Does boiler efficiency get worse over time?

[gerbils]

Trying to weight up the cost/expense benefits of updating our boiler.

Current boiler: GlowWorm Space Saver MKII (1989) - 65% efficiency (when new).
Replacement: Combi - 90% efficiancy
Replacement Cost: £3800
Current annual gas usage / cost: 20,000kWh (£2,000 + £102 standing charge)
Efficiency Saving: 25%
Saving: £500 per year

On the face of it, it seems a bad deal as even at current prices, this would take us just under 8 years to make it cost effective, by which time I am assuming any new boiler will start having issues. I am also sure gas prices will come down in the next 5 years.

However we live in a hard water area so I am sure the existing boiler is full of limescale and wonder if the efficiency would be worse than 65% now that it's 33 years old? If this is the case we could be looking at a better return?

 

[JonathanM]

Current annual gas usage / cost: 20,000kWh (£2,000 + £102 standing charge)
Efficiency Saving: 25%
Saving: £250 per year

25% of £2000 is £500

 

[gerbils]

Oops typo. Corrected. Thank you.

 

[gas112]

the condensing boiler will not run at 90% all the time it will drop to around 82/3ish % after its hot .
the old boiler you have depending on where it is fitted also contributes heat into the room .
You will save a bit of money by not having stored hot water .
The combi will get your house up to temp a lot quicker than the old boiler the rads will usually be hot after about ten mins compared to around half hour for the older boiler house heats up quicker boiler shuts off quicker ,
Various other little difference but yes your right it will take quite a few years to pay for its self .

 

[kidgreen61]

Your current boiler will last 300yrs, keep it, there's so little to go wrong with it.

 

[cdbe]

This is one of the connectors from my glowworm spacesaver MK2, which I removed a few years ago. It was still working but I think would have benefited from a bit of a clean out! We had a primatic? hot water cylinder which I think prevented the use of certain chemical treatments. I think heating up the crud will have affected efficiency.

 

[vulcancontinental]

You have a high mass, high water content boiler with little control of air flow, no modulation, simple working options, flat out or stop.

If stopped it will lose any heat from that big lump of cast and water because it will have an open flue or balanced flue that still works when the boiler is off but takes heat away with it.

What else? How is it controlled? Does it work in a co-ordinated way with the system and prevailing demand adapting to changing heating loads.

Can it adjust temperature to extract maximum heat potential from the fuel?

The gaskets in the two part cast exchanger have done 30 years and there is 30 years of deposits in the waterways which may be negligible or substantial.

If you pay a monthly insurance premium for breakdown cover it's likely to be classified as BER or parts unavailable if it did breakdown so that's a cost to be balanced against a long warranty on a new boiler that will need attention in all likelihood and will definitely need an annual service to maintain the guarantee.

A pre ErP pump will use around 100w/hr, a new ErP pump 39w/hr at max and if fitted within the boiler be covered by the warranty.

20,000kWhrs / year is above average of 12-14,000 so substantial.

I like old stuff and respect it, it owes you nothing but it's up to you.

 

[gerbils]

20,000kWhrs / year is above average of 12-14,000 so substantial.

I thought that too.

But I think 12k is the average for a 3 bed house with 3 people. We are in a 4 bed detached, family of 4. However we are somewhat frugal in as much as 2 people working from home and we don't ever put the heating on until the kids come home.

I am guessing this 20k is largely down to the inefficiency of the boiler and would hope would be more like 14-15kWh with a modern boiler without changing our habits?

We have not had the heating on since May and our usage is as follows:

May: 540 kWh
June: 430kWh
July: 430kWh
Aug: 380kWh
Sep: 370kWh
Oct: 580kWh
Nov: (to date) 280kwh

This is purely 1hr of water heating per day, very occasional cooking and the pilot light. In August I changed the water to heat a lower temp and only 45 mins hence the drop.

I don't understand the spike in Oct as our energy usage is super constant, but we are supplying manual readings as we can't get a smart meter for love nor money around here.

 

[JonathanM]

We are in a 4 bed detached

I am too, and I have a similar age boiler, but my gas usage is quite a lot higher than yours. The house is never above 19C, and upstairs is cooler than that. It's an early 1990s house with standard insulation for the period.

We have not had the heating on since May

Would you be willing to say roughly where you are in the country? I'm in the foothills of the Pennines, in Yorkshire. I think I first put the heating on at the start of October.

 

[gerbils]

I am too, and I have a similar age boiler, but my gas usage is quite a lot higher than yours. The house is never above 19C, and upstairs is cooler than that. It's an early 1990s house with standard insulation for the period.



Would you be willing to say roughly where you are in the country? I'm in the foothills of the Pennines, in Yorkshire. I think I first put the heating on at the start of October.

Our annual usage is 20,000 kWh, it increases hugely once we start heating the house!

We are just south of Oxford - 1989 property, cavity insulated at the time.

We had the heating on Nov 1st but are being frugal just boosting it for an hour a day. Bit silly really as were spending 3x as much on electricity!

 

[JonathanM]

Our annual usage is 20,000 kWh, it increases hugely once we start heating the house!

We are just south of Oxford - 1989 property, cavity insulated at the time.

We had the heating on Nov 1st but are being frugal just boosting it for an hour a day. Bit silly really as were spending 3x as much on electricity!

Based on our nearest respective Met Office weather stations, your average daily temperature for the past couple of months seems to be 2-3C higher than mine. That makes sense. I keep reading people on here who haven't had the heating on yet, and it makes me wonder if I'm a big softee! Until I started looking into boilers a couple of months ago, I had no idea what a variation there was in temperature within England.

 

[gerbils]

Based on our nearest respective Met Office weather stations, your average daily temperature for the past couple of months seems to be 2-3C higher than mine. That makes sense. I keep reading people on here who haven't had the heating on yet, and it makes me wonder if I'm a big softee! Until I started looking into boilers a couple of months ago, I had no idea what a variation there was in temperature within England.

Yeah likewise, never really thought about it until this year, if I am cold I usually stick the heating on.

I have come to realise anything under 16 is uncomfortable in our house; plus we have small(ish) children and I don't want them getting ill from a freezing home so we will look to keep at 18 (usually keep it at 20) when things get colder.

Also worry about mould creeping in if we keep it too cold.

However on the flip-side also a realisation that we don't need it at 20 all weekend and kids can cope with it much lower.

 

[JonathanM]

I have come to realise anything under 16 is uncomfortable

Me too!

Going back to efficiency, I'm really interested in the ways that efficiency has improved, since they made our type of boilers, to the present day ones. We all know that there are gains because of the condensing, but because of the way they do the calculation, I think that only adds about 5%, to the efficiency rating. But we have boilers going from 65% efficient, to 89% efficient, under the SEDBUK rating system. So what are the other gains?

Things I think I have tracked down: Not having a permanent pilot light saves about 4%. Having a fan assisted flue saves another couple of percent, or sometimes a bit more. Adding those, together with the 5% from the condensing, takes a boiler from 65% to maybe 77%. So how have they got the efficiency up to 89%? Is some of that because less heat is escaping into the room from the boiler itself? If so, and your boiler is actually inside your house, is that useful heat, and therefore that heat escape is not wholly inefficient? Is it because the flue gases are being cooled before going out of the flue? I'm not talking about actual condensing here, simply that some of the heat that would have escaped outside is heating the water returning to the boiler. Maybe that is actually the biggest gain?

 

[gerbils]

Me too!

Going back to efficiency, I'm really interested in the ways that efficiency has improved, since they made our type of boilers, to the present day ones. We all know that there are gains because of the condensing, but because of the way they do the calculation, I think that only adds about 5%, to the efficiency rating. But we have boilers going from 65% efficient, to 89% efficient, under the SEDBUK rating system. So what are the other gains?

Things I think I have tracked down: Not having a permanent pilot light saves about 4%. Having a fan assisted flue saves another couple of percent, or sometimes a bit more. Adding those, together with the 5% from the condensing, takes a boiler from 65% to maybe 77%. So how have they got the efficiency up to 89%? Is some of that because less heat is escaping into the room from the boiler itself? If so, and your boiler is actually inside your house, is that useful heat, and therefore that heat escape is not wholly inefficient? Is it because the flue gases are being cooled before going out of the flue? I'm not talking about actual condensing here, simply that some of the heat that would have escaped outside is heating the water returning to the boiler. Maybe that is actually the biggest gain?

Everything I have read suggests it's very unlikely a modern boiler actually runs at 90% efficiency.
Firstly this efficiency is measured in lab conditions after a manufacturer install.

99% of boiler installations will not be that good and they lose between 10-25% of efficiency out of the box (source)

Secondly the efficiency is only when being run at a certain level and time so it's not 24/7.

This is why it's so confusing what to do for people with an old boiler!

 

[JonathanM]

99% of boiler installations will not be that good and they lose between 10-25% of efficiency out of the box (source)

I've read that as well before, but when I tracked down the research, it didn't seem that bad - from memory, condensing boilers in practice were more like 83% efficient on average - I think that quote must be taking in extreme outliers. And the research was done a long time ago now when there was less knowledge. Having said that, I've been told recently by an installer to run a new boiler very hot for maximum efficiency, and my brother was told the same when he got a new boiler installed seven years ago.

It is all incredibly confusing. One issue is that we have a European rating for efficiency, which might be 93%, but the UK rating for the same boiler will only be about 89.5%. I think our figure is adjusted to take into account some real world losses.

That article does illustrate the issue I was talking about above. It states:

2. Why are new boilers so much more efficient?​

All modern boilers are ‘condensing’ boilers. This means they are able to recover heat that was previously lost via the flue to pre-heat the heating system (for the techies out there – this is via a second heat exchanger).

They are not A-rated out of the box however. The heat that previously escaped via the flue is captured by the second heat exchanger and it can only do this if it has turned back into water vapour (hence the term condensing). This requires the boiler to run at a much lower temperature. The lower the temperature, the more efficient the boiler. However most boilers are left at their factory settings (80degC) which means they rarely condense and most useful heat is lost via the flue.

They seem to be saying that the only reason new boilers are more efficient is because they condense. But this is not true. Most of the gains in efficiency have come from other areas. The official calculations take an average of when the boiler is not condensing at all, and when it is condensing a lot, to get the efficiency figure. So, the official efficiency is already taking into account that the boiler will only be condensing 50% of the time. For example, the laboratory tests might typically show that a boiler is 88% efficient when not condensing, and 98% efficient when condensing a lot, and that gives the 93% average. That's why I'm saying that only 5% of the efficiency improvement is coming from condensing in these ratings. And so, even if the boiler never condensed at all, it would only be 5% less efficient than its rating.