Trying to figure out why our heating loop is losing pressure, being dumped down tundish. 

 

One thing I noticed, is that the the systems shuts down/heat demand stops, the pressure jumps up per picture below.

 

Could this pressure jump, cause a pressure valve to trip, dumping down the tundish? 

What pressure is it jumping from? 

 

What pressure is your expansion vessel set at? Has it been checked?

Jumping from about 0.75 bar (I usually keep it at 1.5bar) ,

 

A plumber came out and had a quick look, felt expansion vessels were OK, but that my pressure loss might be due to a pressure vessel being too small for the size of UFH loop (250sqm at 150 spacing, vessel is 18l)... But for me to check. I was also keeping the system at 1.5 bar, but he said drop it to 0.75 bar and see if that makes any difference. 

 

Nothing else jumped out at him got why I'm loosing pressure, but I didn't tell him about this pressure 'boost' which I just noticed.

 

 

I would suspect the EV pressurisation level, my installers left it at the factory setting of 3 bar, far too high. If it is too high or too low it will not be serving any useful purpose - and so appear to be too small.

 

You cannot check it while the system is working, you need first to release the pressure in the primary circuit completely when it is all cold.

 

Then adjust the pressure at the Schrader valve to be just below your chosen system pressure, say 1.2 bar, before re-pressurising the circuit to 1.5 bar or whatever.

 

This ensures the bladder will be slightly compressed by default but capable of accomodating 80% of its nameplate volume in expansion and/or pressure surges.

If draining down, can you use a spanner? 
 

https://www.screwfix.com/p/tesla-expansion-vessel-service-valve-3-4-x-3-4-/442fj?ref=SFAppShare

 

Fit one of these and then you’ll never have to drain down, for fault finding, and for routine annual inspections and checks. 
 

These are great gadgets, and should be commonplace imho. 

If it's a new system then it could be an undersized expansion vessel but if it's been in service for a while the much more likely the expansion vessel has lost its pressure or the diaphragm has failed.

Thanks all, is there any risk to asking the plumber to fit a bigger expansion vessel anyway? For the sake of him coming out to check it, might Kurt replace It anyway? 

37 minutes ago, Andehh said:

Thanks all, is there any risk to asking the plumber to fit a bigger expansion vessel anyway? For the sake of him coming out to check it, might Kurt replace It anyway? 

Nope, you can never have too much expansion. If you’ve got the space on the wall just bang a bigger one in.

Thanks, for the sake of £150 or whatever, I'll give it a go! 

No problem fitting a bigger expansion vessel, but if this EV is in an existing and previously working system, why do you and your plumber think its suddenly become to small??

What is your heat source?

What temp do you run at?

3 hours ago, Dillsue said:

No problem fitting a bigger expansion vessel, but if this EV is in an existing and previously working system, why do you and your plumber think its suddenly become to small??

 

Yes. And if he does not know how to check/adjust the pressure in the current one (see upthread) he won't know how to commission the new one so £150 down the drain and no improvement.

 

Going back to the original post I think there is perhaps a pressure spike when the system shuts down, that's quite likely depending on where the p.v. is in relation to the pump, but correctly set up it should absorb it without difficulty.

ASHP 12kw, with 230sqm of 16mm pipe, at 150mm spacing. 

 

Run temp is prob low 40s/high 30s? 

 

18L expansion vessel.

 

This pressure drop has been around since early on in the post installation phase. We had it for most of last winter, and maaay have had it during our first winter... We just had so many other bigger post move in focus points it wasn't a priority.

 

So it may have been there originally, it may have gotten worse year in year? 

Edited September 30, 20251 yr by Andehh

The ppl who put in my 12kW HP changed the pv from 18 to 25 l, but then I now have 11 upgraded rads on top of ~75 sq m of ufh.

 

Original pv was installed 1996 but is still in good working order so I have saved it with a view to installing it somewhere else entirely.

 

There are online calculators for ufh pipework which it would be worth seeking out to see what the recommended pv size is for your pipework volume. By my very approx calcs you have 230/0.15 = 1500 m of pipe at 0.1 l/m so 150 l system vol + hw coil, HP and primaries etc so 18 litres doesn't sound out of line.

 

I would remove one end of the filling loop to ensure neither isol valves passing.

 

150L system contents assuming cold water temp of 10C and hot of 50C + a 18L EV with precharge/filling pressures of 1.0/1.5 will have a hot final pressure of 1.85bar, even if the pre pressure was reduced to 0bar then a filling pressure of 1.5bar will still only result in a final hot pressure of 2.31bar

 

A system contents of 205L will still only result in a final hot pressure of 2.0bar with 1.0bar/1.5bar pre/fill pressures.

 

Edited October 9, 20251 yr by John Carroll

Thanks John, with your estimations then, my system shouldn't be hitting 3 bar? 

No, it shouldn't, as suggested, the pre charge pressure should be checked first but will require a full or partial system drain down, depending on where the EV is installed, then suggest pre/fill pressures of 1.0/1.5bar.