Keeping house warm

[Andy Champ]

Fascinating. So to get the savings from a condensing boiler, I'll have to change all my rads too. I think I'll keep postponing this move!

Andy

[Andy Hall]

That's not completely true.

With a condensing boiler, the efficiency improves with reducing temperature even above the 54 degree dew point. Below this temperature the *rate* of improvement increases - i.e. there is a knee in the line.

Keep in mind that for much of the year, the amount of heat required is not the same as the peak winter requirement, so the radiator output and boiler temperature can be lower than the peak value anyway. Thus a radiator system sized for conventional 82/70 operation will be able to support reduced temperature running for a lot of the time anyway.

Having said all of that, it is true that if you want to get *all* of the possible savings, the boiler should be run at as low a temperature as possible and to achieve that means designing for as low an operating temperature as makes sense. Normally, it is done for 70 flow and 50 return.

I took a look at this when I sis a boiler change a few years ago. I calculated the heat lossed for the rooms and hence the radiator output required. I knew the outputs of the radiators from the sizes and worked on the basis of 70/50 operation. There were three outcomes:

- In two thirds of the cases, the radiators were sufficiently oversized in the first place that no change was needed

- I was able to take some radiators from larger rooms and use them in smaller rooms

- This left three that were undersized. Changing these for double panel versions of the single panel ones that had been there resolved the rest.

YMMV, but not as much work or cost was involved as was initially anticipated. The boiler seldom runs at a flow temperature exceeding

55 degrees except when it's very cold outside. Today, for example, with 3 degrees outside, it's running with 45 degrees flow and around 27 return and a burn rate of about 5.5kW.

[Andy Wade]

Quite right - as often discussed here in the past, before condensing boilers became mandatory.

By way of illustration I've just uploaded a scan of a plot from CIBSE AM3, showing typical efficiencies v. return water temperature:

[<me9]

To get the maximum savin in extreme weather yes, but most of the time (this week excepted) a condensing boiler will give the efficiency improvement. Radiators are often oversized anyway, due to insulation improvements over the years.

[fred]

In article , John Rumm writes

Huh, Ethane is C2H6 so more carbon than Methane at CH4, are you having a moment :-?

I wonder if short term variations effect the heat output at all as the boiler would need to adjust the air mix to benefit from a higher calorific value, I suspect more 'C' will just result in more CO unless the mix is changed (which the boiler can't do on it's own unless is was designed some kind of closed loop emissions control)

[fred]

Conventional Boilers are now? I ask as I was pleasantly surprised after slagging of someone's choice of a non-condensing boiler that the quoted efficiency was much higher than I expected, I think it was a modern non-condensing Worcester Bosch but can't remember the details.

[John Rumm]

Err. yes looks that way ;-) scrap that theory....

I think boilers tend to run slightly more air than is required for a perfect stoichiometric combustion, for just this reason. You get slightly less than optimal heat out, but with no risk of sooting, and CO production should the mixture get richer as a result in changes of composition of the fuel (or presumably its density).

[AJH]

Yes this excess air makes sure there's enough oxygen present to guarantee completion of combustion to an acceptable level of pollution from products of incomplete combustion. These will be limited to CO and soot from a natural gas fire but a cocktail of chemicals from a wood fire. Excess air is the principle reason that a wood fire will have a lower performance than even a convention gas fire before you start accounting for its inherent lower calorific value and flame temperature. Even on 500kW(t) wood boilers we're often running around

100% excess air (10% O2 in flue gases) and I'll bet a similar sized gas system will be nearer 5% excess.

You still get the same heat out, it's just that it's distributed over a larger massflow and hence a lower temperature.

AJH

[John Rumm]

Yup, I was not intending to suggest you got less heat from the reaction, just that you will be mixing it with more (cooler) air...

[Roger]

The message from Andy Wade contains these words:

decreases all the way down to room temperature. (Wouldn't that require infinitely large radiators?)

Looking at the graph it seems the difference between condensing and conventional boilers above the dew point is largely down to other factors and with the return temperature at 50C the benefit from recovered latent heat is in the range zero to 3%.

[Andy Hall]

There is some degree of truth in that. According to the SEDBUK database, there are a few current model non-condensing boilers achieving 80-82%. Pretty much all of the condensing models are managing 90-91%.

However, it's rather academic since unless there are specific reasons as to why not (fairly tight rules), there is a Building Regulations requirement to use a boiler of 86% or greater. In effect, that rules out non-condensing boilers in most cases.

[AJH]

It's all interesting to me, especially as we have had a few threads about woodburning devices, which is why I commented. I was referring to the actual combustion, of course, given a similar flue gas rejection temperature from the heat exchanger the net effect of excess air is that there is less heat delivered to the room.

I'd guess there's a parallel with petrol (and diesel) engines here in that the highest power is obtained at a point around stoichiometric which produces more pollutants than are acceptable in the absence of a catalyst.

AJH

[Andy Hall]

There are a bunch of factors.

- Generally condensing boilers (certainly nowadays) have a better quality and larger heat exchanger than conventional boilers and are able to work with a higher temperature drop across them.

- Fans are universal, so convection out of the flue is much reduced as compared with older non-fan conventional boilers.

- Modulation of burners reduces operating temperatures and heat loss due to exhaust gas temperature from the flue. There is a lower limit for that for conventional boilers because condensing has to be avoided to prevent corrosion. A conventional boiler should not be operated at a return temperature of 50 degrees. Condensing boilers don't have a lower limit in this sense.

All of this is before the recovery of latent heat is added.

I find with my system that in the spring and autumn, the return temperature is only a few degrees over room temperature anyway. One could go for larger radiators. Mine are designed around 50 degrees return for -3 outside, but it's very rare for even the flow temperature to get to this.

[AJH]

That's what it looks like to me too, i.e. the major difference is in the extra sensible heat in taken from the flue gas by reducing the rejection temperature from, say, 160C to

[Andy Hall]

That is true as far as it goes, but keep in mind that a conventional boiler can't be operated down to 50 degree return temperatures on a continuous basis without condensing which will corrode the internal components.

Therefore, in practical terms one has to look at what a conventional boiler does at its lowest safe return temperature (say 60-65) and compare that with the lowest return temperature for a condensing model

- say 30-35.

Under these circumstances there is about a difference of 10 points on the graph which accounts for about a 12% improvement.

[Andy Wade]

Perhaps if you used one of the CHP boilers (Whispergen, etc.) and used its electrical output to run a heat pump, then you could run the boiler return at below room temperature...

I thought people might find that interesting. 1989 data, yes, but all I have available. The AM3 book is out of print now of course and CIBSE don't seem to have done an updated version. Its raison d'être was to explain and promote condensing boilers at a time when many designers, let alone installers, were still sceptical. It also highlights the good match that exists between weather compensation control (i.e. controlling the flow (or mean) water temperature as a function of the /outside/ temperature) and condensing boilers. I think Andy H's Man Micromat has this feature built-in.

[AJH]

Yes and presumably it's limiting the size of the heat exchanger that guarantees dumping the flue gas at a temperature high enough so that the flue gas never cools to saturation of water vapour.

This may be so if we were comparing non condensing with condensing boilers but we were talking about where the extra heat was coming from in a condensing boiler at a given temperature. As far as I can see it's high school physics, given the same combustion temperature (I'm guessing 1500C for an adiabatic flame temperature of 2000C) entering the heat exchanger and either a flue gas rejection temperature of 200C or 55C it looks like the lower temperature rejects 3.44% of the LHV and the higher 12.5%, giving the theoretical advantage of about 9% of the LHV before condensing starts.

Now I take it from what you say that 55C, at normal pressure, is the point at which the water vapour arising from combustion becomes saturated in the other products of combustion (CO2, O2 and N2, the latter two just enjoying a free ride through the system rather than being true combustion products). So after that the effects of condensing crack in, to contribute some of the difference between HHV and LHV (which I think is 10.7% of LHV for natural gas), below 55C, the contribution to the heating system is the latent heat of the water from combustion that condenses. Not all the water can condense as the flue gas retains the ability to be saturated with water vapour but the amount it can retain lowers as it is cooled further to the practical limit given as 20C. I suppose a large under floor system could return this temperature and still maintain 18C in the living space? In practice I thought I saw return temperatures of ~28C in the only under floor system I have dealt with.

Which is impressive but isn't it a 12% improvement over an 80% efficient usage of the LHV?

AJH

[Roger]

The message from Andy Wade contains these words:

Doesn't it also need a built-in mortgage? :-)

Seriously though ISTR that that boiler is very expensive. I would dearly love such a piece of kit but I doubt very much that such equipment can be DIY installed. But that just might be true for any condenser if they need flue gas analysis to set up in which case I must eventually bite the bullet and get a man in. My current boiler (Potterton Prima circa

1991) worked straight out of the box but that will not last for ever.

[Andy Hall]

Yes it does.

Nah :-) I don't have a mortgage.

I suppose that that depends on your definition of expensive. A good quality large volume boiler such as a Vaillant, which does not have the same degree of control costs around £1000. Those with comprehensive controls such as MAN and Viessmann are listing for around £1600 but can be obtained discounted for around £1200

They can be, but require a flue gas analyser. Those can apparently be rented. AFAIK, for the higher end products, the days of using a manometer and tweaking a screw have gone. The procedure with the analyser is to adjust the gas rate at two points while measuring the flue gas emissions.

[Andy Champ]

It seems to me that the efficiency will keep rising until the exhaust temperature matches the intake temperature - which is outside air. So a lot lower than 20 degrees.

ISTR big power stations use the exhaust gases to preheat the intake air as part of their drive for efficiency. Do any domestic boilers do this? After all, the exhaust is still pretty warm - >50 - and that's energy going to heat the garden!

Andy

p.s. I'm still not going to buy a thousand pound boiler just to save 15% on my gas bills. The payback time is too long.