Heat only boiler

DUTR

Just out of interest those that have a heat only boiler, what kwh rating is it?
For some reason I thought mine was 24kwh but was surprised to see it is only a 11.5kwh it's an old Baxi wallmount 401, the property is a 3 bed semi (bay window type) .
Whilst I'm here, does anybody know if the mfrs keep the same footprint for the boilers? If ever I do change it, was considering the Baxi Solo 15 HE.
TIA for any replies

C_Mababejive

Glow worm spacesaver 50 CF circa 30 years old. Heat input about 11Kw. Does heating and hot water just fine and economically.

DUTR

C_Mababejive wrote: »

Glow worm spacesaver 50 CF circa 30 years old. Heat input about 11Kw. Does heating and hot water just fine and economically.

Thanks, I will still wait until this unit dies before replacing, I was surprised when I looked at the input/output, is that how they measure the effiency?

C_Mababejive

DUTR wrote: »

Thanks, I will still wait until this unit dies before replacing, I was surprised when I looked at the input/output, is that how they measure the effiency?

Well your potentially opening a can of worms there!

Efficiency is a word which is oft abused when talking about boilers. A lot of it is sales talk and really they shouldnt talk about efficiency unless they are more specific. The one thing i think we can all agree on is that with respect to combustion efficiency, my boiler is not as efficient as a more modern condensing appliance.

Why?

Burner technology..my burner is natural draught and operates in a high level excess air environment. Condensing boilers use a kind of forced draught burner which runs close to stoichiometric thus ensuring that the fuel is burned at peak efficiency whilst still ensuring a narrow margin of safety with regard to correct fuel /air mixture.

Heat extraction.

My boiler..natural draught>heats up small cast iron block>products of combustion rise vertically through a flue pipe and are discharged to atmosphere

New boiler...water vapour in the products of combustion surrenders its energy via condensation and much cooler products are then discharged.

DUTR

C_Mababejive wrote: »

Well your potentially opening a can of worms there!

Efficiency is a word which is oft abused when talking about boilers. A lot of it is sales talk and really they shouldnt talk about efficiency unless they are more specific. The one thing i think we can all agree on is that with respect to combustion efficiency, my boiler is not as efficient as a more modern condensing appliance.

Why?

Burner technology..my burner is natural draught and operates in a high level excess air environment. Condensing boilers use a kind of forced draught burner which runs close to stoichiometric thus ensuring that the fuel is burned at peak efficiency whilst still ensuring a narrow margin of safety with regard to correct fuel /air mixture.

Heat extraction.

My boiler..natural draught>heats up small cast iron block>products of combustion rise vertically through a flue pipe and are discharged to atmosphere

New boiler...water vapour in the products of combustion surrenders its energy via condensation and much cooler products are then discharged.

I just found this

Will an efficient new boiler save you money?

Based on a saving of 25% gas usage, the Energy Saving Trust estimates the average saving per household on energy bills after installing a new boiler to be £300. This does not take into account how much installing a new boiler will cost.
The table below illustrates just how much a modern condensing boiler could cut your gas bills by, depending on the size of your house (a boiler that's 15 years old or more is classed as 'old' in these calculations. A lightweight boiler can be attached to a wall; a heavyweight boiler is floor standing.)
Annual fuel costs with different boiler types (source: Sedbuk)Boiler typeEfficiencyFlatBungalowTerracedSemi-detachedDetachedOld gas heavy weight55%£875£1109£1150£1288£1773Old gas light weight65%£759£957£991£1108£1519New non-condensing78%£652£817£846£943£1286New condensing89%£587£731£756£842£1142

These figures are based on a gas unit price of just 4.49 pence per kW (Sept 2011), and a standing charge of £30 per quarter.


So it looks like my boiler is classed as an old lightweight, so at 18.5kwh input and 11.5kwh is near the 65%


So still at the rates shown,it would take a good 7 years+ to repay itself, shall stick as I am :cool:

rogerblack

DUTR wrote: »

I just found this

Will an efficient new boiler save you money?

Based on a saving of 25% gas usage, the Energy Saving Trust estimates the average saving per household on energy bills after installing a new boiler to be £300. This does not take into account how much installing a new boiler will cost.

25% is a truly astonishing improvement in efficiency which is extremely unlikely to occur in reality, for most people.

'Old' gas boilers - of say 15 years old - are not typically anywhere near as bad as 65% efficient.
Sure - if you've got one from 1978 in there - perhaps.

Ben84

C_Mababejive wrote: »

Burner technology..my burner is natural draught and operates in a high level excess air environment. Condensing boilers use a kind of forced draught burner which runs close to stoichiometric thus ensuring that the fuel is burned at peak efficiency whilst still ensuring a narrow margin of safety with regard to correct fuel /air mixture.

I'm a bit confused by this idea. I would have expected that supplying the burner with plenty of air would ensure complete combustion, which yields the most heat from the gas? Supplying more air than needed shouldn't make combustion less efficient. Is it perhaps that limiting the flow of air through the burner so it's only enough to fully burn the gas holds the hot gasses of combustion in the heat exchanger for as long as possible? That would make sense to me - but I might be missing something here?

DUTR

Ben84 wrote: »

I'm a bit confused by this idea. I would have expected that supplying the burner with plenty of air would ensure complete combustion, which yields the most heat from the gas? Supplying more air than needed shouldn't make combustion less efficient. Is it perhaps that limiting the flow of air through the burner so it's only enough to fully burn the gas holds the hot gasses of combustion in the heat exchanger for as long as possible? That would make sense to me - but I might be missing something here?

I think the idea is 'similar' to a car engine running lean or rich, a condenser is like a higher tuned engine, so the 'forced air' is like turbo charging, technically you should get the same heat output for less gas, well more so the gas used is fully combusted. However in practice the final figures whilst maybe improved are not the gains claimed.
Well that's how I understand it now

grahamc2003

Ben84 wrote: »

I'm a bit confused by this idea. I would have expected that supplying the burner with plenty of air would ensure complete combustion, which yields the most heat from the gas? Supplying more air than needed shouldn't make combustion less efficient. Is it perhaps that limiting the flow of air through the burner so it's only enough to fully burn the gas holds the hot gasses of combustion in the heat exchanger for as long as possible? That would make sense to me - but I might be missing something here?

As Dutr said, you really want stoichiometric combustion going on - which means exctly the amount of oxygen delivered (in the air) to fully burn all the gas. In practice, I expect the design delivers a little more air than necessary, which is safer than delivering a little less - but the efficiency drops with any excess air over stoichiometric.

With less air than stoichiometric, then there are all sorts of dangers with co/unburnt fuel in the exhaust gases/sooting and tarring of exhaust flues etc. With more air than stoichiometric, you are simply lowering the temperature of the whole system, since some air is taken in at room temperature, is heated as it passes through for no reason, then exhausted. The temperures after combustion are less, hence a lowered efficiency.

I don't know much about condensing boilers, but googling around, many seem to think the actual heat recovery via condensation within the system only happens under certain conditions - and specifically not after the burner has been lit for several minutes. My neighbour's exhaust from her condensing boiler most of the time throws out steam (which is invisible), shortly followed an inch or two later by tiny droplets of condensed water (the white stuff of clouds) - and this means at that time she's getting no latent heat recovery in her boiler.

Ben84

DUTR wrote: »

I think the idea is 'similar' to a car engine running lean or rich, a condenser is like a higher tuned engine, so the 'forced air' is like turbo charging, technically you should get the same heat output for less gas, well more so the gas used is fully combusted. However in practice the final figures whilst maybe improved are not the gains claimed.
Well that's how I understand it now

I'm certain of one thing, limiting air supply doesn't improve combustion efficiency. A gas burner supplied all the air it can take in, for example the completely open burners on a gas hob will burn with basically 100% combustion efficiency. An excess of air does nothing harmful to combustion efficiency and reducing this excess won't provide more heat for less gas. 100% is the maximum.

What it may do however is reduce the flow of cooler air from the house through the boiler, allowing the hot fumes to hang around and have more time in contact with the heat exchanger before they go up the chimney.

As for car engines, they don't burn fuel in the way boilers do. There's no flames, they just explode fuel to release the energy in a manner that can move a piston back and forth. Their physics are very different. However, a turbocharger is pretty much an air pump to force more air in to the engine so that it can burn more fuel in each explosion. It does the opposite of limiting the air supply.

Ben84

grahamc2003 wrote: »

As Dutr said, you really want stoichiometric combustion going on - which means exctly the amount of oxygen delivered (in the air) to fully burn all the gas. In practice, I expect the design delivers a little more air than necessary, which is safer than delivering a little less - but the efficiency drops with any excess air over stoichiometric.

With less air than stoichiometric, then there are all sorts of dangers with co/unburnt fuel in the exhaust gases/sooting and tarring of exhaust flues etc. With more air than stoichiometric, you are simply lowering the temperature of the whole system, since some air is taken in at room temperature, is heated as it passes through for no reason, then exhausted. The temperures after combustion are less, hence a lowered efficiency.

I don't know much about condensing boilers, but googling around, many seem to think the actual heat recovery via condensation within the system only happens under certain conditions - and specifically not after the burner has been lit for several minutes. My neighbour's exhaust from her condensing boiler most of the time throws out steam (which is invisible), shortly followed an inch or two later by tiny droplets of condensed water (the white stuff of clouds) - and this means at that time she's getting no latent heat recovery in her boiler.

I'm not actually sure just how much energy the latent heat adds up to? The benefits of condensing may be exaggerated if it turns out to not be that much.

As for how often it condenses, your neighbor may have undersized radiators or the boiler thermostat too high. Condensing boilers need the return water to be below a certain temperature to keep the heat exchanger cool enough for the steam to condense on it. Still, the fact that any condensation is noticeable in the fumes is a good sign as it shows the fumes are leaving the boiler fairly cool. My boiler only produces noticeable steam from the chimney in very cold weather. The bigger heat exchangers in condensing boilers are still helpful, even if they're not condensing.