Thu, 30 Apr 2026 06:47:06 +0100 https://www.greenbuildingforum.co.uk/newforum/ Lussumo Vanilla 1.0.3 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=864&Focus=9316#Comment_9316 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=864&Focus=9316#Comment_9316 Tue, 13 Nov 2007 13:21:35 +0000 Rustychain
If I have understood what I have read on this forum, under worked high power boilers are less efficient than fully worked less powerful boilers. Have I got that correct?

The reason I ask is that I was going to spec a Vaillant ecoTec plus 837, due to the reasonable flow rate on the HW (needed for a shower) and the stainless steel heat exchange. But I wonder if this is overkill for my house and whether I could get away with a smaller model. The central heating outputs are as follows:
ecoTEC plus 824 - 6.7 - 19.0kW
ecoTEC plus 831 - 8.7 - 24.0kW
ecoTEC plus 837 - 12.0 - 28.0kW

Thanks

Sean]]> https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=864&Focus=9317#Comment_9317 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=864&Focus=9317#Comment_9317 Tue, 13 Nov 2007 13:31:28 +0000 tony https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=864&Focus=9319#Comment_9319 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=864&Focus=9319#Comment_9319 Tue, 13 Nov 2007 14:30:30 +0000 fostertom Posted By: Rustychainunder worked high power boilers are less efficient than fully worked less powerful boilersGenerally correct - but don't combis get round this? - they have accumulators and stuff so they work flat out and then stop for a decent while, instead of cycling on/off all the time, as conventionals do under part load? Meanwhile with a bigger combi you've got the benefit of approaching-barely-adequate amounts of piping hot water, instead of the inefficiencies of hanging about waiting for the usual combi trickle. Am I wrong?]]> https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=864&Focus=9320#Comment_9320 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=864&Focus=9320#Comment_9320 Tue, 13 Nov 2007 14:42:50 +0000 tony https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=864&Focus=9321#Comment_9321 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=864&Focus=9321#Comment_9321 Tue, 13 Nov 2007 14:55:31 +0000 fostertom https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=864&Focus=9325#Comment_9325 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=864&Focus=9325#Comment_9325 Tue, 13 Nov 2007 17:29:29 +0000 nigel
It is important to size the boiler on the expected heat demand and make sure it is within the modulation range otherwise the boiler will cycle.

The problem with combis and modern housing is that they are sized for DHW and therefore generally oversized for heat output.]]> https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=864&Focus=9326#Comment_9326 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=864&Focus=9326#Comment_9326 Tue, 13 Nov 2007 17:35:48 +0000 fostertom https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=864&Focus=9336#Comment_9336 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=864&Focus=9336#Comment_9336 Tue, 13 Nov 2007 20:12:44 +0000 Rustychain
I've had the cavity walls filled on the existing house and the extension has 100mm dritherm slab cavity walls with 120 Kingspan (not as green as I would have liked, but as is often the case it came down to cost). The house should be fairly well insulated on that basis (oh yes, the loft of the main house is also fully insualted). There are negligible drafts.

Tony - fair comment on the water usage. We had a Vokera in our last house which gave plenty of water (wish I could remember which model it was!). I would rather down spec the boiler to avoid the misses enoying her showers too much!!!

As I want the boiler to be future proof (in case we convert the loft in the future and get another shower in the process), could I spec the larger boiler and put an in line restrictor at the current shower to reduce flow?

A follow on query from the comments. With a lower power model I assume it will take longer for the radiators to reach full operating temp (and therefore longer to bring the house up to a comfortable temp?). If this is true, would using a higher power boiler to bring the house up to the desired temperature more quickly be more/less/the same in efficiency terms. My gut reaction is the same.]]> https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=864&Focus=9337#Comment_9337 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=864&Focus=9337#Comment_9337 Tue, 13 Nov 2007 20:14:46 +0000 jamesingram
the installation manual usely has a graph, with gas pressure/output

though the plumbers I know never bother with it and laugh when I mention it

Jim]]> https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=864&Focus=9342#Comment_9342 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=864&Focus=9342#Comment_9342 Tue, 13 Nov 2007 21:15:34 +0000 fostertom Posted By: Rustychainto avoid the misses enoying her showers too much!!!How many misses do you have?]]> https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=864&Focus=9364#Comment_9364 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=864&Focus=9364#Comment_9364 Wed, 14 Nov 2007 09:14:17 +0000 Chris Wardle