Is there a boiler that preheats the incoming air?

[Andy Champ]

It occurs to me that the current condensing boilers rely on the low temperature of the returned water flow to get their efficiency. Those of us with old fashioned systems designed to run non-condensing have a high return temp., so the boilers won't condense. But there's a nice supply of cold air available to cool the exhaust further - which is the intake. An intake-exhaust heat exchanger would surely recover at least some of the waste heat in the exhaust. Not all of it, firstly because a heat exchanger, even counter current, is never completely effective, and second because the heat capacity of warm steamy air that's condensing is far higher than that of dry, cool air - but it's worth a go, isn't it?

so waffle mode off, does anyone make such a beast? And is it any good?

Andy

[The Natural Philosopher]

THhey are called balanced flue boilers. I am sure there are one or two out there ;-)

[Tony Bryer]

[JimK]

what's the advantage of preheating combustion air? I thought colder combustion air = more dense= more O2= better combustion? or do my elderly A levels let me down somewhere?

JimK

[Andy Champ]

I was not aware that a standard balanced flue boiler had a heat exchanger in the inlet. I know mine doesn't.

Andy

[Andy Champ]

Vague memory of this being mentioned before. And the price-performance was called into question. It's a preheater for the incoming cold water supply though, rather than the incoming cold air.

Andy

[Andy Champ]

You A levels don't let you down, but cold inlet air is a solution to a different problem.

If you have an internal combustion engine, especially one with a turbo- or supercharger, hot inlet air reduces the mass of air available for combustion. So the bang is smaller. Cooling the air wastes energy - but the extra power is generally desirable (if you are a car salesman!).

It won't help us with a boiler, where 10% less output is probably not an issue, but 10% less efficiency matters.

Andy

[The Natural Philosopher]

A balanced flue IS a primitive heat exchanger..;-)

[Tony Bryer]

Nothing like 10% I think - gas delivers 10.8kW/m3. Specific heat of air is

1.25kJ/m3/K = 0.004kW/m3/K, so raising the air temp by 40C saves 0.016kW/m3. This needs to be multiplied by the air/gas ratio. 40 years since I did O-level physics so these numbers probably wrong!

[Doctor Drivel]

It's not its a flue.

[Doctor Drivel]

>>> Vague memory of this being mentioned before. And the price-performance

It will heat incoming air. This box is used for combis, so not applicable for CH. It can also be used to assist in heating a heat bank/thermal store while the boiler operates, using a pump, and increasing efficiency.

They are one of the few eco devices that have a short payback time, along with rainwater recovery.

To warm incoming air you need long flue lengths in concentric flue pipes.

[Doctor Drivel]

What you mean is there a way to use the waste heat from the exhaust of the boiler.

[The Natural Philosopher]

efficiency of a heat engine is pretty much down to the ratio of combustion temperature to exhaust temperature.

That's the rationale behind the condensing boiler.

It doesn't really matter what you cool the exhaust with, as long as its heat that ends up inside the house rather than outside.

I.e. you could pass exhaust gases through a hot air heat exchanger, and us it to preheat ventilation air.

As I said, a balanced flue does act as a rather small heat exchanger, and could be improved with, say, fins..

But my guess is that the gains are very small, or people would have gone that route instead of condensing.

[Tony Bryer]

Surely the rationale of a condensing boiler is that the amount of heat released when the water in flue gases condenses is vastly greater than any amount of heat gained by just cooling them a bit.

[The Natural Philosopher]

I am not sure there is enough water IN the flue gases to make that a dominant factor.

Its more an issue with steam turbines were the steamm is ALL water.

If you think about a typical gas like methane, CH4, then two oxygen atoms make the CO2 molecule, and two make the two H2O molecules. The longer chain hydrocarbons (butane, propane, pentane, heptane etc) make even less water per unit CO2.

AND remember, we are not feeding them a pure oxygen stream anyway, its mainly nitrogen.

I would say, without actually researching it, the bulk of the heat is in the hot CO2 and nitrogen.

Not the water vapour.

[D.M.Chapman]

It will?.. Can't really see how it's much better than a normal flue.

Really??

Alpha claim that it'll save *up to* 52% of the gas used to *heat water* based on 100litres of hot water use a day.

It does nothing to help with heating load - (appart from collecting heat that then is used to help for hot water I suppose assuming hw demand).

Given they seem to be around 700 quid+ VAT for the Gas saver I'm not convinced on the payback...

suggests 170 quid saving a year. I'd imagine that's a best case scenario that most people would never see.

Makes a bit more sense if you can get it in the flowsmart pack at a good price - but paying full price for it will take a long time to payback.

Given the price of flue pipes I'm not all that convinced it'd be a great saving...

Darren

[Doctor Drivel]

Yes really, if on a thermal store and the sums are doen properly.

It can be used heating a thermal store or dedicated heat buffer, to be used when the burner is on, not just for DHW. Whe used this way it gives a good payback. The bigger the house and more gas used the greater the payback. Using one in a one bedroomed flat may never recoup the capital costs. Used on medium to larger homes using thermal storeage they have a decent payback and worth buying.

The saving was not the point. Long flue pipes will warm incoming air.

[Andy Dingley]

Only if you ignore phase changes (and pressure changes too). Assuming that temperature is the _only_ significant factor will mislead you.

Admittedly it is the case for a condensing boiler where you draw the system boundary in the right place, but it won't help you to understand how it's working internally.

[Doctor Drivel]

There is.

[Andy Dingley]

Not since second-generation condensing boilers - they extract all the sensible heat they can at the condenser, then try to avoid cooling the flue gas any further, so as to avoid visible pluming. AIUI, there are even some using double-wall construction to reduce heat transfer in the flue.