Boiler Sizing (Vaillant Combi)?
Discussion
Looking for a bit of a sanity check before buying a new boiler.
Specifics
- 1980 Detached (masonry cavity walls)
- 2600sqft
- 2 bathrooms with walk in rainhead showers
- 4/5 bed
- 2 adults + 2 kids
- 16 new-ish radiators (mostly triple column Acova’s around 1400W each)
- Approx 32 litres per min incoming mains cold water flow
- 22mm gas pipe from meter to boiler (less than 1m run)
- No cavity wall insulation (SW coastal location so appears not recommended)
- 300mm loft
- 40mm above slab ground floor PIR as a token retrospective effort
Currently has a 25 year old Worcester 400 Highflow Combi (floor mounted jobbie) on Tado stat but moving to wall mounted modern boiler for better efficiency and layout change.
Rightly or wrongly have landed on the new Vaillant Ecotec Plus Combi as a replacement?
Largely on account of apparent manufacturer rep, unit size, and what appears to be marginally better flow rates versus something like the Worcester 8000.
Appreciate the advice may be system boiler with cylinder but that isn’t an option here. The boiler appears to have a decent modulation rate (1:10) which I’ll need the Vaillant controls to make use of.
I’m looking at the new 836 (36kw) model (flows 17lpm & modulates down to 3.5-4ish kW) currently for about £1800 but the 840 is only £150 more but maybe unnecessarily oversized for a bit better again flow rate?
I guess the heating only needs 20-25kw or so it’s just trying to make the most of the decent mains flow? Or am I barking up the wrong tree.
Specifics
- 1980 Detached (masonry cavity walls)
- 2600sqft
- 2 bathrooms with walk in rainhead showers
- 4/5 bed
- 2 adults + 2 kids
- 16 new-ish radiators (mostly triple column Acova’s around 1400W each)
- Approx 32 litres per min incoming mains cold water flow
- 22mm gas pipe from meter to boiler (less than 1m run)
- No cavity wall insulation (SW coastal location so appears not recommended)
- 300mm loft
- 40mm above slab ground floor PIR as a token retrospective effort
Currently has a 25 year old Worcester 400 Highflow Combi (floor mounted jobbie) on Tado stat but moving to wall mounted modern boiler for better efficiency and layout change.
Rightly or wrongly have landed on the new Vaillant Ecotec Plus Combi as a replacement?
Largely on account of apparent manufacturer rep, unit size, and what appears to be marginally better flow rates versus something like the Worcester 8000.
Appreciate the advice may be system boiler with cylinder but that isn’t an option here. The boiler appears to have a decent modulation rate (1:10) which I’ll need the Vaillant controls to make use of.
I’m looking at the new 836 (36kw) model (flows 17lpm & modulates down to 3.5-4ish kW) currently for about £1800 but the 840 is only £150 more but maybe unnecessarily oversized for a bit better again flow rate?
I guess the heating only needs 20-25kw or so it’s just trying to make the most of the decent mains flow? Or am I barking up the wrong tree.
Edited by ScottJB on Thursday 7th March 21:44
Edited by ScottJB on Thursday 7th March 21:45
Edited by ScottJB on Thursday 7th March 22:18
Watching this with interest.
We’re in a similar position; 3000 sq feet, five beds, two baths, 2 adults, two kids.
Everyone says it should be a system boiler and a cylinder, but when I spoke to a friend with a 300 litre cylinder he still can’t run two showers at the same time as the incoming mains is only 20 litres per minute or so.
We’ve also been recommended the Vaillant EcoTec Plus storage combi as it has a built in 15 litre hot water tank for instant hot water and to boost the hot flow to 20 litres per minute. So it’s between that, the green IQ and the WB Greenstar 8000; all of them being pretty chunky on the kw side but the plumber said he’d range rate them for heating.
Interesting you mention the modulation needing the Vaillant controls. I want to use my Evohome system so if it won’t modulate with that then it might be a non-starter.
We’re in a similar position; 3000 sq feet, five beds, two baths, 2 adults, two kids.
Everyone says it should be a system boiler and a cylinder, but when I spoke to a friend with a 300 litre cylinder he still can’t run two showers at the same time as the incoming mains is only 20 litres per minute or so.
We’ve also been recommended the Vaillant EcoTec Plus storage combi as it has a built in 15 litre hot water tank for instant hot water and to boost the hot flow to 20 litres per minute. So it’s between that, the green IQ and the WB Greenstar 8000; all of them being pretty chunky on the kw side but the plumber said he’d range rate them for heating.
Interesting you mention the modulation needing the Vaillant controls. I want to use my Evohome system so if it won’t modulate with that then it might be a non-starter.
ScottJB said:
Looking for a bit of a sanity check before buying a new boiler.
Specifics
- 1980 Detached (masonry cavity walls)
- 2600sqft
- 2 bathrooms with walk in rainhead showers
- 4/5 bed
- 2 adults + 2 kids
- 16 new-ish radiators (mostly triple column Acova’s around 1400kw each)
- Approx 32 litres per min incoming mains cold water flow
- 22mm gas pipe from meter to boiler (less than 1m run)
- No cavity wall insulation (SW coastal location so appears not recommended)
- 300mm loft
- 40mm above slab ground floor PIR as a token retrospective effort
Currently has a 25 year old Worcester 400 Highflow Combi (floor mounted jobbie) on Tado stat but moving to wall mounted modern boiler for better efficiency and layout change.
Rightly or wrongly have landed on the new Vaillant Ecotec Plus Combi as a replacement?
Largely on account of apparent manufacturer rep, unit size, and what appears to be marginally better flow rates versus something like the Worcester 8000.
Appreciate the advice may be system boiler with cylinder but that isn’t an option here. The boiler appears to have a decent modulation rate (1:10) which I’ll need the Vaillant controls to make use of.
I’m looking at the new 836 (36kw) model (flows 17lpm & modulates down to 3.5-4ish kW) currently for about £1800 but the 840 is only £150 more but maybe unnecessarily oversized for a bit better again flow rate?
I guess the heating only needs 20-25kw or so it’s just trying to make the most of the decent mains flow? Or am I barking up the wrong tree.
I have this boiler, a few years old now. The size of the house and rads are not an issue. The hot water flow rate is what you need. The most important thing we did, to max the flow rate, was make sure we had a 22mm ring for cold water. This fed everything, including the boiler. The boiler will then pump out 17lpm of hot approximately, and leave plenty of cold still. We have two showers and no noticeable drop in flow. Although, despite what everyone says on the Internet, we probably use both at the same time about once a year. You can though flush every toilet, fill the kettle and have a shower all the the same time. Getting the flow to the boiler is the most important part.
Specifics
- 1980 Detached (masonry cavity walls)
- 2600sqft
- 2 bathrooms with walk in rainhead showers
- 4/5 bed
- 2 adults + 2 kids
- 16 new-ish radiators (mostly triple column Acova’s around 1400kw each)
- Approx 32 litres per min incoming mains cold water flow
- 22mm gas pipe from meter to boiler (less than 1m run)
- No cavity wall insulation (SW coastal location so appears not recommended)
- 300mm loft
- 40mm above slab ground floor PIR as a token retrospective effort
Currently has a 25 year old Worcester 400 Highflow Combi (floor mounted jobbie) on Tado stat but moving to wall mounted modern boiler for better efficiency and layout change.
Rightly or wrongly have landed on the new Vaillant Ecotec Plus Combi as a replacement?
Largely on account of apparent manufacturer rep, unit size, and what appears to be marginally better flow rates versus something like the Worcester 8000.
Appreciate the advice may be system boiler with cylinder but that isn’t an option here. The boiler appears to have a decent modulation rate (1:10) which I’ll need the Vaillant controls to make use of.
I’m looking at the new 836 (36kw) model (flows 17lpm & modulates down to 3.5-4ish kW) currently for about £1800 but the 840 is only £150 more but maybe unnecessarily oversized for a bit better again flow rate?
I guess the heating only needs 20-25kw or so it’s just trying to make the most of the decent mains flow? Or am I barking up the wrong tree.
I have this boiler, a few years old now. The size of the house and rads are not an issue. The hot water flow rate is what you need. The most important thing we did, to max the flow rate, was make sure we had a 22mm ring for cold water. This fed everything, including the boiler. The boiler will then pump out 17lpm of hot approximately, and leave plenty of cold still. We have two showers and no noticeable drop in flow. Although, despite what everyone says on the Internet, we probably use both at the same time about once a year. You can though flush every toilet, fill the kettle and have a shower all the the same time. Getting the flow to the boiler is the most important part.
Edited by ScottJB on Thursday 7th March 21:44
Edited by ScottJB on Thursday 7th March 21:45
Heating you want low and slow so all the combis are massively oversized and as you mention you need to make sure it can modulate right down to avoid lots of cycling (bad for the boiler and efficiency).
For hot water you just have to pick whatever is right for your demand, but again as noted is a whole system thing eh if you’ve got crappy incoming pressure you’d need an accumulator for any system otherwise no showers.
Advice I have read is most boilers in Uk now are oversized due to combi needs…
For hot water you just have to pick whatever is right for your demand, but again as noted is a whole system thing eh if you’ve got crappy incoming pressure you’d need an accumulator for any system otherwise no showers.
Advice I have read is most boilers in Uk now are oversized due to combi needs…
I’ve been messing around a lot with my boiler this week as I felt that even for a 3200sq ft house with UFH on the ground the heating is very peaky, loads of cycling. Boiler is approx 10 year old Worcester CDI 40kW system boiler with probably quite poor modulation ratio - I’m not sure of the stats. The UFH is heavily zoned and I started reading about people de-zoning and reducing flow temps. As it happens the zones are coming on and off all the time so you’re always walking between warm and cold floor surfaces.
As I have switched to using the electric element in my 250l unvented cylinder combined with an EV tariff (cheap electric water heating to the tank stat at ~70C) I figured I could piss around with the flow temp safely without giving myself legionnaires! (Wife and kids are currently away)
I have found the results quite remarkable actually. I’ve had the boiler flow temp down to as low as 40C when it was cold and the house was still cosy, upstairs rads were warm rather than hot but as the UFH was running near permanently downstairs with the boiler not cycling (as far as I could tell) the overall improvement was one of consistent underfoot warmth.
I also found if you open everything up and let the UFH run for a number of hours you get some proper warmth into the floor pad which then stays warm for many hours after heating goes off.
My gas meter is too far from the electric to report usage so short of reading the meter daily I cant tell if I saved anything or not but it seemed like much less stress on the boiler.
My UFH mixers on each manifold are set to about 40C, any lower and they start weeping slightly, so I couldnt experiment with lower temps in the loops.
It got me thinking the boiler is probably massively oversized and that I might be miles better with something modern which can modulate to nothing, weather comp and hot water priority.
Or take the £7.5k and get a ASHP
As I have switched to using the electric element in my 250l unvented cylinder combined with an EV tariff (cheap electric water heating to the tank stat at ~70C) I figured I could piss around with the flow temp safely without giving myself legionnaires! (Wife and kids are currently away)
I have found the results quite remarkable actually. I’ve had the boiler flow temp down to as low as 40C when it was cold and the house was still cosy, upstairs rads were warm rather than hot but as the UFH was running near permanently downstairs with the boiler not cycling (as far as I could tell) the overall improvement was one of consistent underfoot warmth.
I also found if you open everything up and let the UFH run for a number of hours you get some proper warmth into the floor pad which then stays warm for many hours after heating goes off.
My gas meter is too far from the electric to report usage so short of reading the meter daily I cant tell if I saved anything or not but it seemed like much less stress on the boiler.
My UFH mixers on each manifold are set to about 40C, any lower and they start weeping slightly, so I couldnt experiment with lower temps in the loops.
It got me thinking the boiler is probably massively oversized and that I might be miles better with something modern which can modulate to nothing, weather comp and hot water priority.
Or take the £7.5k and get a ASHP
ScottJB said:
May well be wrong on the need for Vaillant controls, just something I read on another forum. May just have meant controls better than simple on/off.
Hopefully someone more knowledgeable comes along shortly to confirm.
unfortunately you can't digitally connect the Tado to a vailant boiler, only use the on / off output from the controller, which is a shame.Hopefully someone more knowledgeable comes along shortly to confirm.
(I believe its theoretically possible with some mythical box sold over seas but its not supported by Vailant or Tado)
dhutch said:
Watching with interest.
As said, any combi you fill will have vastly more power than you need to heat almost any house.
We have a 35kW system boiler (Viessmann 100) on a 250sqm edwardian semi, no wall insulation, drafts, boiler is only ever just ticking over.
I was looking at Viessmann myself.As said, any combi you fill will have vastly more power than you need to heat almost any house.
We have a 35kW system boiler (Viessmann 100) on a 250sqm edwardian semi, no wall insulation, drafts, boiler is only ever just ticking over.
I was thinking I could probably drop to 25-32 but from what you’re saying even that might be overkill!
The capabilities of modern stuff to constantly slow burn and optimise flow temp to demand is very interesting.
I have a Vaillant 837 running EvoHome in my 4 bed, 2.5 bath, 13 radiator house.
It has no issues with the heating, but I can't even run my en-suite shower at the same time as the sink mixer tap otherwise flow to both is seriously reduced.
I always assumed that this was a function of it being a combi in a fairly large house, but it might be down to the fact it was installed by cowboys... either way, it's not a patch on the Worcester I had in my previous (albeit smaller) house. YMMV of course.
It has no issues with the heating, but I can't even run my en-suite shower at the same time as the sink mixer tap otherwise flow to both is seriously reduced.
I always assumed that this was a function of it being a combi in a fairly large house, but it might be down to the fact it was installed by cowboys... either way, it's not a patch on the Worcester I had in my previous (albeit smaller) house. YMMV of course.
Pheo said:
Heating you want low and slow so all the combis are massively oversized and as you mention you need to make sure it can modulate right down to avoid lots of cycling (bad for the boiler and efficiency).
For hot water you just have to pick whatever is right for your demand, but again as noted is a whole system thing eh if you’ve got crappy incoming pressure you’d need an accumulator for any system otherwise no showers.
Advice I have read is most boilers in Uk now are oversized due to combi needs…
People are quick to say that cycling is bad for the boiler and efficiency, but very slow to find any real numbers on the subject.For hot water you just have to pick whatever is right for your demand, but again as noted is a whole system thing eh if you’ve got crappy incoming pressure you’d need an accumulator for any system otherwise no showers.
Advice I have read is most boilers in Uk now are oversized due to combi needs…
Combi boilers cycle a lot with HW, they are designed with that in mind and it doesn't seem to affect their reliability or lifespan.
It wastes a little heat purging the flue at the start and end of every cycle, but I think that's not a big number if flow temps are low.
I have a combi which can be set to keep the heat exchanger pre-heated, this results in extra short cycles, but the cost in gas seems to be somewhere between 'low' and 'negligible'.
It's the kind of thing that mostly matters to makers chasing AA++ ratings rather than reality.
I think the max power needed from the boiler is all about HW and fast turn-on of the heating. Our boiler will run at full power for only a few minutes when switching on the heating from cold, as soon as any warm water starts coming back from the rads, the boiler starts to modulate. Possibly that's because the way our house is set up, we prioritise 3 rooms to get their rads warm first.
IF you are in the habit of leaving the heating off and the nsuddenly wanting warmth, you need more peak heating power than someone who keeps their heating on steadily all winter.
When it comes to steady-state, it's useful to know the most you've ever wanted, before I improved some insulation we managed 170kWh per day in a cold spell, which is probably implying 10kW in the evening? So from a heating POV, I would be happy with a less powerful boiler.
Personally I think the main thing is the controls. We have 'load compensation' on the heating which drops the flow temp as the room temp gets near the target. That is efficient, allows quick heating from cold and gives steady room temp. I wouldn't want an aftermarket bunch of 'smart' controls which messed that up.
Gary C said:
OutInTheShed said:
People are quick to say that cycling is bad for the boiler and efficiency, but very slow to find any real numbers on the subject.
TrueLets face it, for donkeys years this was the only mode of operation.
I think maybe the boiler had a high mode and a low mode? No analogue modulation.
But that was before TRVs were common, everything just went on and off with the room stat I think.
Well rightly or wrongly I went for the new Ecotec Plus 840 with Vaillant controls in the end swayed by the on paper better flow rate versus higher cost.
Versus the 836 it cost about 5% more for 10% better flow rate albeit with a marginally poorer min output (3.7 vs 4.5). Either can comfortably cover the heating demand. Anyone chasing the lowest modulation possible btw, it seems like the Viessmann models are very good in that regard.
Digging out the specs for the 25 year old 400 Highflow it replaces, the old boiler could flow 18lpm and went down to 11kw min.
(Double checked my incoming mains flow (garden tap pre-PRV) and it almost filled a 32 litre bin in a minute (about 30lpm) so I can atleast in theory take advantage of the better flow offered)
Will be fitted in the next few weeks so will see how it works out.
Versus the 836 it cost about 5% more for 10% better flow rate albeit with a marginally poorer min output (3.7 vs 4.5). Either can comfortably cover the heating demand. Anyone chasing the lowest modulation possible btw, it seems like the Viessmann models are very good in that regard.
Digging out the specs for the 25 year old 400 Highflow it replaces, the old boiler could flow 18lpm and went down to 11kw min.
(Double checked my incoming mains flow (garden tap pre-PRV) and it almost filled a 32 litre bin in a minute (about 30lpm) so I can atleast in theory take advantage of the better flow offered)
Will be fitted in the next few weeks so will see how it works out.
Gary C said:
OutInTheShed said:
People are quick to say that cycling is bad for the boiler and efficiency, but very slow to find any real numbers on the subject.
TrueLets face it, for donkeys years this was the only mode of operation.
B'stard Child said:
Gary C said:
OutInTheShed said:
People are quick to say that cycling is bad for the boiler and efficiency, but very slow to find any real numbers on the subject.
TrueLets face it, for donkeys years this was the only mode of operation.
Mind you, when I did my apprenticeship, I lived in an old victorian house with gas fan lights and no central heating. Breaking the ice in the bathroom sink made me really appreciate my parents house.
Gary C said:
Mind you, when I did my apprenticeship, I lived in an old victorian house with gas fan lights and no central heating. Breaking the ice in the bathroom sink made me really appreciate my parents house.
I was going to say, central heating full stop came in within living memory.My parents first house only had open fires when they bought it only 40 years ago, and I am not sure if they ever fitted central heating as they bought their 'family home' when I was crawling. Tiny cottage, also without CH, that became a building project and the house they still live in.
Crumpet said:
Watching this with interest.
We’re in a similar position; 3000 sq feet, five beds, two baths, 2 adults, two kids.
Everyone says it should be a system boiler and a cylinder, but when I spoke to a friend with a 300 litre cylinder he still can’t run two showers at the same time as the incoming mains is only 20 litres per minute or so.
.
You could start with a combi and then add a unvented tank. We did. Similar to you, Started with a 36kw combi (it was here when we bought) same flow rate as yours, same size but 4 floors, and 2 bathrooms, one on the top floor. A combi didn't cut it remotely, unsurprisingly so we had a 200l cylinder and its been fine. We still cant really have 2 showers at the same time but unless you have mega mains you'll need pumps ... an accumulator didn't work for us either.We’re in a similar position; 3000 sq feet, five beds, two baths, 2 adults, two kids.
Everyone says it should be a system boiler and a cylinder, but when I spoke to a friend with a 300 litre cylinder he still can’t run two showers at the same time as the incoming mains is only 20 litres per minute or so.
.
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