One for the plumbers.

I am considering fitting an additional expansion vessel to our heating system. Advice specifies boiler return pipe location. Why?

Reply to
Tim Lamb
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Probably where the boiler pressure gauge/sensor is fitted, and so would change least under normal operation if buffered by the expansion vessel?

It might affect the firing of the boiler if it wildly fluctuated when valves/pumps were turned on or off?

BICBW

Reply to
Fredxx

Because you wouldn't want to pump pushing water *towards* the tank?

Cheers, T i m

Reply to
T i m

It's that bit cooler so less wear on the diaphragm.

Reply to
Jethro_uk

It's that bit cooler so less wear on the diaphragm.

Reply to
Jethro_uk

But it is a dead leg, so not much conduction unless it is very short.

Reply to
newshound

Well it's dead in that it goes nowhere but there is still the flow/return due to the expansion of the water from cold to hot and back. I think that expansion is about 1 l per 100 l 10 to 70 C, guesstimate that's a metre or two of 15 mm pipe.

Reply to
Dave Liquorice

Umm. My boiler modulates but, due to the characteristics of underfloor heating, is mostly returning water at reasonably high temperature. I'll do some checks. There is an 8L vessel in the boiler anyway. A question for another day is how to adapt the *weather compensation provision* to run the system at a lower temperature when there is no call for re-heating the DHW.

The actual issue is that the diverter valve momentarily blocks the pump flow leading to water hammer which opens the boiler relief valve thus losing system water:-) An alternative solution is to fit a bypass valve: to maintain flow during the transition. Soldering access is a pig!

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Reply to
Tim Lamb

Have some relays that switch the pump off when the divert valve is being motored?

That might not stop the hammer unless reasonably open...

Will adding another expansion vessel do anything? After all you have one already and they have quite high interia due to their size. It would have to connected close to the divertor valve an on a short pipe as well.

Surely one of the small "water hammer arrestors" would be better? Screwfix have several, can't persuade Toolstations site to cough anything up.

Reply to
Dave Liquorice

System boiler, internal pump.

The idea was to divert the flow through the hot tank coil with an automatic bypass valve (Honeywell DU144)

Tucked away in the boiler, I don't know which leg it is on. Also 8L is a bit meagre for very cold weather when more heating circuits are running.

Pass. What I hear as water hammer may be the boiler safety valve opening and re-closing. Plumbed in to the condensate pipe so hard to tell.

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Reply to
Tim Lamb

How about a motorised valve opening a bypass pipe a few seconds before the diverter valve operates? I think short delay mains relays are probably a commodity. Though I haven't looked.

Reply to
Roger Hayter

In message snipped-for-privacy@mid.individual.net>, Roger Hayter snipped-for-privacy@hayter.org writes

Heading back towards Y plan complexity which I wanted to avoid. One day, a visiting plumber is going to try making sense of this system! Currently it is W plan with some relays to fudge the underfloor heat control requirements.

I think an automatic bypass would be simpler to understand.

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Reply to
Tim Lamb

As an aside when our new Worcester Bosch combi boiler was fitted I read the installation manual.

If there were more than a certain number of radiators it required an additional expansion vessel to be fitted (beyond the one which came inside the boiler).

I presume because the extra volume of water caused more expansion when heated.

The plumber had never heard of this before. [Was also indecently amused that anyone would bother to read the manual.]

Anyway, no idea about the recommended location. It was a while back.

I assume that the pressure difference between pump out and return might affect the expansion vessel? Pop the release valve if there is one?

Cheers

Dave R

Reply to
David

And how long does that take, and how quickly does a foot of warm copper pipe cool down, especially if it is not lagged. (Actually thinking about it, mine is on a braid covered flexible hose).

Reply to
newshound

No relief valve, relies on the one in the boiler. Difficult to over-pressurise anyway until the diaphragm fails. Probably the schraeder valve that is the weak link.

Reply to
newshound
<snip>

Yeah ... and I think they are worth spending the extra money on, for all the advantages they offer.

Agreed. I've seen too many people twist the plastic caps off the appliance ones because they have seized. Using a small adjustable first is all they normally need to get them moving first though.

We used to have a communal water main that ran across the back of the cottages and off that to each house in some galvanised steel pipe. When isolating my supply via my underground tap I suddenly found I had a pond in the garden. Luckily I knew a guy who worked for the water board who came round and repaired the broken bit (it has rusted away). [1]

When we built the extension it meant it would go over the communal main so we paid to have it bypassed (in plastic) around our extension (to the boundary) and a new main run in directly (plastic, inside some plastic ducting).

Some years later we got a knock on the door saying that there was a burst in the communal bit and I'd have to pay my share. I explained why I didn't and how the leak in their main couldn't be in my garden. ;-)

Daughter is quite practical and I'm not sure the Mrs could get to it with her knees (or not comfortably).

It's a conventional stopcock and I try to work it closed and back once in a while, just to make sure I can ... but a quality ball valve would be nice.

Cheers, T i m

[1] I had a temporary standpipe in the kitchen when I first bought this place but was still living with my parents. I came round after work to do some more work and found the bib tap on the kitchen floor and a rod of ice sticking out of the stand pipe! *Luckily* it was still all frozen so I was able to isolate it outside and put the tap back on with not a drop of water spilled. ;-)
Reply to
T i m

I agree, but I thought there was some risk it wouldn't open fast enough to prevent the overpressure/water hammer effect.

Reply to
Roger Hayter

I can't think of anything acting faster than a pressure senstive bypass valve?

Reply to
Fredxx

Heaven forbid he would actually calculate the volume of water in the system

Owain

Reply to
spuorgelgoog

If it works, that's fine. What acts "faster" is delaying the diverter closing. See my suggestion above.

Reply to
Roger Hayter

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