Comparing costs of running boilers - are my calculations correct?

merchcon55

If I wanted to compare the cost of a new boiler that runs at 90% efficiency against an older boiler that runs at 65% efficiency - would this be the right way to compare:
OLD BOILER: Annual use = 30,000 KW x 65% = 19,500 KW used to heat home
NEW BOILER Annual use of 21,667 KW x 90% = 19,500 KW used to heat home

So Old Boiler 30,000 - New Boiler 21,667 = 8333 KW more with Old Boiler @ 2.7p/KW (potential renewal cost for winter 2021-22) = £225 more for old boiler.

For the example, let's say that monthly service contract for a new boiler (which my understanding is that due to their complexity one should have) is £15 a month, which includes an annual service (£180). My old boiler is given an annual service and costs £75 - so while I would be responsible for any repairs, The new boiler has a higher initial annual outlay of £105.

Overall: Old boiler £225 more in gas - £105 savings for maintenance contract = £120 per year more to run old boiler.

Having spoken with a few friends who have had to work from home this winter and are complaining about their high heating bill for the winter, I am questioning whether their so called newer efficient boiler is really that efficient. Their bills are very similar to mine. I have worked from home for years, as does my wife - so we expect, and allow for, high heating bills in winter. Very surprised though to see that our bills are not that different from some of our friends.

Gerry1

Don't forget that your new boiler won't be 90% efficient in the summer when it's only providing hot water and won't be in condensing mode.

merchcon55

Gerry1 said:

Don't forget that your new boiler won't be 90% efficient in the summer when it's only providing hot water and won't be in condensing mode.

I was not aware of this - so when used to only heat hot water, older boilers and newer boilers use similar amounts of gas?

coffeehound

merchcon55 said:

 I am questioning whether their so called newer efficient boiler is really that efficient. Their bills are very similar to mine. I have worked from home for years, as does my wife - so we expect, and allow for, high heating bills in winter. Very surprised though to see that our bills are not that different from some of our friends.

It's all down to the temperature of the water as it returns to the boiler.  If it is below 55 ℃ (some say 53) then the boiler can extract extra heat from the combustion exhaust gases.  It has been found that many homes run the boiler too hot to achieve condensing mode.  The most efficient mode is when water returns at 30 º where efficiency might reach high nineties.

As Gerry1 said, water heating is less efficient.  In the early stages of heating a hot water cylinder, the boiler will be in condensing range, but as the water in the tank approaches 55º, so will the return flow to the boiler.  

For a combi, the domestic hot water is never heated in condensing mode*, and the efficiency is around mid-80s per cent.

* Perhaps some are better than others, but this has been the case for some I looked at.

David28

I replaced my warm air heating, which had a separate burner for heating the hot water tank which had a pilot light that used 3kWh of gas a day, with a combi boiler  and in the summer I am using the same amount of gas as before so therefore actually using more gas to heat the water directly.

coffeehound

Thought this might help with the estimate.  This is a grab from the ErP data sheet for the Worcester-Bosch Greenstar 8000 30 kW combi with some relevant numbers in yellow



The 94% appears to be an average number like the old SAP figure; not sure of the definition.
The CH efficiency when using a high radiator circuit temperature is 88.5%.   Presumably this includes a portion of condensing operation in the early stages of the firing cycle.
With a low-temperature radiator circuit, efficiency goes up to 98.7%.  I asked W-B the definition for this regime, and it assumes a flow temperature of 50℃ with a 20º temperature drop in the radiators, so a return temperature of 30º.
Instant DHW heating (this is a combi) efficiency is 89%.  I would think heating water in a modern hot water cylinder could be more efficient dependant on a range of factors.

A key takeaway is that a modern boiler will be significantly more efficient than old-school boilers whether in condensing mode or not.

bagand96

Why would you take a service contract out for a new boiler? It will have a warranty anyway, and for some brands if you use an approved installer and have it serviced annually that warranty will be 7-10 years. 

Paying for an annual service on a modern combi will cost way less than a £15/month service plan. 

macman

What he said^
Annual service on mine is £79, that keeps my 10 year Viessmann warranty valid. Very few decent brands now offer less than 7 years warranty as standard: my 3 year extension cost an extra £100.

[Deleted User]

A couple of other things to consider. Replacing an old boiler with a new one isn't like for like. As indicated above,  modern boilers are designed to run at lower flow temperatures than the ones that they replace. All too often, the new boiler is just fitted with no consideration given to the size of the existing radiators. Boilers that run at lower temperatures need bigger radiators if the boiler is to run at an efficient temperature. Secondly, installers favour larger than needed boilers because of water reheating and their perception that consumers want rapid heating of the home. Modern boilers have a built in modulation ratio. For example, my WB boiler will modulate down from 24kWs to 7.2kWs: ideally, I would like it to modulate down to 4kWs. 7.2kWs is just too big for a well insulated home and it results in the boiler cycling.

I didn't choose my present boiler and should I ever need to replace it, I would probably opt for an InterGas boiler under Opentherm control. InterGas boilers have very few moving parts and are favoured by Councils as they are robust; very easy to service, and to repair. Opentherm offers better boiler control than standard TPI thermostats and is mandatory in The Netherlands. The analogy often used is that standard controls are a bit like driving across a city: you will often hit a number of red lights on the way. Opentherm control is a protocol which is designed to control the boiler in such a way that it is green lights all the way. This reduces boiler cycles.

Finally, I agree that a Service Plan isn't necessary; however, based on my experience, I would pay a little more for an engineer visit by a manufacturer's service representative whilst the boiler is under warranty. My WB boiler which must have been built late on a Friday has had the fan; heat exchanger and syphon replaced over the course of two annual service visits.

merchcon55

Thank you for the comments - some very informative stuff here. To be fair I was also questioning why one would take a service contract on a new boiler. So for a high gas user like myself, I'll calculate on £225 more per year. 

Very interesting to read how if you change to a newer style boiler then the existing radiators may not be the correct size for the boiler to achieve maximum efficiency (savings). That could easily explain why my friends with more modern boilers, have similar gas bills to myself.

I do appreciate the want for "rapid heating of the home" - i'm curious, with a more modern boiler, what would be the expected length of time say in the morning to get the house to an agreeable temperature? As example, in our home the heat comes on at 6 AM - by 7AM the house is sufficiently warm. 

I'm from New York - in my mum's apartment they still use steam heating - when that turns on, I'm not sure it even takes 2 minutes for the rooms to be warm - and it's included in her rent LOL. Love it !!