Can I work out my boiler efficiency with this method?

MattHobbes

With the rises in gas prices I’m interested to work out how efficient my boiler is, it’s a maybe 20-30 year old conventional vented gas boiler, it is working fine and I’ve been told in the past if it’s not broke don’t fix it, but that was before all the price rises and those to come.

I was thinking if I know the size of my hot water tank I could measure how much gas is used by reading the meter to heat it from cold to hot if I measure the temperature before and after of the water.

But I’m not sure what formula I would need to plug these values into, is anyone aware of any site that could do this calculation for me?

Then I could decide if getting a new boiler with 90-odd% efficiency I could see how long it would pay for itself.

Verdigris

Yes, you could use that method but there will be quite large margins of error. Try the numbers with 70% efficiency to see if it warrants further investigation.

BUFF

if you have the model name & no. you can probably find what it was rated at when new (efficiency will probably have dropped over time). Also whether balanced flue or fan-flued will have made a difference to efficiency.

[Deleted User]

Boiler efficiency is not linear. Modern boilers are more efficient because they recover heat. The amount of heat recovered depends on the temperature of the return flow to the boiler. Anything below 56C and the boiler will condense. Your present boiler will be operating in non-condensing mode.

The other challenge that you face is that most boilers go to their maximum flow temperature when re-heating a hot water cylinder. That said, depending on the length of the pipe run from the boiler to the cylinder some heat will be extracted from the cylinder whilst the boiler flow temperature rises from room temperature upwards. You will also lose heat in the pipe run unless it is fully lagged.

You will save money by replacing your boiler provided your existing radiators are large enough (and internally clean enough) to cope with a lower boiler flow temperature. I am running my system boiler at 65C (combis can be set as low as is tolerable) which gives us adequate heating whilst protecting the HW cylinder from Legionnaire’s disease. A flow temperature of 65C should give a return flow temperature of 45C (113F) provided that the radiators etc are properly balanced. In a stable rate, I regularly see boiler flow temperatures down as low as 56C.

JohnSwift10

When I replaced my warm air heating and separate hot water heating with a combi and radiators I immediately saved 8 kWh a day because of the pilot lights and my yearly usage went down from 18,000 - 24,000 kWh to a max of 14,500 kWh in the past 5 years.

coffeehound

MattHobbes said:

I was thinking if I know the size of my hot water tank I could measure how much gas is used by reading the meter to heat it from cold to hot if I measure the temperature before and after of the water.

But I’m not sure what formula I would need to plug these values into, is anyone aware of any site that could do this calculation for me?

The formula is Pt = (4.2 × L × T ) ÷ 3600. Pt is the power used to heat the water, in kWh. L is the number of liters of water that is being heated and T is the difference in temperature from what you started with, listed in degrees Celsius.

https://sciencing.com/calculate-time-heat-water-8028611.html

Bear in mind that even a condensing boiler won't always be working at peak efficiency when heating a hot water tank as the temperature in it increases.  (For balance, a combi is never working at condensing efficiency for hot water.)

MattHobbes

BUFF said:

if you have the model name & no. you can probably find what it was rated at when new (efficiency will probably have dropped over time). Also whether balanced flue or fan-flued will have made a difference to efficiency.

thanks, I think its one of these: www. freeboilermanuals. com/assets/pdf/ideal/W2000-RS-30N-40N-50N-60N-IS-1990-2.pdf of the RS60N variety.

Looking at the performance table assuming I'm running at mid output it was 75% efficient when new, is there a ballpark amount of efficiency a boiler will loose over time?

MattHobbes

JohnSwift10 said:

When I replaced my warm air heating and separate hot water heating with a combi and radiators I immediately saved 8 kWh a day because of the pilot lights and my yearly usage went down from 18,000 - 24,000 kWh to a max of 14,500 kWh in the past 5 years.

I'd often wondered how much the pilot is using, and now just remembered my smart meter is sending 30min readings to my supplier so I can see what it costs when the boiler is off. Looks like its costing me £65/y for the pilot at the current price cap... interesting.

Verdigris

Looking at the performance table assuming I'm running at mid output it was 75% efficient when new, is there a ballpark amount of efficiency a boiler will loose over time?

Looks like my 70% was spot on, allowing for decline in eficiency.

Chris_English

MattHobbes said:

JohnSwift10 said:

When I replaced my warm air heating and separate hot water heating with a combi and radiators I immediately saved 8 kWh a day because of the pilot lights and my yearly usage went down from 18,000 - 24,000 kWh to a max of 14,500 kWh in the past 5 years.

I'd often wondered how much the pilot is using, and now just remembered my smart meter is sending 30min readings to my supplier so I can see what it costs when the boiler is off. Looks like its costing me £65/y for the pilot at the current price cap... interesting.

Remember, though, that that pilot light is heating your house, so in the winter, when the heating is running anyway it’s really not costing you anything extra at all.

Verdigris

Remember, though, that that pilot light is heating your house,

With a conventional flue I think most of the heat would be lost.