TMV/water heater issues

[Peter_Griffin]

Good evening gentlemen,

From the start Im not a plumber however I'm good at most things but this is getting the best of me.
Work has asked me to look at an electric water heater in a toilet that produces hot water for 2 sinks in there. Its a Santon unit (quite large) 3kW and is fed of the mains. It gravity feeds the sinks through a TMV which cold side is fed off mains to. Problem was that nothing was working so I assumed TMV was no good and replaced it. Water is now flowing but not exactly a good flow and if you leave for say 5 mins then try the hot tap it feels very stiff as though there is build up of pressure and then water shoots out but then settles back to this slow flow.
I left it for like 15mins to take a call and again tried hot tap, water squirted and then nothing flowed at all!!! Then about 10 secs later slow flow again.

Could mains pressure be too much for certain TMV??
Other heaters in other areas on site are exactly the same but dont have this problem?

Please help as im lost

 

[plumberdarren]

Hi, are you saying that the water heater is wall mounted with the mains feeding a tank in the top which feeds the hot water tank and then gravity feeds to the tmv? If this is the case and you have a mains cold feed to the tmv then there will be unequal pressure and it won't work properly! Also make sure the cold valve isn't stuck shut as all tmv's have a failsafe mechanism which means that when there is no cold water into the tmv the hot will shut of as soon as it gets to the tmv, you would therefore get water through for a couple of seconds and then nothing (as the hot has reached the tmv) this is to prevent scalding in the event of the cold being turned off.

 

[Peter_Griffin]

hey thanks for replying

Hi, are you saying that the water heater is wall mounted with the mains feeding a tank

YES mains pressure feeds into the tank when hot is drawn off, like a float valve situation

gravity feeds to the tmv?

yes hot from the tank is basically gravity down to the TMV (i guess) which is situated directly below the heater (instead of under the basin)

mains cold feed to the tmv

yes. mains that feeds into the top of tank is fed from the same mains pipe that feeds the TMV

then there will be unequal pressure and it won't work properly!

Right OK I can see where your coming from and it makes sense for sure. Only thing though is there must be 4 other heaters on site that are exactly the same config and dont seem to have a prob? Could it just be that the new TMV i've fitted doesnt like this way?

Also make sure the cold valve isn't stuck shut

cold valve as in isolation valve? If so then it is defo not shut.

many thanks for the help so far.

 

[prs1]

Peter does your santon water heater have a tank at the top with a ball valve in it or is it a direct mains supply ie mains cold in hot out-no tank?

 

[Peter_Griffin]

its one of these

[DLMURL]http://www.discountedheating.co.uk/shop/acatalog/santonrunittech.jpg[/DLMURL]

45 litre I think. Not at work now so cant check for sure but it is same style/design. It has an tank/overflow internally at the top that has a ball valve to stop the mains when required. This fills when hot water draws off and this top tank feeds the heater tank in the bottom.

so basically it works similar to this pic

[DLMURL]http://www.aquahot.co.uk/cms/ktml/media/products/R-unit-schematic.jpg[/DLMURL]

except the hot goes through a TMV directly under unit which has a mains feed from cold supply to opposite side of TMV

thanks

 

[prs1]

sorry just reread post

Some tmvs dont like low pressure and obviously depending on how high the heater is on the wall will effect this,we usually use relliance one.
Fit a pressure reducing valve to the mains cold feed to valve if that doesnt resolve it check operating pressures in mi's.
They all vary from manufacturer to manufacturer!
Good luck

 

[prs1]

We fitted some to waterheaters in a school the tanks were only at about 6 foot of the floor-we just could nt get em to work-You could try and get the same tmv that was previously fitted but make sure u tell them it needs to work on low pressure.
Just hope its the mains pressure thats causing the issue as previouly said-Which a PRV will sort!

 

[prs1]

Obviously adjust the PRV down to a similar pressure to the hot-u can get some with a guage on!

 

[Peter_Griffin]

Excellent mate thanks for the advice

Funnily enough this is a school to and yep the heaters are prob no more than 5 foot off the floor. I could not get an orignal valve as they not available or something now so was given a BOSS TMV3.
Will look into fitting a PRV onto the cold side of TMV just hope it will work

In the meantime is there anything else I could be missing or easy enough for me to check?

 

[Peter_Griffin]

hey fellas,

Right went to work today and wierdly....all was OK!!! hot water was flowing from the taps perfectly fine?? I just dont get it. Do TMV's require time to settle in or something? or is the like a "secret handshake" to get it working correctly? LOL

cheers