Sealed System losing pressure on a daily basis

[AndyDalton33]

Hi,
Dumb homeowner just looking for a bit of advice so that I can talk sensibly to a plumber.

Central heating system has been working fine for 4 years but suddenly lose of pressure each day in the evening when the system switches off. Suspect the expansion tank, which sits close to the hot water tank, (white Veram 18L) is failing? Questions:

1) If I'm topping up every day does that mean I have a leak somewhere or can it simply be the symptom of a failing expansion tank?
2) If the expansion tank is failing what's the best course of action:
a) see if the tank just needs repressurizing
b) replace the tank like for like
c) replace with a larger tank

The reason for the last point: neighbour had similar issue (exactly the same system installed in the new builds) - plumber advised new bigger expansion tank next to the boiler. Why would you want to plumb in a new tank in a new location?

I have no idea of the volume of the system but it's a large property (5 beds) with downstairs underfloor heating. Hence the question about a bigger replacement?

Given the existing 18L tank has been fine for 4 years - might it have failed prematurely because it's too small?

My instinct is that I will need to get the tank replaced and should go for something bigger e.g. 25L.

Still not clear though if the underlying issue is a leak or the tank itself?

Help !!!!!

 

[bacon_sandwich]

If you are topping up daily, the water is going somewhere.
1. You have a leak on CH circuit, look for damp patches.
2. You have a leak on a relief valve, look for water / damp around the relief valve exit pipe.
3. You have a leak inside the boiler HX harder to spot for DIYer

You suspect the expansion tank but have not said why, when its been Ok for 4 years. Look for large variations on the pressure gauge from cold to hot. Set at 1Bar with filling loop when cold and report what it rises to or not when system at full temp.
If you have answers to above it should help the GSR heating guy narrow down the issue/
cheers

 

[townfanjon]

Your expansion vessel is “ sort off “ ok but it really aught to be a red vessel designed for heating .the white ones are mainly for potable water.

 

[steadyon]

Can't tell without seeing, but:

1. White expansion vessel near hot water cylinder suggests that vessel is for an unvented hot water system, not for the boiler / central heating.
2. You may well have a system (or even combi) boiler with the expansion vessel inside. If so, then if the expansion vessel has failed there are 2 possibilities:
2a. Change vessel inside boiler. Tends to be on the expensive side.
2b. Fit an external expansion vessel, usually near the boiler. May be what your neighbour had done.
3. Your boiler will probably have a 15mm COPPER pipe going from the bottom of the boiler to outside the property, where it is turned to face the open end against a wall. If water is leaking from this pipe it suggests the boiler's expansion vessel has failed or become filled with water. If the latter it may be possible to drain the water out and refill with air, but this, or replacement, is a job for a gas safe registered engineer (assuming a gas boiler).

 

[AndyDalton33]

Thank you for the advice...it's made me look a bit harder at the system and some more thoughts:

I do have an unvented hot water cylinder so seems likely that the expansion tank next to it is just for that. I did read that these ought to be red (but there are plenty of other things the builders 'ought' to have done !).The tank has a pressure rating of 3.5 bar (again I read H/W tanks are normally 3.5 and CH 1.5).The pressure gauge associated with this tank has a target value of 1.5 bar and when it was installed the engineer told me it should be 1.5.

If this 'white' tank is for H/W only - is it isolated from the CH system? If I release water from a radiator (simulating a leak) the pressure drops. The only place in the house where I have the ability to top up is at this point i.e. the refill taps that feeds this tank.

I have a an Ideal Logic 24 boiler which has an internal expansion tank (according to the manual). This supports the idea that my neighbour's fix to put a new expansion tank next to the boiler was to address an inadequate internal expansion tank.

I think have 4 areas to look for a potential leak: H/W circuit; downstairs underfloor heating manifold; upstairs radiators and the boiler itself. No obvious signs of a leak anywhere but I'll keep checking.

No sign of anything coming out of the 15mm pipe that goes from the boiler to outside.

Gave the system a quick check:
Set pressure to 1 bar
Fired up and saw it immediately rise to 1.2 and then 1.5 after a few mins
Turned it off after an hour and it dropped back to 1 bar

So it looks ok on this test but I suspect I didn't push it hard enough in terms of getting all of the radiators hot and getting all of the underfloor circuits going. I'll do another test tonight (when we normally see the problem - pressure drops in the late evening after everything has gone off).

Just wondering whether I need to test water, underfloor and upstairs radiators separately to see if I can narrow down where a leak maybe?

 

[Riley]

I think you are showing you don’t understand your set up here. The white vessel is for taking up expansion on he water filling your hot water cylinder. Let it run a few hours and see what the pressure rises too

 

[AndyDalton33]

Correct I don't understand my set up - hence the questions, such as "would releasing water from a radiator impact the pressure on this tank?" It does - I just wondered that would be expected if that tank only serves the hot water.

 

[Riley]

The pressure gauge you are seeing is the pressure of water in your central heating system it has no bearing of the pressure in the cylinder which will be controlled by a pressure reducing valve

 

[AndyDalton33]

Ah ok got it, but is this gauge telling me something about my central heating or about my hot water or both? Sorry for being thick!

 

[Riley]

Do you understand how your hot water is warmed?

 

[Pickwickpick]

The hot water that heats up your radiators also goes through a coil inside your hot water cylinder, it heats up your potable hot water without actually coming into contact with it. Hence the two guages/pressures. One is the pressure of the water in the cylinder, the other is the pressure of the water in the central heating/rads&coil inside the cylinder.

 

[AndyDalton33]

great - I think I get it, at last. The 'central heating' system is in fact the the 'central heating & hot water system' - one thing which I'll call 'the system' from now on. Hot water is formed in the water tank via heat-exchange from the system. White (should be red) expansion tank is merely managing the cold/hot expansion in the hot water tank. (I was under the impression the CH and HW were two independent systems with their own pressures - not true).

So the pressure problem I have with the system is likely due to a leak somewhere. (Rads, Pipework, boiler, expansion tank inside the boiler).

Just putting the hot water on takes the pressure from 1 bar to 2.5 bars. Switching it off after a couple of hours returns it to 1 bar. All seems normal so not sure what/when it drops daily to < 0.25 bar. Will let the wife & kids take their showers and figure out if it happens after then).

Thanks for your patience!

 

[Riley]

Let it run and see if the pressure keeps going up. Don’t worry safety devices will kick in at 3 bar. If the pressure goes to 3 bar and above that will be where your waters going if not it’s a boiler or pipework leak

 

[Ric2013]

My hunch is that either the expansion vessel is buggered or needs refilling with air. That is probably why the pressure is rising to 2.5 bar: there is insufficient air to allow for expansion. If it has genuinely worked okay for 4 years then something has obviously changed recently, such as the expansion vessel going flat or puncturing.

I would suggest: bleed all radiators while system off and cold; put plastic bag under the copper pipe on outside wall or else something that will prove whether it lets water out (it may not be letting water out now, but it might do later); fire up system for a couple of hours heating all radiators up and the hot water.

My suspicion is that at some point the copper pipe terminating against the outside wall will fire out a shot of water to relieve the excess pressure; as the system cools, the system pressure will fall due to contraction of the water.

 

[SimonG]

Pictures would help.

 

[AndyDalton33]

Thanks - spot on. I'm seeing an occassional discharge from the boiler via the outside copper pipe so have asked a plumber to come and check my boiler expansion tank; and if appropriate stick an external expanion tank on.