Gas Boiler Efficiency and Hot Returns

[Andy Champ]

Because I've got old radiators, the return to my boiler when the heating is running is pretty high. Which means that condensing boilers, which like a return temperature below 55, wouldn't be running at peak. In fact most of the time probably won't be condensing at all!

But it occurs to me that there's a nice cold fluid floating about in the system - the incoming air. If the exhaust was used to preheat the incoming air, the exhaust would be cooler and the pre-burn mixture hotter - both Good Things. Power stations have been doing this for decades.

Anyone know of a domestic boiler manufacturer that uses this trick? Of course, it would probably be a convenient side effect of a good long balanced flue...

Andy

[YAPH]

But it would still be more efficient than a standard-efficiency boiler.

Alpha and Baxi have adopted the z-something (forget the name) flue gas heat exchanger that pre-heats DHW or a thermal store. But the value of the return on money & materials is debatable.

[dmc]

Alpha call theirs the "gassaver" IIRC. We have one fitted and I was impressed at just how much the incoming cold is preheated. Of course, it does nothing to help when the heating is running though - just the hot water.

Seems these things sell for about 700 quid which seems ridiculous - no way can it be worth anything like that. 700 quid gets you a lot of gas as well...

Darren

[YAPH]

wot I said :-)

[Andy Champ]

I'm inclined to the same view on swapping the boiler for a condenser.

How much better *would* a condenser be, given too hot returns? Certainly nowhere near the "book" figure.

The hot water isn't the issue - the tank's less than a year old, and it's quite noticeable that the boiler no longer cycles when heating the water. And the summer bills tend to point to heating being the lion's share of the fuel.

Andy

[YAPH]

Indeed: unless you're heating a draughty, poorly insulated mansion which you can't improve the efficiency of it's not worth while throwing out a working standard-efficiency boiler and replacing it with a condensing one. Not even for a 65% SEDBUK dinosaur.

If you have to replace the boiler anyway, for other reasons (e.g the dinosaur is falling apart with corrosion) then the energy saving is a nice bonus. However to then spend the price of a second boiler on a gizmo to improve the fuel efficiency by a marginal amount makes singularly little sense whatever the circumstances.

[Doctor Drivel]

The Zenix is one of the few eco products available that does have short return.

[Doctor Drivel]

When using a thermal store/heat bank, it can heat the stored water in the cylinder at all times the burner is on. The stored water may be used for DHW and CH. Well worth doing.

[Doctor Drivel]

What figures do you have on this?

You need to do some figures. Going from 60% to 92% saves a hell of a lot of expensive gas.

[Doctor Drivel]

A good condenser lowers the burner to achieve a low return temperature. 90% of run time a boiler is on part load - full output not required.

Some boilers may have long concentric flues - efficiency is lifted when these are used.

[YAPH]

65% and 90% SEDBUK efficiencies, circa £2K for a boiler replacement and take your choice of interest rates and gas prices averaged over the next few years.. The rest is O-level maths.

Where do you get 60% and 92%?

But in any case you've got to compare your 'hell of a lot' with the other hell of a lot of expensive boiler replacement.

[Doctor Drivel]

approx £1,500 per ann. gas bill on 60% efficiency. That saves approx £500 a year. £2K to replace a boiler to 92% efficiency and the payback is 4 years - on current gas prices. As the gas prices go up over the next 3 years the payback shortens. Sounds good to me and you have a new boiler with maybe a 3 to 5 years guarantee. After the payback period you have a cheaper to run boiler, a selling point if selling the house, and maybe lots of space released.

The Zenix heat recovery top box is one of the few eco additions that really does do what they say. Solar panels can have a payback of 10 years, while these top boxes can take 3, especially if coupled to a thermal store when heat is recovered when the burner is on at all times, not just when DHW is called.

[Doctor Drivel]

Ravenheat use it too.

[RobertL]

You could try altering the way the radiators are fed. Rads usually seem to be connected across the bottom with internal conection used to spread the heat within the radiator. AIUI if you feed in at the top and out at the bottom (diagonal opposite) you will get a lower return temperature for the same heat output (but lower flow rate).

Robert

[Andy Champ]

When the heating is running they're hot all over.

When it starts up they do seem to heat top down. I don't know whether this is internal piping or just convection.

Andy

[Doctor Drivel]

You could try altering the way the radiators are fed. Rads usually seem to be connected across the bottom with internal conection used to spread the heat within the radiator. AIUI if you feed in at the top and out at the bottom (diagonal opposite) you will get a lower return temperature for the same heat output (but lower flow rate).

[Andrew Gabriel]

It won't make any difference. When bottom fed, the hot rises to the top in the first couple of channels, and then it's like a top fed radiator. It's important the return is from the bottom, but the flow can be either.

[Tim S]

Andrew Gabriel coughed up some electrons that declared:

I wonder - how much difference would it actually make if the flow was at the bottom and the return from the diagonally opposite top?[1] If you have drops from ceiling to all your rads, as I will have, it would help clear the air from the system.

I suppose it partly comes down to how fast is the water flow through the rad - if it's fast enough to beat convection, perhaps it won't make much difference...

Stupid thought for the day ;->

Cheers

Tim

[RobertL]

I take your point, but I suspect that if flow and return are both at the bottom there will be a significant flow of hot straight across the bottom of the rad which will not be there if it is diagonally fed. I do agree completely that return from the bottom is essential.

That would not be good at all. The hot would convect to the top and you'd draw it striaght off up the return pipe. Much of the rad would remain cold. Robert

[Bob Mannix]

-I take your point, but I suspect that if flow and return are both at

-the bottom there will be a significant flow of hot straight across the

-bottom of the rad which will not be there if it is diagonally fed. I

-do agree completely that return from the bottom is essential.

Mine are all bottom in and bottom out and there is no significant flow straight across - you can feel it as it heats up, unless, perhaps, the pump speed is set too high. The flow does exactly as AG describes. Top in, bottom out is worse as there is a dead area at the bottom end below the flow connection.

I have had both and bottom in bottom out is better for heat and better for less sludge collection.