs a closed circuit; i.e. liquids are fundamentally uncompressable. Therefore the actual pressure anywhere in the circuit will be identical. Ergo, it doesn't matter where you site a safety valve it would blow off once its preset pressure was exceeded.

How would massive over-pressure build up if the boiler didn't cycle but ran continuously?

I would think that what would happen in practise, would be that the rads would run at overheat until the TRVs cut them out, then probably, the boiler water jacket would split and put the flame out! [:D] You do have some nice thoughts![6]

There must be another safety component here, surely? Probably some form of thermal fuse which would cuts power to the injector pump if it overheats: which it would as the air temp in the casing would be enormously high! Also, I have a feeling that boiler stats fail safe, not dangerous.

 

[Michael]

 Paul Re reading over what i wrote.The tee for installing the[ pressure valve].suppose to read pressure vessel..   If the stats and pump failed as you say and the burner  ran on, the pressure relief /safety valve would would/should be set and blow at 3 bar on a normal domestic system.The pressure/expansion vessel and pressure relief valves are two totally separate items.The safety valve or pressure relief valve are the same beast and as i said earlier should be sited on the boiler or flow pipework With the pump on the return it makes no difference long as the pressure/expansion vessel is piped into the inlet/suction side of the pump.Summing up you say your pressure relief valve is sited on your flow pipework, well thats correct,Your expansion vessels allso on your heating flow well thats your choice.if you have any problems any time put it on the cooler return, specially if its the robofill type with the combined guage/pressure reducing valve/filling loop.michael.