What is the situation with blow off pipe if no outside wall for cylinder?

28 Nov 2010
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United Kingdom
I am thinking of having a direct unvented cylinder fitted but there is not really an outside wall a pressure relief pipe can go to very easily

A drain valve can obviously discharge into waste pipe but can the blow off/emergency pipe? Is this ok with a tundish? Accessing grey large diameter plastic drain is what seems possible.

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It can, but you have to comply with the regulations for unvented pressure discharge and with the regulations for soil pipe air containment and ventilation. You can't use an ordinary tundish direct to the soil pipe because it doesn't have a trap (u-bend). To be compliant you'll need soemthing like a Hotun

You'll also need to check the temperature rating of the waste pipe
There are tight rules regarding D1 and D2. You need to get an appraisal from a fitter with UV ticket.
There is "always" a way, but it has to be done right and with whatever costs it will incur.
Thanks. Hoping to have a some visit's this week from experienced trades.

Are systems with cold water tanks really a thing of the past? An option I have is still utilising the cold water tank and buying a more modern vented type cylinder
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Thanks. Hoping to have a some visit's this week from experienced trades.

Are systems with cold water tanks really a thing of the past? An option I have is still utilising the cold water tank and buying a more modern vented type cylinder

What made you consider uv?
I guess it isn't vital but I was thinking mains pressure to a new shower mixer (it's an electric shower just now) and just not having the cold tank.

I'm going all electric replacing an old back boiler. Dilalio, I think you reploed to a post I made a week or so ago. I was wondering about options to replace the old Baxi Bermuda but can't have a flue to the front of the building and don't want to have to put a liner in the chimeny (it's very high to the roof) You can't even seem to buy a replacement very easily. Probably got 15mm gas pipe presently going to the baxi and not at all easy (even pumped) to get a condensate ran.

So, at present thinking electric flow boiler for CH and was thinking pressurised cylinder.

Just a small flat so cost of running leccy boiler should be manageable.
If this is the rental flat, remember unvented cylinder requires an annual safety check, so that's an ongoing expense and an inconvenience for the tenant having a tradesman visit.

Electric boilers for CH lack the controllability of individual heaters in each room and have the disadvantages common to all wet systems of leaks, bleeding radiators etc.

If you have the space you could consider a thermal store, which heats up using off-peak electricity but can run wet central heating and a mains pressure shower. The store itself is usually vented, so no unvented regulations apply. They can be bulky though, and have quite a high initial cost. Tenants (and a lot of installers) may not understand them.
Thanks, it's pretty tricky deciding what's best. I was actually going to go for modern electric radiators which as you say do at least have the advantage of 100% controlability. However, I see appeal in the electric boiler from the point of view of still being able to say the flat has "central heating" with normal radiators etc. The bedroom is really small so it won't take a lot of heating. The main room and kitchen to one end will be the only real area to heat.

The flat is already piped for CH from the Baxi back boiler Looks like must be a manifold onto 8mm pipes going to rad's.
What would thoughts be on re-using this? There doesn't seem to be any leaks or anything from it anywhere. Getting old though being the age of the old Baxi.
It might spring a leak under pressure, but at least with a manifold you'll be able to isolate the affected branch.
Yes, one installer mentioned the possibility of this last week. I'm now leaning towards just using electric radiators rather than trying to keep it a wet system. They seem likely to be no more costly to run. Still think a new unvented cylinder and new mixer shower a good idea.

Four installers visited to quote and unfortunately I feel faced with differences of opinion on how to run the blow off.

-One was going to take it into a raised level white plastic waste pipe which drops down from the cold storage tank (copper goes into it, looks like some sort of blow off from the old cylinder I think..)

-Two simply said nothing other than taking it outside was possible (I don't see this as entirely true but accept there is a right way to do it..)

-One was going to take it into the grey 4" soil

The first one above doesn't sound the worst of ideas (to me anyway) because it goes to the 4" with nothing else in front of it (no sink. baths etc).

Knowledge is good but if you didn't have a clue you wouldn't know who to trust. I guess I still don't know who's right!

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