Can you estimate old boiler efficiency from figures in the installation manual?

[JonathanM]

I have a 32 year old floor standing Baxi.

I have found the installation manual online.

It gives two figures, one is for "Maximum Input" (30 KW), and the other is for "Maximum Heat Output" (23.4 KW). Dividing the output by the input gives 78%.

Is this the efficiency of my boiler? If not, is there a way to estimate the efficiency from these figures? I have read, in another post, that you sometimes need to knock off 4% for the permanent pilot light.

 

[ericmark]

Basic answer must be no, as we exceed 100% efficiency with heat pumps. So using a sterling engine on a gas supply we can exceed 100%.

But in real terms we are looking at the amount of fuel used to maintain the home as a comfortable temperature and much depends on how we transfer the energy into the compartment requiring heating.

So this boiler seems to be wasting heat, but if it was not heating the carriages then the heat would simply be released into the atmosphere, so heating the carriages while at the station is not wasting a single Joule of heat.

There are 360 joules in a watt/hour, I have never understood why we use kWh, very misleading as it has nothing to do with time.

But what wastes the most energy is hysteresis, where the temperature is not kept stable, we want the room at for example 20°C not 18 to 22°C and ensuring the temperature is as wanted is far more important than the losses due to boiler efficiency.

The radiator has two methods to control the transfer heat into the room, one is control air flow, other is the radiator temperature so either we have a thermostatically controlled fan, or a thermostatic radiator valve (TRV) the fan is faster in response time to the TRV, so the fan in general works better, however the fan does not control the return temperature of the water in the same way as the TRV does, so with the fan we really want radiators in series, and with the TRV in parallel and in the main homes have the radiators in parallel so easier to use TRV heads to control the boiler output.

However there is a trade off between speed and installation cost, and nearly every central heating system is a compromise, the ideal system is rare, so banging in a condensing boiler which actually never boils anyway, with a system using fan assisted radiators may be less rather than more efficient.

What we need to look at is the system as a whole, not just one small part. And to only heat rooms when required to only the amount required even with a 50% efficient boiler is likely to use less energy than heating the rooms be they used or not to 10% hotter than required 24/7 even with a 120% efficient boiler (which does not boil anything so is not really a boiler to start with, and simply transfers heat from outside to inside).

 

[vulcancontinental]

I cant remember the old SEDBUK figures but depending on the system 60-70%. You can improve on the efficiency but no on the reliability.

 

[Roddy Young]

Watt is a unit of power.
747,000 Watts (747kW) is one Horse Power.
To write "watt/hour" is like writing "Horse Power per hour".
That has no meaning.

1Kw of power used over 1 hour will take 1kWh of energy.

 

[Madrab]

What Baxi is it? If it's one of the old Bostons then it's in @ around ~55%, the Bermuda's/solo's came in at around ~65%

 

[vulcancontinental]

Interesting point about sometimes knocking off 4% because there is a pilot flame. 4% of 30 is 1.2kW, that’s quite a pilot light, I recall them being more like 200W.

Your boiler will not freeze in cold weather, it does not need a frost stat which becomes useless in the event of a power cut.

Heat emitted by the pilot flame is absorbed into the cast iron exchanger, the loss of efficiency is only the heat contained in the passage of air through the exchanger when the appliance is idle so temperature difference and flue flow through a balanced not powered flue is very low.

Ah, the good old days.

 

[fixitflav]

Watt is a unit of power.
747,000 Watts (747kW) is one Horse Power.
To write "watt/hour" is like writing "Horse Power per hour".
That has no meaning.

1Kw of power used over 1 hour will take 1kWh of energy.

Your units are somewhat out there.

747 watt (0.747 kW) is one Horse Power. Usual figure is 746 watt, to be a little pedantic.

 

[JonathanM]

Interesting point about sometimes knocking off 4% because there is a pilot flame. 4% of 30 is 1.2kW, that’s quite a pilot light, I recall them being more like 200W.

Your boiler will not freeze in cold weather, it does not need a frost stat which becomes useless in the event of a power cut.

Heat emitted by the pilot flame is absorbed into the cast iron exchanger, the loss of efficiency is only the heat contained in the passage of air through the exchanger when the appliance is idle so temperature difference and flue flow through a balanced not powered flue is very low.

Ah, the good old days.

The pilot light takes 4.21 KW almost every day, with a tiny variation some days. I think that makes your figure pretty close. I suppose the reasoning for the 4% is that, averaged out over the year, the boiler will be running at a lot less than 30 KW. I think that makes sense?

 

[JonathanM]

What Baxi is it? If it's one of the old Bostons then it's in @ around ~55%, the Bermuda's/solo's came in at around ~65%

Are those SEDBUK figures? I've become really interested in the history of efficiency, but can only find very limited information. Were boilers tested for efficiency before SEDBUK?

It's an FS 801 RS. It has been given the default SEDBUK rating of 65%, presumably because it has never been through the prescribed SAP test. But it looks like more than a thousand other boilers have been given that same 65% rating. I did the same calculation in my OP on another Baxi, and its Output/Input was only 71%, compared with my 78%, but it still has the same 65% default rating.

 

[Roddy Young]

Your units are somewhat out there.

747 watt (0.747 kW) is one Horse Power. Usual figure is 746 watt, to be a little pedantic.

Thanks for correcting me - I was still half asleep!

 

[fixitflav]

Thanks for correcting me - I was still half asleep!

No problem

 

[Madrab]

Are those SEDBUK figures? I've become really interested in the history of efficiency, but can only find very limited information. Were boilers tested for efficiency before SEDBUK?

SEDBUK ran from 99 - 2015, then it was replaced by ErP, most of these would be covered by the SAP standard. Prior to that then it's was basically the manufacturer's rating.

I use this site as a basic reference when needed

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