Expansion vessel for potable water for a combi boiler

[CLC plumbing]

Hi,a customer of mine has a combi boiler and they don't have an expansion vessel for the potable hot water supply. I know there's an expansion vessel for the closed-loop central heating circuit but I doubt the combi boiler has one for the potable water. I was thinking to put one in the airing cupboard on the domestic hot water line which would service the faucets in the house. However, when installing one on an unvented cylinder it's usually on the cold water feed to the cylinder. Some advice from the experts would be greatly appreciated to help me determine if my method would suffice.

 

[ShaunCorbs]

Don’t need one for a combi

What’s the make of boiler ?

 

[Chuck]

Why do you think you need an expansion vessel for the 'potable hot water supply'? Is this, for example, a system where the combi is used as a heat source for an unvented cylinder? (If so, you need a G3 (unvented HW) qualification to modify the system.) Are there other heat sources in the system?

 

[CLC plumbing]

There is no cylinder only the combi boiler heating the potable water. I was making the comparison of when plumbing an unvented cylinder that there's an expansion vessel for the potable water but in the case of using a combi boiler there's none. Is there no need for one with a combi boiler?

 

[ShaunCorbs]

No as the hot water isn’t heater until you open the tap

 

[CLC plumbing]

No as the hot water isn’t heater until you open the tap

That makes sense thanks for your response,much appreciated

 

[John.g]

The above is interesting.

The DHW shutting off is what tells the boiler to stop firing and if the combi cold water supply is via a PRV then it effectively becomes a trapped volume of water so there must be a rise in pressure?. I might have misread it but I thought I saw where some manufacturer specified a E.vessel in this case.

Is pump overrun used in DHW mode? and if not, why not, as the boiler will generally be firing at 100% output prior to shutdown. If it is used then in the PRV case above it could raise the DHW pressure even more?.

For interest, does anyone know the heating surface of the secondary Hx in, say a 30/35kw combi?, it may be labeled on the Hx.

 

[SJB060685]

The above is interesting.

The DHW shutting off is what tells the boiler to stop firing and if the combi cold water supply is via a PRV then it effectively becomes a trapped volume of water so there must be a rise in pressure?. I might have misread it but I thought I saw where some manufacturer specified a E.vessel in this case.

Is pump overrun used in DHW mode? and if not, why not, as the boiler will generally be firing at 100% output prior to shutdown. If it is used then in the PRV case above it could raise the DHW pressure even more?.

For interest, does anyone know the heating surface of the secondary Hx in, say a 30/35kw combi?, it may be labeled on the Hx.

Picture a small under sink unvented hot water heater. What is it under 10 or 15 litres in volume then a vessel isn’t needed if there’s 2m of pipework? It’s a similar case for a combi. The volume of water in each part of the counter current hex is going to be a lot lot less than 10 litres and the pipe work after probably won’t equate to that either. Yes when the outlet closes there will still be some heated water but I imagine the pipe work would accumulate this expansion with little pressure rise. Of course when the tap is running you won’t notice this, much like an unvented cylinder.

 

[John.g]

Picture a small under sink unvented hot water heater. What is it under 10 or 15 litres in volume then a vessel isn’t needed if there’s 2m of pipework? It’s a similar case for a combi. The volume of water in each part of the counter current hex is going to be a lot lot less than 10 litres and the pipe work after probably won’t equate to that either. Yes when the outlet closes there will still be some heated water but I imagine the pipe work would accumulate this expansion with little pressure rise. Of course when the tap is running you won’t notice this, much like an unvented cylinder.

The 2M length is to the nearest cold water draw off to ensure that any expanded hot water doesn't come out the cold tap when opened, 2M of 1/2 ins piping will have a vol of 0.25Litres and 15 litres of water at 60C will expand by 0.24 litres (10 litres by 0.16 litres). If any NRV in the cold supply then a EV must be installed. My neighbour couldn't figure out for years why the 6 bar PRV was lifting on his attic 10 litre heater until he found a NRV on the cold mains just inside his house for reasons unknown, he just removed it. He also told me that the PRV would start passing within 4/5 minutes of the heating element switching in.
Obviously the expanded water in a combi secondary Hx may only be, maybe 50ml so will probably be absorbed without any problems.

 

[Matt0029]

Don’t need one for a combi

What’s the make of boiler ?

Do most fit the little 0.5 litre expansion / shock arrestors on the cold feed to the boiler. If a water meter is installed at the boundary stop tap externally. Surely there is enough pipe work to absorb a slight pressure increase?

 

[Matt0029]

I tested the pressure in a system with a combi as the hot tap is shut the pressure rise by 0.5 bar. That was tested near the boiler at the cold fill point if the filling loop.

 

[John.g]

I wouldn't think its common practice to install a arrestor on the mains but if so it would certainly act as a expansion vessel as well for the minute amount of expanded water.

The 0.5bar you are referring is the boiler (primary) water Expansion Vessel which must be fitted, otherwise the boiler safety (PRV) will lift each time the the system water (primary) heats up, this system can easily be 100 litres and will expand by ~ 1.8 litres so this expanded water must go somewhere.

 

[ShaunCorbs]

I do the only reason why is quarter turn taps can stress the internal components with a shock wave of closed fast eg plastic

 

[Matt0029]

I have when there has been a water meter internally near the boiler. When at the boundary to date I haven't as I figured there would be enough in the pipework to absorb a rise. I am seeing them fitted more and more though.

 

[John.g]

Why do you install it with a water meter?.

 

[Matt0029]

Why do you install it with a water meter?.

A check valve in the water meter so it doesn't allow expansion back down the mains. Have started fitting ideal are asking for it now.

 

[John.g]

That's interesting, from the Ideal User guide for Logic Combi C. 24 30 35

EXPANSION
NOTE. If a water meter is fitted into the incoming water mains
there may be a requirement for a domestic hot water expansion
vessel Kit. Contact a Gas Safe Registered Engineer or in IE a
Registered Gas Installer (RGII).

 

[Matt0029]

I did speak to one of ideals engineers and he said he have seen the plate hex blow off. Due to a nrv near the boiler, in a pressure reducing valve. And the pre heat beging left on.

 

[John.g]

I suppose shouldn't be too surprised as water is incompressible and especially with NRV close by, has no where to go.