Combi boiler costing more than previous system boiler.

annie42

In January we swapped our system boiler for a combi having been advised it would save us money. Compared to the same months in 2023 we are using significantly more gas and electricity so much so Octopus is increasing our DD. We can’t lower the water temperature to reduce costs as, for a comfortable temperature, it’s set at 48 degrees which is actually less than the minimum 50 degrees recommended. Initially we set the water temperature to the recommended 60 degrees but it was unbearably hot. We are wondering if there is a problem with the boiler? Thanks. 

tacpot12

Have you spoken to the person who advised you it would save you money? I would try to engage them to review whether the system is using more gas. There could be a range of reasons why you are using more gas; cooler weather, having the heating on a different schedule, using more hot water, cooking different sorts of meals (if you have a gas hob or gas oven).

It would be unusual for a new boiler to increase your electricity cost. A new boiler will tend to have a very efficient pump.

Do you know if the boiler has weather compensation?

When you say you are adjusting the water temperature, do you mean that you are adjusting the temperature of the water at the taps? If so, you can turn the temperature down futher if the water is still hot enough at the taps, but I doubt that this is the problem with your high bills. 

annie42

Thanks. I haven’t spoken to the company yet. It was only tonight I compared our usage from the same period last year. The timings haven’t changed. It’s been a milder winter than last year. We are actually using an air fryer a lot this year rather than the electric oven and hob. We perhaps use more hot water in the bath as previously the tank contents didn’t fill the bath as much as I’d have liked. I don’t think the boiler compensates for the weather. I’ve adjusted the water temperature to 48 degrees at the boiler. 

FreeBear

Your talk of water temperature of 48°C is, I assume, for water out of the taps ?

Heating water is a relatively small part of your annual gas consumption. Much of it will be for heating. To get good efficiency out of a boiler, you need a low return temperature so that the system runs in condensing mode. But a low flow/return temperature requires larger radiators to get the same heating effect compared to your old system. If you run at very low flow/return temperature, you also need larger pipes in addition to bigger radiators.

When I had a new boiler last year, new pipework was installed (22mm for the feed & return, with short lengths of 15mm to the radiators). Also fitted new radiators some 30-50% larger than a heat loss calculator suggested. Currently running with a return temperature around 40°C which should give me an efficiency of about 94%. Been adding instrumentation to my boiler, and once I can get realtime gas consumption, I'll be able to measure real efficiency (need a spell of cold too).

With the above in mind, a few questions.

Did you have larger radiators fitted ?

Any changes made to the plumbing ?

Has the system been balanced properly ?

What flow temperature are you running at ?

ThisIsWeird

annie42 said:

Thanks. I haven’t spoken to the company yet. It was only tonight I compared our usage from the same period last year. The timings haven’t changed. It’s been a milder winter than last year. We are actually using an air fryer a lot this year rather than the electric oven and hob. We perhaps use more hot water in the bath as previously the tank contents didn’t fill the bath as much as I’d have liked. I don’t think the boiler compensates for the weather. I’ve adjusted the water temperature to 48 degrees at the boiler. 

Make and model of combi, Annie?

Was your ol' system boiler getting close to the point that it needed replacing in any case? I mean, you didn't swap to a combi boiler just to try and save a few bob?

Although you should expect a combi boiler to provide some energy savings due to the simple fact that it only heats the water you actually use, any savings will likely be minor, depending on how modern your system DHW tank is and your DHW use. Eg, if you constantly heat a whole tank to a high temp, but only use small quants, then expect that to be a wasteful method, and a combi to be noticeably more efficient. But if you heat up your modern DHW tank and pretty much use up what you heat, then a combi is unlikely to provide noticeable savings over this. But I cannot think of any circumstance where a combi would consume more fuel for the same CH and DHW provision as a 'system'.

You are definitely comparing actual gas usage, and not the monthly 'bill'? This is certainly weird.

Keep_pedalling

Those deeper baths are likely to be costing you, and lowering the HW temperature does not really help as that just mean you need more from the hot tap and less from the cold. What is the flow temperature of the central heating set at?

Can you provide us with the actual monthly usage you used to compare this and last year please?

I doubt if the new boiler is affecting your electric costs, but an air fryer is not always the cheapest method of cooking, it might be if there are just two of you but not if you are preparing meals for a larger family. 

Albermarle

Keep_pedalling said:

Those deeper baths are likely to be costing you, and lowering the HW temperature does not really help as that just mean you need more from the hot tap and less from the cold. What is the flow temperature of the central heating set at?

Can you provide us with the actual monthly usage you used to compare this and last year please?

I doubt if the new boiler is affecting your electric costs, but an air fryer is not always the cheapest method of cooking, it might be if there are just two of you but not if you are preparing meals for a larger family. 

In any case even when only cooking for one or two, the savings from using an airfryer are widely exaggerated.
One or two pounds a week in reality.

OP- If you are using more gas then it is more likely to be an issue with the central heating than the hot water.
What flow temperature is this set at ?

annie42

Thanks for all your replies. Will try and answer all questions. We now have an Ideal Vogue Max 32 which replaced an old system boiler with a Gledhill tank. The HE suggested a combi as there are just the two of and he said it would be cheaper. He gave us the option of a 32 or 40 but explained he’d need to uplift carpet to install new pipe work for the bigger boiler. He said the 32 would be ok so we went with that to avoid the inconvenience. There’s been no change to the plumbing. No new larger rads. The water temperature is 48degrees and the flow temperature to the rads is 60. We are in our seventies and not of the generation that has daily showers and baths. We have ten double rads and three single ones but seven are generally switched off partly because we have a large multi fuel stove in the kitchen the heat of which permeates other rooms. 

Feb -March 2023  Elec. 276.7kwh  Gas - 230 units.

Feb -March 2024 Elec 264.3. Gas 284 units. 

March -April 2023  Elec- 205kwh  Gas - 101units.

March - April 2024 Elec - 278kwh Gas. 166 units.

I have just checked all the rads and discovered my husband had turned on four of the ones normally off because of the continual damp weather which has cause a bit of mould around our draught sash windows. They were on but only so low I didn’t notice. So we’ve been heating ten instead of six. 🙈 Have I answered my own question? 

casper_gutman

How long do you have the heating on for each day? Are you heating the house 24/7, or only at certain times?

ThisIsWeird

annie42 said:

Thanks for all your replies. Will try and answer all questions. We now have an Ideal Vogue Max 32 which replaced an old system boiler with a Gledhill tank. The HE suggested a combi as there are just the two of and he said it would be cheaper. He gave us the option of a 32 or 40 but explained he’d need to uplift carpet to install new pipe work for the bigger boiler. He said the 32 would be ok so we went with that to avoid the inconvenience. There’s been no change to the plumbing. No new larger rads. The water temperature is 48degrees and the flow temperature to the rads is 60. We are in our seventies and not of the generation that has daily showers and baths. We have ten double rads and three single ones but seven are generally switched off partly because we have a large multi fuel stove in the kitchen the heat of which permeates other rooms. 

Feb -March 2023  Elec. 276.7kwh  Gas - 230 units.

Feb -March 2024 Elec 264.3. Gas 284 units. 

March -April 2023  Elec- 205kwh  Gas - 101units.

March - April 2024 Elec - 278kwh Gas. 166 units.

I have just checked all the rads and discovered my husband had turned on four of the ones normally off because of the continual damp weather which has cause a bit of mould around our draught sash windows. They were on but only so low I didn’t notice. So we’ve been heating ten instead of six. 🙈 Have I answered my own question? 

Very possibly. :-)
All the info regarding the boiler sounds fine - yes, the larger model would have required a bigger gas supply pipe, but 32kW should give you a perfectly good hot water flow for very nice showers. 
And, yes, your HE's recommendation of a combi since it's just for the two of you also makes sense.
The Ideal VMax should be pretty much as efficient as its competitors, and both your CH and DHW flow temp settings are nicely low, so it should be running pretty much solidly in 'condensing' mode = as efficiently as it can.
So, that seems all good!
So, why the higher unit consumption? The first possibility you may have just discovered for yourself :-)
Any room you do not use for long periods, simply crack open every window in there to 'vent' setting, or even wider if it's dry outside and security isn't a concern, and shut the door to it; it should not develop any damp or condensation issues, and won't cost a penny to heat. Sorted.
Perhaps another possibility is that the boiler CH output temp is a tad too low, since you are using it with the original rads, which were designed for a higher running temp? So the rooms are now struggling to get up to temp due to the lower water temp, and the boiler therefore runs for much longer periods than it should? (As I write this, it doesn't make sense to me, as surely it's a case of lower temp for longer periods vs higher temp for shorter, so the latter will always cost more? I'm confusing myself...)
Annie, does the CH system actually work ok? Ie, when you turn it on or up in the morning, does it get your home as warm as you need, just like the old boiler? Or do you feel that it takes longer to do this? Do you think the new boiler is running for longer periods between going 'off' when 'satisfied'?
Another thought - did the HE fit new controls, such as a Programmable Thermostat, or even a Smart type (with phone App control)? If so, has how you run your CH now changed? How did you control your old boiler - was it by a manual room stat where you turned a dial up and down as required? If so, what settings did you use for 'day', 'eve', and 'overnight'? And, how does this compare with now?

BarelySentientAI

ThisIsWeird said:

annie42 said:

Thanks for all your replies. Will try and answer all questions. We now have an Ideal Vogue Max 32 which replaced an old system boiler with a Gledhill tank. The HE suggested a combi as there are just the two of and he said it would be cheaper. He gave us the option of a 32 or 40 but explained he’d need to uplift carpet to install new pipe work for the bigger boiler. He said the 32 would be ok so we went with that to avoid the inconvenience. There’s been no change to the plumbing. No new larger rads. The water temperature is 48degrees and the flow temperature to the rads is 60. We are in our seventies and not of the generation that has daily showers and baths. We have ten double rads and three single ones but seven are generally switched off partly because we have a large multi fuel stove in the kitchen the heat of which permeates other rooms. 

Feb -March 2023  Elec. 276.7kwh  Gas - 230 units.

Feb -March 2024 Elec 264.3. Gas 284 units. 

March -April 2023  Elec- 205kwh  Gas - 101units.

March - April 2024 Elec - 278kwh Gas. 166 units.

I have just checked all the rads and discovered my husband had turned on four of the ones normally off because of the continual damp weather which has cause a bit of mould around our draught sash windows. They were on but only so low I didn’t notice. So we’ve been heating ten instead of six. 🙈 Have I answered my own question? 

Very possibly. :-)
All the info regarding the boiler sounds fine - yes, the larger model would have required a bigger gas supply pipe, but 32kW should give you a perfectly good hot water flow for very nice showers. 
And, yes, your HE's recommendation of a combi since it's just for the two of you also makes sense.
The Ideal VMax should be pretty much as efficient as its competitors, and both your CH and DHW flow temp settings are nicely low, so it should be running pretty much solidly in 'condensing' mode = as efficiently as it can.
So, that seems all good!
So, why the higher unit consumption? The first possibility you may have just discovered for yourself :-)
Any room you do not use for long periods, simply crack open every window in there to 'vent' setting, or even wider if it's dry outside and security isn't a concern, and shut the door to it; it should not develop any damp or condensation issues, and won't cost a penny to heat. Sorted.
Perhaps another possibility is that the boiler CH output temp is a tad too low, since you are using it with the original rads, which were designed for a higher running temp? So the rooms are now struggling to get up to temp due to the lower water temp, and the boiler therefore runs for much longer periods than it should? (As I write this, it doesn't make sense to me, as surely it's a case of lower temp for longer periods vs higher temp for shorter, so the latter will always cost more? I'm confusing myself...)
Annie, does the CH system actually work ok? Ie, when you turn it on or up in the morning, does it get your home as warm as you need, just like the old boiler? Or do you feel that it takes longer to do this? Do you think the new boiler is running for longer periods between going 'off' when 'satisfied'?
Another thought - did the HE fit new controls, such as a Programmable Thermostat, or even a Smart type (with phone App control)? If so, has how you run your CH now changed? How did you control your old boiler - was it by a manual room stat where you turned a dial up and down as required? If so, what settings did you use for 'day', 'eve', and 'overnight'? And, how does this compare with now?

Tall skinny stack vs short wide stack.  Same number of bricks.  The boiler only has to put in however much heat has come out of the radiator, so if both times the room is going from the same starting temp to the same finishing temp, the actual heat is the same.

Ignoring losses, which you probably can for this sort of rough analysis.