Air brick heat exchanger

[Theo Markettos]

A non-sealed-flue[1] gas boiler means there needs to ventilation in the room, which is provided by air brick(s) (on the opposite wall to the gas voiler). This tends to produce a howling gale of cold air in winter, especially when the wind is blowing against the wall. Is there any kind of heat exchanger that would attempt to recover some of this heat without impeding air flow too much?

I'm guessing it wouldn't work without forced airflow (which is possible, but complicates matters)? Or is there some kind of passive (eg convection) solution?

What sort of airflow is required for safe combustion in the average gas boiler?

Thanks Theo

[1] I don't know the exact term, but it's the type of gas appliance which operates like an open fire, in that the oxygen comes from the room, rather than outside, but the combustion gases go up the flue.

[YAPH]

No.

Not possible. Where are you going to recover the heat from? The cold air is coming in to replace air being burned in the appliance and going up the flue as Carbon Dioxide and Water (as steam). There isn't a corresponding flow of warm air out of the room, as there is with mechanical heat recovery ventilation units.

The sensible energy-efficient approach is to replace the inefficient and potentially-unsafe open-flued boiler with a high-efficiency room-sealed one.

Approx 5cm^2 per kW of heat input to the appliance, less 7kW of the appliance's rating (a certain amount of ventilation is assumed to be available "adventitiously", i.e. from natural draughts). So for a 14kW appliance you need 7 * 5 = 35cm^2.

Heating-only boilers (which heat hot water in a cylinder, rather than instantly as a combi does) tend to be rated 10-15kW.

Open flue or conventional flue.

[fred]

Often abbreviated to CF when looking for details on boiler models with multiple flue options

I wonder if there isn't some wobble room here to provide a ventilated cabinet for the boiler to separate it from the room?

I am thinking of ducted ventilation to the outside or ventilation from underfloor (although I believe that ventilation (area) requirements are higher from such sources). Likely extras might be high level ventilation requirements (to the room) from a cabinet to release boiler heat, maybe low level too but arguably that is being provided by the external source.

All this dependant on current regs and the manufacturer's requirements in the installation instructions and of course the o/ps ability to accommodate such a construction without it looking like a dog's dinner.

[Rick Hughes]

Recover what ? .... as you mentioned the air coming in is cold ... you are not losing heat out through the air brick .... now you are losing heat elsewhere or no air would be drawn in.

If it is as bad as you say ... change to a room sealed boiler, a small balanced flue model may solve your problem.

[Dave Liquorice]

Quite, you can get "draught free" free flow ventilators though. Try a google on "black hole ventilator" (other makes and models are available).

[Harry Bloomfield]

YAPH was thinking very hard :

There is, but it goes up the chimney or flue.

One old solution, was to have an under-floor air vent where the air supply came in close to the point where it was to be burned - rather than at the back of the room, where the cold air would travel past the room's occupants.

[Chris J Dixon]

Indeed so. My parents used to have a pair of gray plastic "Tele-vents" in the floor either side of a coal fire. When pulled up, they allowed a gale to blow through, rather than howl across the room from under the door.

Chris

[Theo Markettos]

Ah, good point.

This isn't practical in this situation, much though it's obviously the best option.

The main thing is to reduce draughts while not interfering with combustion. Does such an arrangement draw in cold air by convection? So would, for example, a louvred path be OK because the combustion causes lower pressure in the room that would suck in fresh air? Or would the boiler happily churn out CO before it causes a significant pressure difference?

I'm a bit puzzled by the units. That means a grille 7cm x 5cm, presumably? But what's the amount of air input you need to the boiler (ie in cm^3 per second)?

Theo

[YAPH]

It will be necessary in due course when the existing unit packs up.

The important criterion is that the air vent be unobstructed by e.g. fly grilles. Commercially-available vents for the inside wall generally have louvres, but that won't do much to help if there's a stiff breeze blowing. A properly installed appliance should have its air supply arranged so as

*not* to produce an unpleasant draught across living areas, e.g. by being ducted under the floor to come up beside the appliance (precisely because it may encourage occupants to block it up) but this is only a recommendation rather than a requirement of the British Standard and more honoured in the breach.

The free area of ventilation is what's specified in the gas installation regs. The volume rate is something you (or someone else with a better recollection of school physics & chemistry than me :-)) can work out.