Re-pressurise combi - quick question.

[David.WE.Roberts]

Herself mentioned to me this morning that the radiator in the spare bedroom didn't seem to be working.

Quick check of forcing CH on by upping the target temperature then opening the TVR up full.

Oops!

Warm right along the bottom but cold from there up. So full of air and needs bleeding.

Quick turn of the bleed screw to confirm and loads of air rushing out.

Quick sanity check on spare capacity of system vs. volume of radiator and swift trip to the combi - after removing half the contents of the airing cupboard.

Yep - pressure down to zero.

Open up the filling loop to get it back up and running, with the intention of final re-pressurise after it has cooled down.

Back up quite quickly to 1.5 bar on the dial, but no further.

Another round of bleeding and re-pressurising and again it comes up to 1.5 bar and no further - no sound of water flowing either so I don't think it is cracking a pressure relief valve.

Implication is the static water pressure is around 1.5 bar.

Is this a reasonable assumption?

[I have a pressure tester in my kit somewhere and last time I checked it I thought it was higher than that on the outside tap. So - does anyone routinely fit a threaded take off/tap on the internal cold water (as I didn't) to be able to check static water pressure?]

Cheers

Dave R

[Tim+]

The issue of static pressure in a sealed loop has often puzzled me. In an open U tube, clearly the pressure at the bottom is higher than at the top. If the U tube is made into a sealed loop though with no air in it, is this still true? If the tube walls are rigid I'm guessing that the pressure no longer varies from top to bottom?

In practice, I suspect that all the radiators and pipe work have a certain compliance which will mean that the downstairs ones will bulge slightly and the upstairs ones potentially get sucked in a bit (if the system pressure is low).

As my boiler is in my loft (2 storey house) and the expansion vessel in in my garage I'm never sure what indicated pressure I should be charging the system to.

Tim

[Gazz]

Washing machine or dishwasher tap?

wondering if there is a lot of air in the rads, and you are not leaving the fill tap open long enough for the water to compress all the air to raise the pressure.... bleed all the rads till they are totally free of air, then try to re-pressurise again? tho 1.5 bar sounds about normal for the cold pressure.... could there be a pressure limiter in the boiler to stop muppets opening the fill valve untill the gauge reads 6 or 7 bar... then something pops.

[John Rumm]

Yup... if you open the tap and it flows, slows, and eventually stops when it gets to 1.5, that would suggest it was the static pressure of the main (at the time). You may get different results at different times of day.

Yes, and I bet you probably have one feeding your washing machine ;-)

(but outside tap would be fine as well)

[David.WE.Roberts]

The outside tap isn't a good guide because I have a water softener in the loop, which is 'protected' by a pressure limiting valve which I keep meaning to take out.

So there are two significant possible pressure reductions.

I can open the bypass for the water softener but this still leaves the 'prssure limiting valve' in the way.

I really must remove this because if the pressure is well below 3 bar this doesn't bring much to the party.

Oh, and both dishwasher and washing machine are well buried in the kitchen units so a pain to get at the connectors.

On my list of Tuits is a measurement of the flow rate from outside the tap and various inside ones.

I think something is slowing down the water flow.

Again a simple tap on a cold feed would make this much easier.

Everything is hidden behind mixer valves (some thermostatic).

Cheers

Dave R

[John Rumm]

You can get adaptors for the pressure test kits (mine came with them[1]) that let you clamp it onto the spout of a tap...

[1] one old "hose end type" and a clamp on with screw down for mixers etc.

[Colin Stamp]

No. Sealing the system doesn't alter the pressure change with "depth". The pressure changes by around a quarter of a bar per floor in most houses, sealed or not.

They'll all bulge a tiny amount compared to empty rads, with the downstairs ones bulging more. This doesn't affect the pressure at-all. They'll only get sucked in a bit if you get a partial vacuum in the system, which would be a bit weird.

I'd tend to try to get the boiler where it wants to be, bearing in mind that the pressure in the expansion vessel will be about half a bar higher. Where is the PRV?

Cheers,

Colin.

[Dave Liquorice]

Sound awfully low to me. 1 bar is roughly one atmosphere or about 32' of water. Doesn't leave an awful lot to get the water up into the loft storage tanks with any decent rate of flow.

Though 0.7 bar (7 metres static head) appears to be the minimum allowable pressure.

ssure/

I like that idea. Donno if any boliers do have an inlet pressure limiting valve tho',