Hi, I’m removing my old gravity fed hot water tank and 12kW heat only oil burner and was going to replace with a combi as I have very little internal space. My OFTEC installer wants to fit a 21kW or even 26kW grant vortex pro combi (suggesting 26kW for better DHW). My heating system is about 8kW of rads. I spoke to Grant and they are suggesting this is massively oversized. You can’t really get significantly a smaller combi than 21kW. Is this a bad idea? I’m concerned about excessive short cycling. The heating volume is about 130l plus the boiler which might help buffer the heat. Or is unvented plus heat only my only real option?
- Thread starter andrew700andrew
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- May 23, 2015
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I have a non combi 20kW 20 year old Firebird, the heating demand is often down to ~ 3kW and the boiler does cycle "excessively" but it hasn't required a single replacement part during this 20 years so I wouldn't worry too much about this, oil fired boilers by their very nature will always cycle since they don't modulate, for the same reason combi oil fired boilers have (primary) stored HW, thermodynamically, a 12kW combi will give a continuous flowrate of 4.9LPM with a dT of 35C, a 15kW, 6.1LPM, a 21kW, 8.6LPM, but will give a far greater flowrate for a few minutes due to the stored HW until it "runs out". Some manufacturers quote the equivalent (cylinder) vol of HW available.
Called up firebird and Worcester, both do a 12kW combi which I’m waiting on quotes for. Apparently the quoted hot water figures (something like 15l/min for 120l and 90l respectively) are the same regardless of rating. I’m only looking for an 8l a min shower so these should be fine. Thanks for the replies
I would ask both, and especially Firebird, how they calculated these outputs, see below, as my calcs give only ~ 50/60% of these outputs.
The Firebird (primary) store is 40L and is maintained at 78C, they claim that this will equate to a 120L cylinder & 12LPM at a dT of 40C, I can't figure out how they come up with these numbers.
If a 40C DHW temperature is required then because of the plate heat exchanger, the lowest temperature the primary store can fall to is probably around 50C to maintain a DHW temperature of 40C. Using a average winter mains temperature of say 7C, then that 40L store in falling to 50C,will give, 40*(78-7)/(50-7), 66L, IF the store temperature can fall to 45C to maintain a DHW temp of 40C then the 40L store will give, 40*(78-7)/(45-7), 75L. 66L will give you a 8.26 minute shower at 8LPM+40C and a maybe possible 75L, which will give a 9.38 minute shower at 8LPM+40C, IMO.
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- May 23, 2015
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Boiler will reheat at the same time eg kicking into the store 16kw also 12/18 will normally be nozzled / factory set at 15-16kw
A few calcs below, (if correct, are interesting) in that a 16kW combi which will only give a continuous flowrate of 6.6LPM at a dT of 35C will give a flowrate of almost 14LPM for 10 minutes before the DHW temperature starts falling.
Unfortunately can't post the excel file even if I zip it.
Unfortunately can't post the excel file even if I zip it.
I would suggest ensuring that if you go for a 12kW boiler that the oil jet size can be increased to give up to 20kW, the Firebird I think is ranged, 12 to 20kW but only has a 20L store. If you do opt for the 12kW then that will only give a continuous 4.9LPM at 40C in the winter and 8.3LPM for 10 minutes but you then have to wait for 5 minutes or so to reheat the store. 20kW should give 8LPM continuously and ~ 11.5LPM for 10 minutes.
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