Boiler sizing from cubic area, or actual usage

[Kaymo]

Christ! I think I’ll miss one of those out at that price!!!

Its well worth the money, catches all the metal sludge that would otherwise continue to circulate around and block up your system.

 

[Mottie]

Still think it’s it’s a tad expensive at £100,000 - you could buy a hundred boilers for that.

 

[Mottie]

..The person who did the quotes should have sized the cubic area of your house as a minimum, and if they didn't you should probably ask them how they came to the conclusion of which boiler is needed.

Thanks. I've had no quotes as of yet. I have a British Gas 'Heating Advisor’ coming round next Tuesday. I thought I’d start with them first - I just thought they’d look at what boiler I’ve got and give me a price to replace it. I wondered why they said the appointment was a 90 minute one and I was expecting 85 minutes of hard sales! Based on what you say, I assume they will be taking measurements/making calculations and will come up with a suitable size boiler to replace mine?

Well, here’s what I can expect, apparently!

I just want a quote for a boiler change, like for like, I don’t need to know what types of heating systems there are on the market etc etc.

Okay, just for fun, how much do you reckon BG will want for (a) a 18Kw (or thereabouts) heat only boiler and (b) how long do you think the heating advisor visit will last?

 

[Kaymo]

Still think it’s it’s a tad expensive at £100,000 - you could buy a hundred boilers for that.

Ok, I get it now, I didn't notice the 'k' I accidentally added.

 

[fixitflav]

Hi D_Hailsham,
At rated heat output and high temperature regime η4 % 87,7
The spec I took this from gave the rated heat output as 24kW, i.e. the maximum heat output of that particular boiler. So, I took this % to be the efficiency when running at 24kW and a high temp.
At 30 % of rated heat output and low temperature regime η1 % 99,4
I therefore took this, to be 30% of 24kW, so when running a heat output of 7.2kW, and a lower temp (presumably at the temp where the boiler is condensing)

Is this wrong?

And, as Bernard says greater than 100% efficiency is not possible, so what does net mean in this context?

Thanks.

Gas CV can be stated in 2 ways, nett (NCV) which assumes the water vapour produced by combustion remains in the form of vapour, and gross (GCV) where the vapour is ondensed, so recovering the latent heat of vapourisation.
If the heat output of the boiler is divided by input based on GCV, it's always less than 100%. If it's divided by input based on (NCV) it can be above 100%.

 

[Kaymo]

Gas CV can be stated in 2 ways, nett (NCV) which assumes the water vapour produced by combustion remains in the form of vapour, and gross (GCV) where the vapour is ondensed, so recovering the latent heat of vapourisation.
If the heat output of the boiler is divided by input based on GCV, it's always less than 100%. If it's divided by input based on (NCV) it can be above 100%.

So, are you saying that condensing boilers can be more than 100% efficient in reality, in the sense that combustion is 100%, or nearly 100% efficient, and the latent heat is pushing the heat output over the 100% mark, so that there is more energy coming out than going in. i.e. 10kW of energy in the form of gas is supplied to the boiler, resulting in say 10.1 kW of heat output to the inside of the house?

If that is the case, why on earth would there be a value that presumes the water vapour would not be condensed, when in a condensing boiler it is clearly going to be condensed?

I had always presumed that, using the example of the WB Greenstar 30i that I am looking at, there is a figure of 88% given for heat output in high temp regime, and this was the efficiency of the boiler when run too hot for the gases to condense on the return. The figure for low temp regime of 99.7% was the figure including the recovery of latent heat.

I know that a condensing dehumidifier is effectively more than 100% efficient as a heater, because all of the electricity is being turned to heat, + by condensing you get a latent heat effect. However, with combustion, aren't you only recovering the energy that you have already used to produce the water vapour in the first place?

 

[John D v2.0]

So, are you saying that condensing boilers can be more than 100% efficient in reality, in the sense that combustion is 100%, or nearly 100% efficient, and the latent heat is pushing the heat output over the 100% mark, so that there is more energy coming out than going in. i.e. 10kW of energy in the form of gas is supplied to the boiler, resulting in say 10.1 kW of heat output to the inside of the house?

Not exactly, "energy in the form of gas" is only determined by how much heat you can get out of it by burning it. So it turns out the energy in the gas is higher than originally assumed because the latent heat was originally ignored as not recoverable.

why on earth would there be a value that presumes the water vapour would not be condensed, when in a condensing boiler it is clearly going to be condensed

Because the standard was set will before condensing boilers came into being. Also it won't necessarily be condensed, it depends on the temperature of the combustion gas leaving the heat exchanger.

However, with combustion, aren't you only recovering the energy that you have already used to produce the water vapour in the first place?

You could look at it that way.

 

[Mottie]

So, is it better to have a boiler specced so that it is running at maximum power or a spec a higher powered one running at say 75% power to get the best economy?

 

[John D v2.0]

So, is it better to have a boiler specced so that it is running at maximum power or a spec a higher powered one running at say 75% power to get the best economy?

It's better to have it running at minimum water temperature, boiler itself won't be so significant and if you over size the heat exchanger itself too much it will cycle on and off more.
Even if you did want to size your system to run at 75%, that would only apply two days a year. Heating demand varies from next to nothing up to 30 degrees difference to outside.
Better to spend the money on insulation (and maybe a weather compensating thermostat if youryo that way inclined) if you want efficiency

 

[fixitflav]

So, are you saying that condensing boilers can be more than 100% efficient in reality, in the sense that combustion is 100%, or nearly 100% efficient, and the latent heat is pushing the heat output over the 100% mark, so that there is more energy coming out than going in. i.e. 10kW of energy in the form of gas is supplied to the boiler, resulting in say 10.1 kW of heat output to the inside of the house?

Yes, if the energy in the gas is based on NCV. But the difference is more than 1%, latent heat is about 10% of the total, and most of that is recovered in a condensing boiler. Roughly in line with your figures below.

If that is the case, why on earth would there be a value that presumes the water vapour would not be condensed, when in a condensing boiler it is clearly going to be condensed?

Condensing boilers are relatively new, about the last 20-25 years, so it didn't really arise. And using NCV gives a higher quoted efficiency, so manufacturers are going to prefer that.

I had always presumed that, using the example of the WB Greenstar 30i that I am looking at, there is a figure of 88% given for heat output in high temp regime, and this was the efficiency of the boiler when run too hot for the gases to condense on the return. The figure for low temp regime of 99.7% was the figure including the recovery of latent heat.

I know that a condensing dehumidifier is effectively more than 100% efficient as a heater, because all of the electricity is being turned to heat, + by condensing you get a latent heat effect. However, with combustion, aren't you only recovering the energy that you have already used to produce the water vapour in the first place?

Yes, but it doesn't make any difference. You can't avoid the water being initially in the form of vapour. If you do two tests to measure heat produced, one with the water remaining as vapour, the other with it condensing, you still get a higher heat figure when it condenses.

Put my answers in bold to distinguish them from the rest of the text. I still can't get the hang of multi-quote, perhaps you or somebody would be good enough to explain how to do it!

 

[fixitflav]

It's better to have it running at minimum water temperature, boiler itself won't be so significant and if you over size the heat exchanger itself too much it will cycle on and off more.
Even if you did want to size your system to run at 75%, that would only apply two days a year. Heating demand varies from next to nothing up to 30 degrees difference to outside.
Better to spend the money on insulation (and maybe a weather compensating thermostat if youryo that way inclined) if you want efficiency

Yes, I doubt it makes much difference to efficiency whether it runs at 75% or 100% (but somebody might correct me). If you want high efficiency, better to oversize the rads, so you get the required heat output with lower water temperature. Then the flue gas is cooler, so more heat is recovered from all of it, not just by increasing condensation of the water vapour, specially if, as you say, you go for weather compensation.

 

[Kaymo]

Ok, I think I am there now. I had always presumed that the calorific value would only consider 'conventional' combustion, and latent heat would be considered as 'extra', which I now understand is NCV. So I guess if a boiler was able to get around 91% efficiency without condensing, then another 10% from the latent heat, then it would be 101% NCV. I presume then that as you will always lose some heat from the vent, even with the most efficient boiler, its unlikely that you would get much more than 100% ncv, maybe 105% - are there any boilers in the UK market that go above 100% ncv?

I suppose though, what is important for the consumer is, which does the gas supplier use, when they bill me for a kWh of energy is that ncv or gcv?

 

[fixitflav]

Ok, I think I am there now. I had always presumed that the calorific value would only consider 'conventional' combustion, and latent heat would be considered as 'extra', which I now understand is NCV. So I guess if a boiler was able to get around 91% efficiency without condensing, then another 10% from the latent heat, then it would be 101% NCV. I presume then that as you will always lose some heat from the vent, even with the most efficient boiler, its unlikely that you would get much more than 100% ncv, maybe 105% - are there any boilers in the UK market that go above 100% ncv?

I suppose though, what is important for the consumer is, which does the gas supplier use, when they bill me for a kWh of energy is that ncv or gcv?

I would say 105% is about maximum, most I've seen are a bit lower. Mine claims 95.8% with mean water temp 70°C, 102.5% with mean water temp 40°C (which seems unlikely) but it is 20-odd years old.

As you might expect, suppliers charge on the GCV, it varies a bit but currently 39.3MJ/m3.

 

[Kaymo]

This is interesting/enlightening.

So suppliers are charging on the GCV, but boiler manufacturers advertising the NCV value, which is potentially quite misleading when trying to compare to other forms of heating. The variety of different values being quoted is baffling for me and I have a technical background, so the average member of the public would have no chance. IMHO it seems wrong that suppliers be allowed to quote net values, if the customer is being charged gross by the suppliers.

Looking at the spec sheet for the WB 30i the quoted 99.5% efficiency looks great compared to my current 30 year old Ideal Mexico which is 73%, except that the 73% is a SAP value, which I presume is a gross value, and the 99.5% is presumably a net value. Having now looked up the SAP value for the WB 30i it turns out is only 89.8, which is not quite so impressive and doesn't look quite so attractive given the 3k boiler replacement cost. Particularly so as I understand a modern boiler life is only 10-15 years, and for a ~16% reduction in gas unit cost, rather than what I thought would be closer to 26%, an ROI looks even less likely!

 

[fixitflav]

This is interesting/enlightening.

So suppliers are charging on the GCV, but boiler manufacturers advertising the NCV value, which is potentially quite misleading when trying to compare to other forms of heating. The variety of different values being quoted is baffling for me and I have a technical background, so the average member of the public would have no chance. IMHO it seems wrong that suppliers be allowed to quote net values, if the customer is being charged gross by the suppliers.

Looking at the spec sheet for the WB 30i the quoted 99.5% efficiency looks great compared to my current 30 year old Ideal Mexico which is 73%, except that the 73% is a SAP value, which I presume is a gross value, and the 99.5% is presumably a net value. Having now looked up the SAP value for the WB 30i it turns out is only 89.8, which is not quite so impressive and doesn't look quite so attractive given the 3k boiler replacement cost. Particularly so as I understand a modern boiler life is only 10-15 years, and for a ~16% reduction in gas unit cost, rather than what I thought would be closer to 26%, an ROI looks even less likely!

Charging for gas based on GCV looks a bit underhand, but it’s supposed to be a competitive market (ha-ha) so if they used NCV they would charge ~ 10% more per kWh. But as you say, much care is needed in comparing boilers, an aspect I hadn’t thought of before.

Many years ago (1973-on for a few years) I had a New World CH501 boiler. I still have the instructions, it says input 1.82m3/h, CV 37.96MJ/m3 natural gas, giving 19kW. That’s GCV, clearly non-condensing at that vintage. Output 14.6kW giving efficiency 77%, not bad for 1973. The CV is a bit dubious, as the data is first given as 65ft3/h and 1000BTU/ft3. 1000BTU/ft3 looks a suspiciously round figure, and to be fairly precise, 1000BTU/ft3 = 37.259MJ/m3.

So your old Mexico 73% is almost certainly based on GCV.

BTW what is SAP, is that the German business systems corporation?