UFH/DHW bodge

was just at a neighbour's to investigate a chronic problem (newly renovated old farmhouse, single mum, not much common sense... and she didnt have much of a clue either)

her tundish had been running water for ages & ages, and massive electric bills in cold weather caused her to take an interest.

this is as much as I could work out:


I noticed there was no pressure reducing valve on the cold inlet to the DHW cylinder so found the one that had come with the cylinder and fitted it immediately. this *appears* to have solved the problem of the T/P blowoff valve constantly letting plenty water run thru the tundish.

however water was still dripping in the tundish.
I then located the valve on the pressure vessel expansion tank, and whilst checking the pressure there, noticed that water (rather than air) was squirting out of the valve if I depressed it.
call me old fashioned but isnt this indicating a shagged pressure vessel?

next q, I noticed the pri flow is going into the DHW cyl ABOVE the pri return.
I always thought that hot water went IN the bottom and OUT the top...???

next q, there appears to be 1 TRV on the pri flow, but no TRV on the flow to the UFH. shouldnt there be one on each?

there is absolutely no remote control to the system, looks like a half done job.

the professionals have been called, but its a 2 week wait (offshore island). just wanted to get her going with hot water. the situation is not exactly dangerous, as its oil fired, and the plant is in a leanto outbuilding.

she doesnt have an electric shower either so is having to traipse across fields for a shower at the neighbours.[/img]

SparkyTris said:

call me old fashioned but isnt this indicating a s*****d pressure vessel?

Click to expand...

Yes. And that will cause the pressure relief valve to open when the water heats up, hence the trickling tundish.

When you say TRV, do you mean zone valve?
If the ufh has actuators on the manifold then no zone valve required.
However the zone valve to the cylinder is also a safety device that should be wired in series with a high limit stat on the cylinder to cut the supply if a overheat fault develops with the boiler.

And there should also be a pressure relief valve after the pressure reducing valve.

To the OP!

You cannot check the air pressure at the air valve unless the cylinder has been depressurised! ( Otherwise you will just be measuring the water pressure! ).

Furthermore anyone working on an unvented cylinder should have an appropriate G3 qualification.

Tony

The way you've drawn this diagram, that installation looks dangerous and its use should be discontinued immediately until the problems are rectified. Get someone with appropriate qualifications in to look at it

I'm no expert but ... ;

Surely if there is no power to the zone valve then it will remain closed and there will be no primary flow to the hot water cylinder.

The obvious question is - does the hot water cylinder get hot?

Laurie
(why would she have a big electricity bill for an oil fired boiler?)

lauriehurman said:

I'm no expert

Click to expand...

Hmmm...


The assumption (possibly incorrectly) is that the zone valve has been wedged open to allow the boiler to heat the cylinder, having said that the high electric bills suggest that it is currently being heated by the immersion rather than the boiler

Laurie - massive electric bill because she was running immersion heater to get hot water but of course it wasnt staying in the tank, as the T/P valve was letting off.

dean7445 - yes sorry I did mean zone valve, not TRV. and I think in the P-reducing valve Ive just fitted there is an integrated P.relief. and a blanked takeoff.
OK, yes there are actuators on the manifold. I did wonder this. so no valve required on the UFH leg. Yes I absolutely take your point, the valve on the DHW should be wired to a limit on the cyl; there is none fitted. but the cyl has a TP relief that is clearly operational. danger? scalding at the tap? the TP should take care of the cyl exploding.

muggles - yes correct, zone valve manually locked open to try to get some energy into the cylinder. and yes, immersion heater on.


I know you cannot accurately check the pressure with mains water pushing up into the bottle, but I believe I am correct in that water coming out of the valve indicates a ruptured balloon inside the expansion vessel.

a good local company are coming to sort out the mess, dont worry. but you will appreciate the situation - remote island, single mum, winter,already been heartily rag'd bag'd and sh*g'd by the "professionals" already etc etc.I'm just trying to be practical. the company cant get here for 10 days.

at the moment my only question is why the boiler does not seem to be heating the water in the cyl at all. I can feel the pri flow inlet port to the cylinder is getting hot but there is no heat whatsoever in the pri return port. I would expect the return pipe to be a little warm, indicating some flow.
my working theory is that with no circularing pump in the DHW circuit, the hot flow is stalling and expanding through the UFH manifold in preference to working its way downwards through the exchanger to the cylinders return port...
is that rule about hot flow must go below return codswallop or gospel?
the electric immersion heats the water OK.

The potable ( white ) expansion vessel is almost certainly just needing repressurising unless water comes out of the air valve continuously when the pin is pressed. Often a little water will come out and then stop.

On most systems the air should be pressurised to the same value as the cylinder operating pressure usually about 2.5 bar to 3.5 bar.

Usually difficult with a car foot pump to get that pressure. A compressor is usually needed.

Water in the EXV should be removed with the cylinder pressurised. Pressurising with air can only be done with the cylinder depressurised.

Tony

If there is no flow through the coil the zone valve must be shut, if she is running of the immersion I would leave it like that until rectified as the immersion stat will have a high limit safety cut out.
If you put the zone valve over to manual you will have no control over hot water temps plus you would be by-passing a safety valve.

Hi SparkyTris

Think about what is happening in a gravity-fed hot water cylinder.
The water comes into the primary coil from the boiler really hot. It then gives up it's heat to the secondary water in the cylinder and so cools down. It cools and gets more dense and falls downwards back towards the boiler again.

So the input (flow) goes into the tank above the return.

Laurie