Discharge pipework from system boiler

20 Oct 2009
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United Kingdom
Hello - safety discharge pipework from a system boiler or an unvented hot water cylinder:

Right next to where I plan to have a system boiler and unvented hot water cylinder (in the loft) is the 100mm stink pipe, which is fitted internally. After the tundishes, can the discharge pipework run into this 100mm stink pipe, as opposed to being run to the outside and then down the wall?

If I have to run pipe(s) outside down the wall, can I combine both tundishes into one outside pipe (22mm min, depending on unvented hot water discharge calcs)?
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In theory you should not but if you first pass it through a fanny flap then apparently you can!

From a practical perspective, will you actually be able to see any flow go into the soil stack? not much point having the tundishes if you won't be able to see it. Whereas if its outside, i guess it will be more prominant

I was thinking along the viewable lines, and have the discharge pipes go into a 2 inch waste to the stack. The 22mm dischrge pipe would be a loose fit into a 50-32-22mm reducer, so that smells would not exit, but the assembly could be lifted for viewing.
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If it's fitted in the loft it would be better if they terminated in a visible location, otherwise you could find that, particularly with the UV cylinder, you could have the discharge running continuously and not know about it for ages, leading to water wastage and high bills. It may also be interpreted as a contravention of the regulations for the UV cylinder as neither the tundish nor the termination point is technically visible, although if you use your loft regularly you could possibly argue that you will see fairly quickly when it is discharging.

If you do wish to take this risk, and run into the stack, you may provided you stick to the correct method of doing so. This must be via a HepVo waste valve with tundish adapter, which will then connect into 32mm MUPVC high temperature waste pipe that will carry it all the way to the stack. Hunter waste pipe is MUPVC as standard, most other makes are not and are therefore not suitable.

The decision on whether to terminate in the stack, given the possible issues with compliance, ultimately rests with your G3 engineer as it is he who must sign the commissioning paperwork. I personally would be generally reluctant to have the termination point out of sight where the cylinder and tundish are in the loft.

Hope this helps
Thank you all for your advice.

It looks like it has to be outside then.

Can I use just one external 22mm pipe into which both internal tundishes connect to?
One pipe will be fine (there's not actually a requirement to have a tundish on the boiler, so you could just connect this after the tundish). It's size depends on the length of run, 9 metres max less I think 0.7 metres for every bend
I am having further thoughts on this. I have spoken to my plumbing chap who has the unvented ticket.

I asked him about curling the discharge pipe close by the wall exit so that it points back into the wall. His preference is for a pipe that runs down the wall, stops near the bottom, and now with a mesh guard to protect people getting too close to the pipe in case it vents boiling hot water.

I have now flipped. Let us study this from a health and safety point of view. You either have boiling water spewing out at high level (curled into wall), or spewing out close onto a pathway running down the side of the house. That same pathway running down the side of the house that my 2 toddler boys play. So in the event of a fault, there is chance my 2 boys will get scalded. Crazy, dangerous, short sighted and irresponsible. The alternative? close off the pathway running down the side of my house to pedestrians? Utter madness.

Let me calm down, press rewind, and approach this from first principles, and not from narrow minded interpretation.

The discharge pipe is there to safely dispose of discharged water. There is also a requirement to know if water has been discharged to alert to a fault with the system.

The safest way is to discharge into the stack, as described by muggles above, in the loft.

QUESTION 1: can I use a water sensor in the discharge pipe to signal when water is present? The sensor would trigger an audible alarm, and backed up by a visual LED. The trigger would be latched so that a small burst of discharged water would cause a permanent alarm (until manually cancelled). The alarm and LED would be mounted on the ceiling of the upstairs landing so that it would be more readily noticeable (like a smoke alarm).

QUESTION 2: Unvented cylinders appear to require a 2 port motorised valve on the feed pipe from the boiler. Is the 2 port valve to be fitted in addition to a conventional 3 port mid position motorised valve, or can the unvented cylinder be fed directly from the 3 port valve from the boiler?
Or should I adopt a 2x 2 port valve system in place of a 3 way mid position?
here's just my opinion.

The best way would be to use a kumkle or sentinel valve, these are often used in steam turbines to avoid over pressurizing the casings. When the pressure gets too high, they lift and you hear a loud squeal.

Unfortunately, that will not comply with building regulations.

If you do decide not to comply with code and frankly most installations I go to don't in some way shape or form, then this will at least give you an audible warning.

Your installer is quite correct, the preferred termination of a UV cylinder discharge is 100mm above the ground with a mesh guard over the end to protect it and anyone around it. Terminating by turning the discharge back to the wall at high level is generally not allowed; it may be considered when all other options are exhausted but as you appear to have a clear path down to the ground this would not be permitted. This is a regulation and not your installer being awkward.

In answer to your questions...

Answer 1: Most of the time there is not a 'burst' of water but an intermittent drip-drip-drip, as a lot of the time discharges are caused by expansion vessel / space faults, and occur as the cylinder heats up. There are two relief valves on an unvented cylinder - a pressure relief valve (PRV) which will discharge at 6-8 bar, and a combined temperature and pressure relief valve (TPRV) that will discharge at 10 bar and 95ºC. In order for the TPRV to discharge (the one that is more likely to give a burst of water), both the PRV and the overheat stat must have failed. That is to say, the TPRV is a third-stage safety device. Therefore unless your alarm is able to react to odd drips, and can still function after many months or even years of doing nothing, it would be unsuitable for indicating a discharge. Also, if it created any kind of restriction in the discharge pipework it would be unsuitable.

Answer 2: The 2-port is wired to the overheat thermostat on the cylinder. Should the cylinder reach 80-85ºC, the overheat stat will trip and cause the zone valve to close. This will not be able to re-open again until the stat is manually reset. It is, therefore, a safety device and must be fitted regardless of whether you have a 3-port or use it as one of two 2-port zone valves. I'd favour converting to a 2×2-port (S-plan) system
Thank you muggles for your reply.

Point 1, I understand you about the drip drip. My idea should be able to react to that. Dare I say better than an external discharge pipe where the drip drip would have most likely evaporated off the pathway before I realised there was a fault?

Is this drip drip mode of failure caused by an expansion vessel that has lost its precharge? Is that not part of an annual service check?

In addition to a green LED 'status on' similar on my mains powered smoke alarms, my idea would also incorporate a test feature - a button press, similar to a smoke alarm test, that would trigger the alarm. A further test could be performed on the pipe contacts directly. These tests could be written into the annual service plan for the boiler and UV at the very least. When was the last time we all checked our smoke alarms?

Point 2 - understood.
You are quite correct that the dripping is caused by the loss of charge of the expansion vessel (or loss of air gap in a Megaflo), yes it is part of the annual service but very few people actually get their unvented cylinders serviced annually in my experience.

Other than that your idea seems sound (pardon the pun) but I must re-iterate that it cannot cause any kind of blockage or restriction in the discharge pipework. Also, 'should' react isn't really good enough, it must react every time, an alarm that works intermittently is arguably worse than no alarm at all. I test my smoke alarms weekly, but that's getting a little off-topic...
Thank you again, muggles.

I agree, 'should' was the wrong word to use. I now know the mode of failure, and can simulate that to ensure my idea works reliably. The ultimate test will be satisfying my installer. I know he is not awkward but merely goes by the book - when it comes to safety, I am glad he is that way!

Not causing a blockage is clearly understood.
Glad to be of assistance. Your installer may, in his haste to tell you that your idea is unsuitable (I might be persuaded that it could work...) suggest that it won't work in the event of a powercut. Well no, it won't, but then neither will the boiler or the immersion heater so the cylinder won't be heating up. I'm sure you'd have worked this out anyway but just a heads-up to get you on your way ;)

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