Megaflow CL120 Backflow issue.

[Tom41]

Hi,

Hi wonder if anyone could help?

I have a customer that has recently had backflow problems with her cylinder. She gets very hot water coming through her cold taps in her 3 floored property. I recently changed her ground floor basin taps and in doing so had to drain down the cylinder as a ball-a-fix valve started leaking circled in yellow. Since completing that job she has had very hot water flowing through the cold taps for 30 odd seconds before going to cold. Obviously this isn't convenient and I am concerned with potential legionella risk.

I believe it could be a faulty non-return valve just after the balanced cold feeds and before the cylinder in photo circled blue.

I you could get back to me that would be much appreciated.

Kind regards,

Tom.

 

[Rtplumb]

A couple of questions.

Does the mains in go directly to the combination valve first then the 3 bar balanced side feed the cold to the property?
Did the hot water through the cold taps only happen after you replaced the basin taps and the 22mm iso valve?
Does this problem only happen when the heating is on?
Is there a hot pipe running next to the cold supply ?
Have you checked the non return valve (circled in blue)?
Is that orange pump connected to the balanced cold main?
What does that notice say next to the iso valve going into the cylinder?

I'm wondering if that 22mm iso valve you replaced should be a 22mm NRV?

It's easy to check. Run the hot water and if the mains into the cylinder with the iso on gets hot then that iso should be a NRV.

Sorry for the long winded thought process. Hope it helps.

 

[Tom41]

A couple of questions.

Does the mains in go directly to the combination valve first then the 3 bar balanced side feed the cold to the property?
Did the hot water through the cold taps only happen after you replaced the basin taps and the 22mm iso valve?
Does this problem only happen when the heating is on?
Is there a hot pipe running next to the cold supply ?
Have you checked the non return valve (circled in blue)?
Is that orange pump connected to the balanced cold main?
What does that notice say next to the iso valve going into the cylinder?

I'm wondering if that 22mm iso valve you replaced should be a 22mm NRV?

It's easy to check. Run the hot water and if the mains into the cylinder with the iso on gets hot then that iso should be a NRV.

Sorry for the long winded thought process. Hope it helps.

Hi Rtplumb

Thanks for getting in touch.

- So yes the mains goes into the monobloc and comes through balanced at 3 bar. then feeds 2 cold tee offs for upstairs and downstairs. Then goes through a NVR into the cylinder.

- The customer had this issue before but after draining down the cylinder it seems to have made it worse.

- No I don't think the problem occurs when heating is on but when they demand hot water on the programmer it seems to cause it after a duration.

-Hot pipes are running alongside the cold and judging by the age of the installation I doubt any of it is lagged. But I'm doubtful it is this as the cold tap is steaming hot when ran a few seconds.

- With the check valve is there a way to check if its failed? I do suspect this could be the problem.

-The orange pump is the secondary return to the cylinder for the hot water. (brass pump).

- The notes says hot water isolation valve.


I'm wondering if that 22mm iso valve you replaced should be a 22mm NRV?

It's easy to check. Run the hot water and if the mains into the cylinder with the iso on gets hot then that iso should be a NRV.

Sorry for the long winded thought process. Hope it helps.

Yeah that's what I'm thinking. I do think the Isolation valve I replaced is like for like I don't think its a NVR but I think the NVR that's circled in blue is the issue and has failed.
But I don't know if there is a way to check if it has failed?

No worries for the questions its good to get the bigger picture as I might've missed something.

 

[John.g]

- So yes the mains goes into the monobloc and comes through balanced at 3 bar. then feeds 2 cold tee offs for upstairs and downstairs. Then goes through a NVR into the cylinder.

Tom, how does the water expand into the expansion vessel if there's a NRV between it (the EV) and the cylinder inlet??.

 

[Tom41]

The only expansion vessel is on the monobloc before the NRV. There is no expansion vessel on the hotside after the NRV.

 

[Tom41]

Tom, how does the water expand into the expansion vessel if there's a NRV between it (the EV) and the cylinder inlet??.

Apparently the cylinder has a bubble to take up expansion. I can add

Tom, how does the water expand into the expansion vessel if there's a NRV between it (the EV) and the cylinder inlet??.

Hi John,

Thanks for getting in touch,

The only expansion vessel is on the monobloc before the NRV. There is no expansion vessel on the hot side after the NRV. I looked up the the instructions online apparently there is an expansion bubble in the cylinder itself. It also mentions an additional expansion vessel could be used? I've added screenshots.

 

[John.g]

Yea, I have recharged one of those Magaflo's for a neighbour of mine a few times, a external EV can be installed but if I am reading the photo correctly then it will do nothing in that position for the HW expansion due to the NRV position and if pipe2 is the cold inlet to the UV? what is pipe1?., is it a (the) secondary return.
It doesn't seem correctly plumbed to me as the combination valve has or should have a expansion relief valve set well below the T&PRV setting so maybe set to 6bar, this can't provide protection IMO with the position of the NRV. However if pipe1 is the secondary return then it will lift at 6bar but seems a very unusual installation but maybe meets regulations.
The NRV if leaking could be the problem.

 

[Tom41]

Yea, I have recharged one of those Magaflo's for a neighbour of mine a few times, a external EV can be installed but if I am reading the photo correctly then it will do nothing in that position for the HW expansion due to the NRV position and if pipe2 is the cold inlet to the UV? what is pipe1?., is it a (the) secondary return.
It doesn't seem correctly plumbed to me as the combination valve has or should have a expansion relief valve set well below the T&PRV setting so maybe set to 6bar, this can't provide protection IMO with the position of the NRV. However if pipe1 is the secondary return then it will lift at 6bar but seems a very unusual installation but maybe meets regulations.
The NRV if leaking could be the problem.

So Pipe 2 after the NRV is the cold feed to cylinder and secondary return.
Pipe 1 I assume is balanced cold feed to floor downstairs. The cylinder is on the 1st floor. The property has 2 floors in total (ground floor too).

I've attached another photo showing where expansion vessel is. Green is PRV discharge. Red circled is NVR before tee off to inlet of cylinder and secondary return.

I have added another photo showing the secondary return pump and that set up.

 

[John.g]

The megaflo schematic above shows the NRV installed just after the recirc pump, your installation shows it on the cold feed but the reason for this is probably because a combination valve normally has the balanced cold taken off just after the PRV but before a inbuilt NRV in the combination valve, your balanced cold(s) are taken off after the combination valve hence the reason for the NRV position. I still can't figure out what the external EV is doing as it will have no effect and also now that we know where the secondary return is tied in then the expansion relief valve will have no effect IMO.
I wonder was this the original installation?.

 

[Rtplumb]

The megaflo schematic above shows the NRV installed just after the recirc pump, your installation shows it on the cold feed but the reason for this is probably because a combination valve normally has the balanced cold taken off just after the PRV but before a inbuilt NRV in the combination valve, your balanced cold(s) are taken off after the combination valve hence the reason for the NRV position. I still can't figure out what the external EV is doing as it will have no effect and also now that we know where the secondary return is tied in then the expansion relief valve will have no effect IMO.
I wonder was this the original installation?.

Looks like that EV isn't doing anything as you say John. It should be sited after the NRV. Back to the original problem. I'd replace the NRV with the blue circle around it.

 

[Tom41]

I done some digging and found an installation schematic online. It does show an EV but after the balanced cold feeds?

Edit: I've drawn out a current diagram of the system.

 

[John.g]

That shows the balanced cold taken off before another combined NRV and expansion relief valve but don't see a EV connection, the expansion relief valve is correct.
Going back to your photos, it shows a PRV (pressure relief valve) on the EV with the waste pipe and the inlet/outlet but can't see where this is teed into the system, it might be after the NRV, you can check that out as its not clear to me from the photos, I wouldn't be too worried about the EV as its not a safety device but the expansion relief valve is another matter, if this is required by the regulations then it should be relocated.

This is a fairly good schematic of another UV.

 

[Tom41]

So I can't really recall where in the system the EV is located but I think judging from another photo I have it tops the pressure up on the CH before the primary pump. I've added a photo. Hope this coveys well?

 

[John.g]

Ah, now its clear, that EV has nothing to do with the UV cylinder, its for the CH system, the bubble takes care of the UV expansion, if the pressure is only showing ~ 1.5bar then definitely for the CH.

 

[Tom41]

Ah, now its clear, that EV has nothing to do with the UV cylinder, its for the CH system, the bubble takes care of the UV expansion.

Do you think its worth fitting an external EV anyway for the UV cylinder?

 

[John.g]

No, the UV cylinder volume is fixed so if the bubble is restored properly then its fine, sometimes the internal diaphragm breaks up leading to frequent recharging of the bubble, I can't think of any other reason for a external EV.

 

[Tom41]

No, the UV cylinder volume is fixed so if the bubble is restored properly then its fine, sometimes the internal diaphragm breaks up leading to frequent recharging of the bubble, I can't think of any other reason for a external EV.

Ok, I may have to recharge the bubble then? So with the backflow issue do you think its just a case of replacing that NRV? Also the customer gets very bad limescale was thinking of fitting an electrolytic inline scale reducer too.

 

[John.g]

Yes, I would just replace the NRV and restore the bubble exactly as shown.

 

[Tom41]

Yes, I would just replace the NRV and restore the bubble exactly as shown.

Ok, Thanks John & Rtplumb