Highlandladdie Posted February 27, 2017 Report Share Posted February 27, 2017 In the process of having new oil heating installed. The plumbers have spec'd a 26kW oil combi boiler. I currently have a 32kW LPG combi boiler. I'm a bit worried that this is slightly under sized? More worried about the hot water supply to my upstairs mixer shower, I don't want a trickle! Boiler has to be fit for a 4 bed 1 1/2 story hoose with 2 bathrooms, 12 rads. House is cavity block built with minimal insulation, just some glasswool in the roof. Would that be sufficient? The next option is a 36kW, but that seems like quite a jump. Any ideas? Quote Link to comment Share on other sites More sharing options...
millrace Posted February 27, 2017 Report Share Posted February 27, 2017 Run small run hard to max efficency runnin large with add capicity never works hard enough..... Quote Link to comment Share on other sites More sharing options...
Tim Kelly Posted February 27, 2017 Report Share Posted February 27, 2017 The only difference will be about 1 litre a minute in flow rate of the hot water. You will not notice it. Worth checking what the heat output is in CH mode, as with most gas combi boilers it's lower. Don't know about oil boilers. You must have a lot of outside walls if you have 4 bedrooms so worth doing a heat loss calc for the building to see if the heating will manage. I expect it will though, if the radiators are sized correctly. Quote Link to comment Share on other sites More sharing options...
figgy Posted February 27, 2017 Report Share Posted February 27, 2017 (edited) Your combi is rated higher for the DHW the CH side will be much lower, if it will be sufficient at 26kw it will use burn less oil when running the water. The heating side should be adequate too, as it will still get upto temp just may take a tad longer to get there. I have a 40kw system boiler when 34kw was all I needed, due to the fact that the 40kw boiler modulates down to 7kw minimum output the 35kw boiler would only modulate down to 9kw, so when only ticking over for underfloor heating etc the bigger boiler saves me money. Check what your needs are and what the boiler can and can't do. You want high domestic hot water flow rate, some makes are better than others. Edited February 27, 2017 by figgy Quote Link to comment Share on other sites More sharing options...
simonb Posted February 27, 2017 Report Share Posted February 27, 2017 When I began in the heating industry so many years ago at the very start of the 'combi' age an old heating installer told me the best system for hot water supply is a stored hot water supply, It would seem from what you say that may be your best answer. As far as the shower is concerned why not install an electric independent power shower? Combi boilers? another European idea and I wouldn't give 'em house room (no pun intended) Maybe an oil fired system boiler with an unvented cylinder which requires no cold water storage or flow and expansion tank and works on mains pressure with normally a 20 mm cold water supply .just one cylinder can supply hot water to (depending on the cylinder) as many 'draw off points as required including multi use and will supply hot water for your central heating. Google will point you to loads of unvented cylinders and boilers. Aint going to be a budget job though Remember many 'heating installer's prefer a combi as it is less install time.Stored hot water all the time unless space is really limited. In smaller homes a combi is OK but I would guess not in your case. Be sure though the installer has the correct building regs qualifications to install an unvented cylinder. Used to be G4....good luck Quote Link to comment Share on other sites More sharing options...
seph234 Posted February 27, 2017 Report Share Posted February 27, 2017 What combi boiler has he specified? whats the overall size of your house in metres length, width height (aprox) Quote Link to comment Share on other sites More sharing options...
seph234 Posted February 27, 2017 Report Share Posted February 27, 2017 Yes Unvented will always beat a combi hands down. depending on your water pressure. but he should of checked the pressure for a combi too. Quote Link to comment Share on other sites More sharing options...
Highlandladdie Posted February 28, 2017 Author Report Share Posted February 28, 2017 Thank you for the replies. House is approx 10mtrs x 8.5mtrs, detatched. Boiler spec'd is a http://www.grantuk.com/product/vortex-pro-internal-combi-condensing-range/ I see that the from the spec sheet the 26kW will modulate down to 7.8kW at 30% load, and the 36kW down to 10.8kW respectively. Flow rates are 17 l/min and 20 l/min for the 36kW. Heed is burst trying to decide, its a lot of money for the system install at £5k (moving boiler position too) so want to get it right. Quote Link to comment Share on other sites More sharing options...
Highlandladdie Posted February 28, 2017 Author Report Share Posted February 28, 2017 Just did this http://calculators.baxiknowhow.co.uk/boiler-sizing/ came out a 14.5kW. Maybe 26kW will be OK after all? Quote Link to comment Share on other sites More sharing options...
figgy Posted February 28, 2017 Report Share Posted February 28, 2017 If your spending a chunk of money. Look at how much hot water you use. I will always go for invented mains pressure hot water cylinder unless I was in a very small house or flat. Cylinder will last years and years with little maintenance, system boilers don't go wrong as much and are cheaper to replace. Quote Link to comment Share on other sites More sharing options...
Highlandladdie Posted March 2, 2017 Author Report Share Posted March 2, 2017 Thanks for the replies gents. After much consideration I've gone for the 26kW. It was the best balance for everything and should be the most economical Quote Link to comment Share on other sites More sharing options...
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