Water pressure reducer

[diy mad]

Hi,
I recently fitted a mains water pressure reducer and set the pressure to 3 bar.
When the cold taps were turned on and then off, the pressure reading returned to 3 bar, but with the hot tap it was 4 bar.
About 3 months after fitting the pressure instantaneously rises so that the guage spins completely round when the hot tap is turned off.
This is the second reducer I have fitted as the original one stuck on the maximum reading.
Any ideas anyone?
Many Thanks.

 

[oldbutnotdead]

Combi boiler? Water meter? If yes to both you need an expansion vessel on your dhw pipework. Quick explanation- water expands when it gets hot. Turn hot tap on, water heated in combi, expands, goes out of hot tap, cold comes into combi, all good. Turn hot tap off- water stops. Water that has just entered the combi gets hot and expands. Except it has nowhere to go so pressure increases instead.

 

[diy mad]

Many Thanks Oldbutnotdead, makes perfect sense.
One thing though, how does a water meter affect this?
Also, apart from the fact that the pressure guage on the reducer will eventually give up due to its constant battering against the stop, is it a problem worth worrying about?

 

[Crezzer]

Water meter has a none return valve in it so the water can't expand back up the main.

 

[diy mad]

Thanks Crezzer.
Is it a thing worth worrying about?

 

[oldbutnotdead]

Yes, it won't be doing your boiler any favours (you can end up with fatigue cracks from the frequent pressure changes, specially abrupt ones). Some mfrs now advise expansion vessels in dhw (so if a meter is fitted after boiler is installed it won't cause your problem).

 

[diy mad]

Can the expansion vessel be fitted at any point in the hot water piping?

 

[flameport]

Can the expansion vessel be fitted at any point in the hot water piping?

No.
It's installed in the cold water supply.

 

[diy mad]

Oh I see, thanks.
The supply comes into the house where there is the stopcock but no more room before the pipe disappears into an internal wall.
There would be ample room on the cold feed pipe under the kitchen sink, does this sound OK?

 

[oldbutnotdead]

Oops, my bad- yes it goes in the cold. Under sink should work, it's where I put mine . Doesn't need to be a massive thing, it only has to cope with the expansion of a fairly small volume of water (not like a sealed heating system where you generally have 100 litres or so expanding every time the heating comes on). Make sure you fit one for drinking water (white or blue), not for heating systems (usually red)

 

[diy mad]

Thanks OBND. I've seen items online with a capacity of 0.16L. Does this sound adequate?

 

[diy mad]

Or I've seen "mini-resters" which are small cylindrical items. This would be much easier to fit but would it be big enough?

 

[oldbutnotdead]

It'd probably be ok, me I'd go for the 2 litre (especially if your combi has that mode where you can have the hot preheated)

 

[Madrab]

Or I've seen "mini-resters" which are small cylindrical items. This would be much easier to fit but would it be big enough?

The mini resters aren't for expansion, they're to absorb the shock pulse associated with water hammer.

You need to use a mini vessel, that has a membrane within it

 

[oldbutnotdead]

If space is critical, have a look at your boiler manual, see if it suggests an appropriate size. Just had a look at some sums, (bit suspicious of the site I've found, it seems not to differentiate between Deg C and Deg F) but taking worst case, long as the hex or storage tank in your combi is 3 litres or less then that 0.16 jobbie will be ok (sums based on heating from 10 to 65 Deg C, which is fairly pessimistic)