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Author Topic: How large should the F&E overflow be?  (Read 1253 times)
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« on: May 13, 2017, 06:53:02 PM »

I am getting conflicting information on the above question so thought I would seek yet more opinions!

I will have a 400L thermal store heated by ASHP, PV/Immersion and a WBS (Dunsley Yorkshire with BB), this will have a 22mm pipe to the F&E tank in case of problems. What size should the overflow from the F&E tank be and should it have a tundish on it?  It was proposed to use 22mm copper for the overflow (WBS is an uncontrolled source so could effectively be 100C water), however I have now been told by the stove fitter that it should be 28mm.  Because the overflow will discharge into a soil pipe the overflow pipe will most likely have a HepVO (waterless trap) on it to prevent any smells from the soil pipe getting out. What size pipe should be used?

I am told there is no requirement for a tundish on the overflow, but since if it is ever in use we are highly unlikely to know unless there is a tundish I am tempted to fit one - views?

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« Reply #1 on: May 13, 2017, 09:23:50 PM »

I would agree to fit a tundish so that you could see if its in use.   The overflow capacity needs to match the potential inflow rate. Given that the WBS, (pumped I presume) potential for 100'C water temperature and water expansion over sizing to 28mm makes sense as it gives a safety margin.


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Backache stuff!!

« Reply #2 on: May 13, 2017, 09:50:02 PM »

An over flow on a F/E tank (or more correctly, Cistern) is there to provide a visible warning in case of failure of the float valve, not an over heat or boil over issue. Therefore it needs to visibly discharge somewhere that causes you untold grief so that you notice it and then fix it, no tun dish needed, and 19mm minimum diameter. An over heat or boil up issue of the system it serves determines how big the cistern is what material it is and the size of the vent pipe, nothing to do with the overflow. If you think about it a boil up will cause the fluid to vent over the cistern and then drain back into the system, it goes round and round rather than out the overflow.


still a crazy old duffer!
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« Reply #3 on: May 13, 2017, 10:03:58 PM »

I think such an overflow has to flow to atmosphere (i.e. an open-ended pipe, not directly into further plumbing). Flow through it should also be noticeable because it is a warning pipe. So discharge outside is OK, discharge into a WC or sink is OK, and discharge into a tun dish is OK as long as the tundish is somewhere flow will be noticed.

I don't know what size it should be. I would ask your stove fitter to show you the rules that require 28 mm, since that's a lot bigger than I would have thought necessary.
« Last Edit: May 13, 2017, 10:07:14 PM by djh » Logged

Cheers, Dave
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