Condensing boiler in cellar?

[Nick Holmes]

I need to put in a new CH/DHW boiler. I inherited a gas combi but the present one is conventionally flued via an 8.5m chimney (with bends). I am reasonably happy with a combi as long as it is sufficiently powerful - there seem to be a number which will do the job.

The current site is bad for balanced flue access and for condensate drain/ pressure relief valve discharge pipe. I would also like to lose the requirement for ventilation into the room (cold draughts).

There really aren't any good sites above ground from my point of view - obviously possible ones which usually involve destroying something else I value and/or running flue pipes across rooms.

I would like to resite the boiler to the cellar where we have good flue access plus good access to water and gas (though it does require a hot water pipe to be run).

However, I am struggling with the twin issues of condensate drain and pressure relief discharge. If I understand correctly, the condensate could be run into a syphonic trap with a pump to periodically discharge it. Can this discharge be connected to an external cast iron soil pipe (which in our 1906 build carries both bath(shower) outflow and foul water from 2nd floor toilet (incidentally Cambridge's edwardian/victorian houses have combined surface and foul water connections to mains drainage which would not be permitted in a modern property)).

I suspect the answer may be no. If not we may have to dig up the concrete outside - there is a drain inspection cover a metre or so away but no other pipes or accessible drains nearby (nearest is 6m including

3m outside and with a doorway between).

Also what about the requirement for the discharge pipe. Will this need a different trap and pump and pipe - the type of possible flow being v different?

As usual any help and advice most welcome.

best wishes

Nick

---------------------------- Nick Holmes

[bof]

Re combi in the basement

I have one here, the condensate goes via gravity into a small tank and is periodically pumped out by a small electric motor, the unit's something like 8"x8"x4". The condesate is pumped through a narrow bore, (about 1/2" ext dia) flexible tube under the floorboards and out through the wall into an external drain. I was a bit dubious before installation but it's been working well for the last two years.

[Jules]

Do you mean "can" is in will it work? Or "can" as in is it legal?

Like bof, we've got a unit of about 8x4x4" with a pump which traps condensate and pumps it up about 8' (and sideways about 10') to the cast iron waste pipe via a narrow bit of yellow-tinted translucent tubing. Seems to kick on for a couple of minutes a couple of times a day.

Can't comment on the legality though as a) I'm in the US and b) we have a private septic system...

cheers

Jules

[bof]

In message , Jules writes

Here the flex tube runs about 7' vertically up, then 12' under the floorboards and is connected to a rigid plastic pipe fixed to an exterior wall that takes the condesate another 6' to the drain.

It was installed by a CORGI here so it must be OK(!)

[David WE Roberts]

I am a bit puzzled by this - when I put a second bathroom in my 1930s house around 2001/2 I had to run a new soil pipe to an existing manhole and I fed both grey and black water into the same stack.

Building control were perfectly happy with this.

Is this a recent change, or does it not apply when working with older properties?

Cheers

Dave R

[dennis]

I don't think I would pump it into a cast iron pipe. Its mildly acidic and you can just dump it into a small soak away and not risk corroding the pipe.

[dennis]

A lot of newish (less than about fifty year old) properties have separate drains and sewers. If you connect a sewage outlet to a drain you are in trouble and rightly so as you are dumping untreated sewage into the rivers. It stops the sewers from flooding when there is a storm and makes treatment easier as there is less to treat.

[The Natural Philosopher]

More a function of your local water authority really.

But he was talking 'white' water, Rain collection.

All used water must be treated, black, brown or grey.

All rainwater ideally should end up in the subsoil or river system.

[Nick Holmes]

Thanks for this. What about the pressure relief discharge pipe though?

[bof]

There's a 15mm copper pipe goes out next to the flue, whether this is for pressure relief or no I don't know

[David W.E. Roberts]

Still confused - he talked about the stack taking bath(shower) water and foul water so I assumed that the next para. he meant 'grey water'.

If the two paras. are not directly related then fine - I understand what is going on!

[Nick Holmes]

Sorry I have confused the issues. Probably it makes no difference where you are, bath and toilet waste gets combined. In our case gutter flow also gets run into the same waste pipe (though not the particular soil stack I mentioned).

In answer to another post, we cant easily go to soakaway (property boundary is 1m from cellar ext wall and external ground on our side is concrete for 15m in both directions).

Locally, we have exemption for older properties with dual purpose sewerage ('white' and grey/black).

best wishes

Nick

---------------------------- Nick Holmes

[Nick Holmes]

It sounds as though it might be - does how does the pipe terminate?

Doesn't the pressure relief discharge pipe have to be terminated so that there is no risk of it spewing hot pressurised water onto anyone and doesn't it need to run downwards from the pressure release valve (unless through a trap and pump)?

[bof]

This runs down from the boiler then up 3' and out through the wall terminating in an elbow about 6" above, and pointing at, the ground

[YAPH]

As others have said this application calls for a condensate pump. You shouldn't pipe acidic condensate into a metallic drain system. In the case of cast-iron I suspect this is as much arse-covering in the event that the CI finally rots through and the aggrieved owner scapegoats the boiler installer, rather than a century's natural corrosion. If you are both installer and owner you may decide not to worry, but you may still find it hard to get your installer to commission such an installation. A way around might be to use a condensate-neutralising gadget (I think Salamander do one).

That's more of a problem: it must not go uphill (otherwise when it does discharge there's no way for the pipework to empty, and the water in it will gradually dry out leaving dissolved salts behind, I guess - anyway I'm pretty sure if you check your boiler manufacturer's instruction you'll find it says no uphill runs). I suppose it could discharge via a tundish into some sort of sump which is emptied via a sump pump, as for basements prone to filling up with water, but I've never heard of such an arrangement and it would be expensive. One way would be to fit a heating-only boiler with a separate expansion vessel and PRD which could be located higher up in the CH system where it could discharge downwards and out. However that would rule out a combi (because in practice all combis are system rather than heating-only types - have a look at the article in the DIY FAQ wiki for an explanation of the different types).

In principle one could remove the PRD from a combi and run a pipe from its connection point within the boiler to a point conveniently higher up and re-fit the (or a) PRD there, but this would almost certainly not meet with the manufacturers' approval (or that of the commissioning installer).

Might be worth talking to your boiler mfr about this - I'd have thought the question must have arisen. If there's a satisfactory answer please let us know back here!

[ARWadsworth]

What if you connect the boilers PRD into the CH flow or return and use a seperate PRD higher up? I have seen it done but I do not know if it is correct

Adam

[Andrew Gabriel]

Sounds like a way to get very badly burned, if you suddenly get 30+kW worth of steam blasting into an inclosed space such as a cellar.

I don't recall anything which would have prevented doing this in the installation instructions for my boiler (although I didn't need to, and I haven't gone back to check them).

I think that to make this safe, I would say:

1 Pipework from boiler to PRV must be all metal (it's the route which carries super-heated water and steam when boiler controls fail, which is hotter than plastic pipe is designed for). 2 If the run is more than a couple of feet, increase to next pipe size so as not to add undue flow restriction to the escape route. 3 No valves or isolation devices to be fitted between boiler and PRV. Something I did as a result of previously observing the effects of steam in pipework - make sure the pipe path from boiler to PRV and to outside is clamped firmly enough to prevent soldered joins melted by the steam from blowing the joins apart inside the house (again, where the ejection of super-heated water and steam might cause injury).

[YAPH]

You get tundish arrangements on unvented HW cylinders (ptui!) so that's presumably OK. Or maybe not (ptui^2).

[Doctor Drivel]

I have seen this done. It is not the same as capping the Pressure Discharge Valve and putting one elsewhere on the system. It relieves pressure at the boiler to protects its components (where the [pressure builds up at the burner) diverting the pressure to another point on the system where it can be relieved to atmosphere.

[Doctor Drivel]

It is supposed to point back at the wall.