Hi All
Im new here so go easy on me please, and apologies for the lengthy initial post
We've currently got an old (very old!) Baxi back boiler with gravity fed hot water tank which is all well past its sell by date, so when we extend the house this autumn we intend on getting a new combi system installed in the extension, plumbing onto the existing rads.
Firstly can anyone advise on how best to go about accurately sizing a combi? Our house is a marginally bigger than average 3 bed semi in Hampshire, built in the 1950s. We have concrete floor, (old-ish) UPVC double glazing, good loft insulation and reasonable cavity insulation - we couldn't get the entire house cavity insulated because the first floor front and back walls are tiled so don't allow access to drill/fill, but most of it is done.
Using a Whole House type boiler size calculator, it says we only need around 10-11Kw of heating output but I've seen it mentioned that this calculation doesn't work for Combis? Im guessing though that I can use that figure to spec a combi that modulates below ~10Kw on CH? If so, does that mean the size of the boiler doesn't make a lot of difference, at least in terms of it heating the house? Is there an efficiency issue still though, or will a 30kw boiler running at 30% capacity to supply 10Kw be as efficient as a 20kw boiler running at 50%?
Regarding hot water requirement, we currently have two adults living in the house, 1 bathroom with electric shower which may become a Mira type shower run from the combi when we re-do the bathroom next year. We rarely if ever run a bath at the mo but if kids come along in the future that may change. Downstairs there's obviously the kitchen sink to supply, plus another two sinks in the extension (utility room and washbasin in the loo), although I don't see them being used more than one at a time very often at all.
We've had a plumber friend-of-a-friend around to have an initial look and he said he'd be recommendeding something like a 30Kw Gloworm CXi for our requirements, but having become an "internet expert" by reading various sites like this (8)), the boiler that seems most recommended is the Vaillant Ecotec Plus series, which I think would point us at an 831 (31kw DHW?) if wanting one roughly equivalent to the Glow Worm CXi that's been recommended?
Im happy to spend the extra cash on a good boiler as the Glow Worms seem to have mixed reviews, and being a techie type I'd like to go with the all singing/dancing control options with weather compensation etc. The question is really though, is a 30Kw boiler roughly what we'd need, and is main sizing consideration more for the increased hot water flow rate of the bigger boilers rather than CH capacity?
Regarding the choice of the Vaillant, the concern we have going from a hot water tank to combi is losing the reliable/quick supply of hot water we have now, and also how often the combi system loses pressure and shuts off. We previously lived in a house with a cheap combi and that put us off a little, as it took a while to run hot water and also lost pressure quite regularly and shut itself down without notice until I re-pressurised it again.
The hot water speed issue I know can be helped with the "Aqua Comfort" option on the Vaillant pre-heating the water heat exchanger every so often, but I can't find anywhere that can quantify how much additional gas this would use? I believe though this can be controlled via the control box (VRC430) so it's only running at times when hot water is most likely going to be required, but Ive also seen comments saying its not really worth having on anyway?
Regarding the pressure issue of combis, I know this is a bit of an unknown and will need doing on all combis some time or other, but how often/quickly should you expect the pressure to drop, because our old one seemed to leave us with a cold house about once a month because it had lost pressure and shut down? Can the higher spec boilers like the Vaillant monitor this and give a warning beforehand via the control unit or even re-pressurise themselves, or do they all just shut down without any warning too and rely on the user to keep them primed?
Finally regarding the VRC430 weather compensation controller, Ive seen it mentioned a couple of times on here and elsewhere that you need a north facing wall to mount the external temperature sensor? Our house is on the diagonal if you like, so the three exterior walls face NE, NW and SW, with the boiler external wall facing NW, is this likely to cause a problem?
I know some will say that my plumber/heating engineer should be advising on all this but although CORGI registered and very experienced, he's a bit "old school" and Im not sure he's totally clued up on the latest higher spec boilers and may just be recommending based on what he always fits, so I'd like to form my own opinions as well. That probably suggestd that I should change my engineer for someone with more expertise/knowledge on the boilers Im interested in, but I always like to give work to friends / personal recommendations where at all possible so would prefer to continue with him if I can.
thanks in advance
Im new here so go easy on me please, and apologies for the lengthy initial post
We've currently got an old (very old!) Baxi back boiler with gravity fed hot water tank which is all well past its sell by date, so when we extend the house this autumn we intend on getting a new combi system installed in the extension, plumbing onto the existing rads.
Firstly can anyone advise on how best to go about accurately sizing a combi? Our house is a marginally bigger than average 3 bed semi in Hampshire, built in the 1950s. We have concrete floor, (old-ish) UPVC double glazing, good loft insulation and reasonable cavity insulation - we couldn't get the entire house cavity insulated because the first floor front and back walls are tiled so don't allow access to drill/fill, but most of it is done.
Using a Whole House type boiler size calculator, it says we only need around 10-11Kw of heating output but I've seen it mentioned that this calculation doesn't work for Combis? Im guessing though that I can use that figure to spec a combi that modulates below ~10Kw on CH? If so, does that mean the size of the boiler doesn't make a lot of difference, at least in terms of it heating the house? Is there an efficiency issue still though, or will a 30kw boiler running at 30% capacity to supply 10Kw be as efficient as a 20kw boiler running at 50%?
Regarding hot water requirement, we currently have two adults living in the house, 1 bathroom with electric shower which may become a Mira type shower run from the combi when we re-do the bathroom next year. We rarely if ever run a bath at the mo but if kids come along in the future that may change. Downstairs there's obviously the kitchen sink to supply, plus another two sinks in the extension (utility room and washbasin in the loo), although I don't see them being used more than one at a time very often at all.
We've had a plumber friend-of-a-friend around to have an initial look and he said he'd be recommendeding something like a 30Kw Gloworm CXi for our requirements, but having become an "internet expert" by reading various sites like this (8)), the boiler that seems most recommended is the Vaillant Ecotec Plus series, which I think would point us at an 831 (31kw DHW?) if wanting one roughly equivalent to the Glow Worm CXi that's been recommended?
Im happy to spend the extra cash on a good boiler as the Glow Worms seem to have mixed reviews, and being a techie type I'd like to go with the all singing/dancing control options with weather compensation etc. The question is really though, is a 30Kw boiler roughly what we'd need, and is main sizing consideration more for the increased hot water flow rate of the bigger boilers rather than CH capacity?
Regarding the choice of the Vaillant, the concern we have going from a hot water tank to combi is losing the reliable/quick supply of hot water we have now, and also how often the combi system loses pressure and shuts off. We previously lived in a house with a cheap combi and that put us off a little, as it took a while to run hot water and also lost pressure quite regularly and shut itself down without notice until I re-pressurised it again.
The hot water speed issue I know can be helped with the "Aqua Comfort" option on the Vaillant pre-heating the water heat exchanger every so often, but I can't find anywhere that can quantify how much additional gas this would use? I believe though this can be controlled via the control box (VRC430) so it's only running at times when hot water is most likely going to be required, but Ive also seen comments saying its not really worth having on anyway?
Regarding the pressure issue of combis, I know this is a bit of an unknown and will need doing on all combis some time or other, but how often/quickly should you expect the pressure to drop, because our old one seemed to leave us with a cold house about once a month because it had lost pressure and shut down? Can the higher spec boilers like the Vaillant monitor this and give a warning beforehand via the control unit or even re-pressurise themselves, or do they all just shut down without any warning too and rely on the user to keep them primed?
Finally regarding the VRC430 weather compensation controller, Ive seen it mentioned a couple of times on here and elsewhere that you need a north facing wall to mount the external temperature sensor? Our house is on the diagonal if you like, so the three exterior walls face NE, NW and SW, with the boiler external wall facing NW, is this likely to cause a problem?
I know some will say that my plumber/heating engineer should be advising on all this but although CORGI registered and very experienced, he's a bit "old school" and Im not sure he's totally clued up on the latest higher spec boilers and may just be recommending based on what he always fits, so I'd like to form my own opinions as well. That probably suggestd that I should change my engineer for someone with more expertise/knowledge on the boilers Im interested in, but I always like to give work to friends / personal recommendations where at all possible so would prefer to continue with him if I can.
thanks in advance