Under-Sink heater issues

12 element · 12-11-2019 9:20pm #1

I have an ATC under sink heater that I've never been happy with. I've had 3 of them develop leaks over the last 6 years and 3 different plumbers look at it and I still don't think it's right. It was originally installed without and expansion tank!

Could someone advise is the relief valve in the wrong location here? The pressure rises to 6 bar and the relief valve starts letting out water at 5.5 bar.
looking in the manual it states not to fit the safety valve directly to the heater. it this stopping the expansion tank taking the pressure?

no water coming out of the Expansion tank valve and I've checked its at 3bar with the system open.

wizardman · 12-11-2019 9:26pm

Yes, it's installed incorrectly... Most are. See it all the time.

12-11-2019 9:29pm

The Chrome fitting needs to be replaced with a 6 bar safety valve fitted as close as is practical and with copper pipe discharge outlet that terminates in a safe manor.

The Chrome safety valve maybe a check valve and which can prevent the expansion vessel from doing its job.

12 element · 12-11-2019 9:53pm

wizardman wrote: »

Yes, it's installed incorrectly... Most are. See it all the time.

Is it the valve being in the incorrect location that's the issue?

gary71 wrote: »

The Chrome fitting needs to be replaced with a 6 bar safety valve fitted as close as is practical and with copper pipe discharge outlet that terminates in a safe manor.

The Chrome safety valve maybe a check valve and which can prevent the expansion vessel from doing its job.

The chrome valve is actually a pressure relief valve (6bar) it's hard to make out but there's an orange pipe attached to it that drains off to a white waste pipe.

12-11-2019 10:26pm

12 element wrote: »

Is it the valve being in the incorrect location that's the issue?

The chrome valve is actually a pressure relief valve (6bar) it's hard to make out but there's an orange pipe attached to it that drains off to a white waste pipe.

Yea, I know:D, I’m the technical representative of another manufacturer of under sink water heater.

If you look at one you will find the is a arrow showing the direction of travel.

If a unvented device is lifting the safety valve it usually indicates expansion is not being dealt with, if I had called to your installation I’d be giving you the above advice based on past experiences.

12-11-2019 10:32pm

Also the rubber hose on the safety valve is not something I’d have in my home, you can get very hot water/steam, a rubber hose isn’t going to last long.

12 element · 12-11-2019 10:34pm

gary71 wrote: »

Yea, I know:D, I’m the technical representative of another manufacturer of under sink water heater.

If you look at one you will find the is a arrow showing the direction of travel.

If a unvented device is lifting the safety valve it usually indicates expansion is not being dealt with, if I had called to your installation I’d be giving you the above advice based on past experiences.

There is surely an arrow on it! Is the location the problem? It was supplied with the heater! Looking at safety valves online there doesn't seem to be any inline versions available might need to start looking for yet another plumber!

12-11-2019 11:43pm

12 element wrote: »

There is surely an arrow on it! Is the location the problem? It was supplied with the heater! Looking at safety valves online there doesn't seem to be any inline versions available might need to start looking for yet another plumber!

Unvented heaters are covered under G3 UK water regs:eek: by insurance companies who use it as the deciding factor for any claims, G3 regs look for a labelled 6bar safety valve(which is available in all good plumbing merchants) which allows you to pipe copper to direct the hot water/steam to a safe place.

The heaters sold in Ireland only meet Irish safety standards (err...none) which means the heaters are sold with a crap safety valve which is also a non-return valve that is often found in other European countries who don’t care about the safety of their country men/women.

If you look at the installation instructions it indicates the valve is a non-return valve and the expansion vessel should be plumbed between the heater and the valve.

John.G · 13-11-2019 9:37am

12 element wrote: »

Is it the valve being in the incorrect location that's the issue?

The chrome valve is actually a pressure relief valve (6bar) it's hard to make out but there's an orange pipe attached to it that drains off to a white waste pipe.

I had a good look at one of these PRVs a good few years ago and here is my take on them.
They are fitted where the expansion volume is taken up by the cold feed pipe and consist of three components combined in one fitting..... the PRV itself which is in the side branch, you can see a little plastic "lifting gear" which can be used to manually open the PRV or maybe can be used as a air release if the cold water supply has failed or has been switched off. the second component is a NRV which is fitted in the main body of the valve and opens (downwards) to allow cold water to enter the heater when there is a hot water draw off. I think its main purpose is to act as a anti syphon valve in the event of the cold water supply failing/draining down and keeps the heater full of water, the third component is another NRV which is inside the "main" NRV and opens in the opposite direction to allow expanded water to escape into the cold mains, main NRV is ~ 8mm dia and the smaller NRV is ~ 4 mm.
Assuming a empty heater and the cold water supply is opened then the main NRV (containing the smaller NRV) will open and allow the heater to fill., the main NRV will then close and as the water expands in the heater then the smaller NRV will open at ~~ 1 bar and allow the expansion into the cold feed.
This chrome pressure relief valve should be (obviously) installed with the arrow pointing downwards.

You say that originally no E.vessel was fitted, the reason that the above valve couldn't release the expansion into the cold feed was/is because of the pressure reducing valve fitted acts as a NRV so the expansion had no where to go.
The E.vessel should be between the heater and the pressure relief valve but I.m still a bit puzzled as to why its pressurizing the E.vessel to 5.5/6 bar, maybe each time it expands the smaller NRV closes and causes a build up after each expansion.

I would, at the minimum, change the plumbing to have the expansion immediately before the heater as suggested by gary71 but "better" solution would be to install a 6 bar conventional pressure relief valve although purely from a interest point of view I would love to see the result if you just re plumbed using the existing valve.

For interest:
Assuming a 10 litre heater and a 2 litre E.vessel with a prepressure of 3 bar and allowing 3% expansion should give a expansion volume of 0.3 litres and a final E.vessel pressure of 3.7 bar. Other numbers heater 20 litre/2 litre E.vessel
= exp vol of 0.6 lires and final pressure of 4.71 bar.
with a 4 litre E.vessel the final pressures would be 3.32 bar & 3.71 bar.

If you post the heater and E.vessel volumes I can calculate quite accurately the final pressure.

12 element · 13-11-2019 11:24am

John.G wrote: »

I had a good look at one of these PRVs a good few years ago and here is my take on them.
They are fitted where the expansion volume is taken up by the cold feed pipe and consist of three components combined in one fitting..... the PRV itself which is in the side branch, you can see a little plastic "lifting gear" which can be used to manually open the PRV or maybe can be used as a air release if the cold water supply has failed or has been switched off. the second component is a NRV which is fitted in the main body of the valve and opens (downwards) to allow cold water to enter the heater when there is a hot water draw off. I think its main purpose is to act as a anti syphon valve in the event of the cold water supply failing/draining down and keeps the heater full of water, the third component is another NRV which is inside the "main" NRV and opens in the opposite direction to allow expanded water to escape into the cold mains, main NRV is ~ 8mm dia and the smaller NRV is ~ 4 mm.
Assuming a empty heater and the cold water supply is opened then the main NRV (containing the smaller NRV) will open and allow the heater to fill., the main NRV will then close and as the water expands in the heater then the smaller NRV will open at ~~ 1 bar and allow the expansion into the cold feed.
This chrome pressure relief valve should be (obviously) installed with the arrow pointing downwards.

You say that originally no E.vessel was fitted, the reason that the above valve couldn't release the expansion into the cold feed was/is because of the pressure reducing valve fitted acts as a NRV so the expansion had no where to go.
The E.vessel should be between the heater and the pressure relief valve but I.m still a bit puzzled as to why its pressurizing the E.vessel to 5.5/6 bar, maybe each time it expands the smaller NRV closes and causes a build up after each expansion.

I would, at the minimum, change the plumbing to have the expansion immediately before the heater as suggested by gary71 but "better" solution would be to install a 6 bar conventional pressure relief valve although purely from a interest point of view I would love to see the result if you just re plumbed using the existing valve.

For interest:
Assuming a 10 litre heater and a 2 litre E.vessel with a prepressure of 3 bar and allowing 3% expansion should give a expansion volume of 0.3 litres and a final E.vessel pressure of 3.7 bar. Other numbers heater 20 litre/2 litre E.vessel
= exp vol of 0.6 lires and final pressure of 4.71 bar.
with a 4 litre E.vessel the final pressures would be 3.32 bar & 3.71 bar.

If you post the heater and E.vessel volumes I can calculate quite accurately the final pressure.

Thanks for your reply! That's what's confusing me seems that even if it is in the wrong location the expansion tank doesn't seem to be doing its job anyway. the pressure rebuilds quite quickly once I release it (with the black PRV lifting gear) this happens even when the heater element is off (light not illuminated)

The heater is 10 litres and the EV is 5 litres.

John.G · 13-11-2019 12:08pm

Well that points to a problem with your mains pressure/pressure reducing valve if its building up when cold!. Is your cold water suppply from the the mains or have you your own pumped supply?. If your mains/pumped pressure is ~ 5 bar and if the pressure reducing valve isn't tight under no flow conditions then the pressure will build up to the upstream pressure, there are two different types of pressure reducers, one will hold its setpoint pressure under static/noflow conditions....the other type which I think is called dynamic will not hold the setpoint pressure under no flow conditions but will fall to its setpoint once there is a flow.

What is the setpoint pressure? and once you release the pressure with the PRV lever can you shut off the cold water supply and see if the pressure remains steady.
You could then switch on the heating element and see if your final pressure is close to the numbers below.

Final pressure after expansion with a 10 litre heater, a 5 litre E.vessel prepessurised to 3 bar and with a cold water pressure of 3 bar after the pressure reducing valve will be ~ 4.2 bar with the present location of the PRV and 3.2 bar if the expansion is re routed to be nearest the heater.

Edit: The type of pressure reducing valve you require is known as "drop tight" https://heatingcontrols.honeywellhome.com/products/Valves/Pressure-Reducing-Valves/D05-Pressure-Reducing-Valve/

12 element · 13-11-2019 7:40pm

John.G wrote: »

Well that points to a problem with your mains pressure/pressure reducing valve if its building up when cold!. Is your cold water suppply from the the mains or have you your own pumped supply?. If your mains/pumped pressure is ~ 5 bar and if the pressure reducing valve isn't tight under no flow conditions then the pressure will build up to the upstream pressure, there are two different types of pressure reducers, one will hold its setpoint pressure under static/noflow conditions....the other type which I think is called dynamic will not hold the setpoint pressure under no flow conditions but will fall to its setpoint once there is a flow.

What is the setpoint pressure? and once you release the pressure with the PRV lever can you shut off the cold water supply and see if the pressure remains steady.
You could then switch on the heating element and see if your final pressure is close to the numbers below.

Final pressure after expansion with a 10 litre heater, a 5 litre E.vessel prepessurised to 3 bar and with a cold water pressure of 3 bar after the pressure reducing valve will be ~ 4.2 bar with the present location of the PRV and 3.2 bar if the expansion is re routed to be nearest the heater.

Edit: The type of pressure reducing valve you require is known as "drop tight" https://heatingcontrols.honeywellhome.com/products/Valves/Pressure-Reducing-Valves/D05-Pressure-Reducing-Valve/

Thank again! I think you're onto something! It's connected directly to the public mains. I'm not sure of the mains pressure here or the set point so I drained all of the hot water out of the tank with the heater off then closed the tap and the gauge reads just over 3 bar (is this the set point?). I then shut off the mains feed just before the Pressure reducing valve and turned on the heater. The water fully heated without any rise in the pressure on the gauge (presume the tank was taking up the expansion). I then opened the mains and the pressure started creeping up to 5.5 bar again!

Looks like replacing the valve with a drop tight version should solve it? I will replace the safety valve with the recommended type while I'm at it also.

John.G · 13-11-2019 8:33pm

That's great news, and a new drop tight pressure reducing valve will do the trick, also, even though the PRV is ok, its certainly prudent to replace it.
When you get a chance you might post the make/model of the existing pressure reducing valve, it would be interesting to see if this was a drop tight type and developed a fault or if the incorrect type has been fitted from day one.

You have excellent mains pressure of 5.5 bar which a lot of other people would love to have and yes, the pressure reducing setpoint was set at 3bar so the EV prepressure of 3 bar is also correct.

Under-Sink heater issues

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