Hot water, not cold!

[simonkev]

Are you sitting comfortably? Good, now here's a story.

Called to a larger property with a big unvented cylinder (Megaflo). About a year ago it needed a new immersion heater so the cylinder was drained and a new one fitted (not by myself). Following this in the mornings after the system is left standing overnight when any cold tap draws water, hot water comes through for a few moments, after which the cold comes through. If the system is left untouched for a couple of hours subsequently then the same thing happens. I suspected the combination valve but although not plumbed in correctly (tee fitted afterwards for hot and cold split - balanced out on combination valve blanked uhhh???!) water is not back flowing from the cylinder. When I arrived the system was cold and sure enough if a cold tap was opened the hot supply pipework from the cylinder heated up. The cold tap delivers very hot water for a few moments. A non return valve has been added below the combination valve recently, but again I don't believe this would help. Obviously hot is being drawn back somewhere, and short of installing NRV at every output (very difficult with shower valves in tiled bathrooms!) I am at a loss to resolve this.

The customer assured me they had lived with the system for some eight years and this had not happened until the immersion was changed. I don't believe the immersion has anything to do with it but I see her point of view.

If anyone has any suggestions they would be greatly appreciated.

 

[galaxy_plumbing]

is there any mixer tap? is T for balanced cold water between combination valve and non return valve?

 

[mountainman]

My thought so far FWIW.
If the air bubble in the Megaflo is lost then the excess water pressure should trigger the pressure relief valve.

But

What if the non return valve is faulty and allows backflow of HW into the cold supply at a pressure lower than would operate the PRV?

To all intents and purposes the Megaflo would appear to be working normally but the expanded hot water would appear in the initial draw off of the balanced cold.

 

[chris watkins]

Can you post a photo of pipework around combi valve / cylinder, this is a problem we come across from time to time, as you suspect it is to do with the balanced cold / check valve not being in the correct positions. Got to be careful though if fitting additional non-returns if you get them in the wrong location to the expansion safety valve you can isolated it.
First thing in the morning after heating has taken place at night, the hot side will be at a greater pressure (expansion) than the cold incoming mains so when cold is opened by flushing wc or a tap water flows from the cylinder back along the C/F to the balanced cold supply.
The only other thing is, does the large system have a secondary return system installed ??
P.S. have you got your Part G ticket Simonkev???

 

[SimonG]

Have you stripped the combi valve and nrv down and cleaned it out? Might be a bit of muck wedged from when it was drained down.

 

[glenno1]

Have you checked expansion vessel for water

 

[chris watkins]

Have you checked expansion vessel for water

Simonkev has searched high & low but can't find one on this Megaflo cylinder !! LOL

 

[kay-jay]

Simonkev has searched high & low but can't find one on this Megaflo cylinder !! LOL

Lol
Im with mountainman on this drain cylinder and refill to re-establish air bubble. Check operation of nrv

Sent from my GT-I9100 using Tapatalk 2

 

[simonkev]

Thanks all so far.

Mountainman - if I put my hand on the cold feed to Megaflo and draw cold water, hot water is not going backwards, cold feed remains cold. If I hold the hot water output pipe from top of Megaflo it heats as you would expect if a hot draw off was made. Therefore I do not think its a faulty NRV.

Chris Watkins - I did not take piccies, but as per mountainman there is no demonstrable backflow through cold feed. Good point re secondary return. I don't think it has, but I should check. Sleep easy, I have Part G.

SimonG - as per above, hw not back feeding.

Glenno - I did not check bubble (he said diplomatically!), but really that should not have any affect.

I guess if I ask Heatrae to get involved they would say correct pipework first before they would look. I guess all I can do is replumb as per HS instructions and see what happens. But, as ever, should I get paid for making no difference??

Any more thoughts???

 

[SimonG]

Poorly fitted pipework somewhere? Cold in contact with hot or htg pipework?

 

[mountainman]

Poorly fitted pipework somewhere? Cold in contact with hot or htg pipework?

Should have been a problem for the last eight years, OP says that this is a fairly recent problem.