epsilonGreedy Posted March 9, 2018 Share Posted March 9, 2018 (edited) After deciding on an LPG combi boiler for my new build spec I had assumed that an exotic subject like hot water re-circulation was a luxury I could not have. Now having read a detailed post on a DIY forum I believe there is a valid design for connecting a combi boiler's DHW output with a re-circulation circuit. My knowledge of hot water re-circulation has gone from zero to only somewhat confused in the last 24 hours, I am beginning to think that a well insulated example will behave like a low capacity linear hotwater tank. See post 13 at... https://community.screwfix.com/threads/suitability-of-combi-boilers.66102 Edited March 9, 2018 by epsilonGreedy Link to comment Share on other sites More sharing options...
ProDave Posted March 9, 2018 Share Posted March 9, 2018 I am not sure a gas combi can do this. A lot (most?) oil combi's maintain a small store of hot water that is ready to be drawn instantly. I think they do this as the start up time of an oil burner would otherwise make them suggish. This I guess is what could allows a re circulation system to work. Link to comment Share on other sites More sharing options...
epsilonGreedy Posted March 9, 2018 Author Share Posted March 9, 2018 4 hours ago, ProDave said: I am not sure a gas combi can do this. A lot (most?) oil combi's maintain a small store of hot water that is ready to be drawn instantly. I think they do this as the start up time of an oil burner would otherwise make them suggish. This I guess is what could allows a re circulation system to work. The narrative at that forum discussion I linked to needs a diagram, my interpretation is that the return of the re-circulating loop feeds back into the cold water input of the combi. A one way valve would be required prevent cold mains water being drawn up the wrong side of the loop. Link to comment Share on other sites More sharing options...
Nickfromwales Posted March 9, 2018 Share Posted March 9, 2018 On 09/03/2018 at 15:43, epsilonGreedy said: After deciding on an LPG combi boiler for my new build spec I had assumed that an exotic subject like hot water re-circulation was a luxury I could not have. Now having read a detailed post on a DIY forum I believe there is a valid design for connecting a combi boiler's DHW output with a re-circulation circuit. My knowledge of hot water re-circulation has gone from zero to only somewhat confused in the last 24 hours, I am beginning to think that a well insulated example will behave like a low capacity linear hotwater tank. See post 13 at... https://community.screwfix.com/threads/suitability-of-combi-boilers.66102 I don't know how many pages that lot went on for but could just about bring myself to complete page one. What a bunch of nuggets. One guy talking a 'bit' of sense and another dismissing an UVC because they explode and will blow up a car garage. WTF. If you do as those geniuses suggest then you'll probably half the life of the combi. Your warranty would instantly be voided, unless your happy to de-plumb everything every time you need to have it fixed again, and your legionaries risk would be off the chart. The pump speed ( litres per minute circulation rate ) would have to be perfect in order to get very hot water circulating. If the speed was too high then the water would never get hot, instead it would be warm, unless it's a beast of a combi. Every time the pipework cooled down you'd get another DHW 'pulse' which would shut down the general heating, activate the diverter switch, reignite the flame and run flat out until the pipe stat was satisfied, probably 10mins cooling and a 2 minute burn ( pulse ) for every cycle. The extra boiler labouring and subsequent fatigue would be ridiculous. Not even a starter afaic. IF you could access all of the hot and hot return pipework and lag the ? out of it you may halve the pulse rate and then you'll only take around 25% of the boiler life away. Large property with long runs to multiple locations = UVC ( the non exploding ones, don't forget that important detail ) plus a hot return circuit and dedicated pump with a system boiler. Not one of them mentioned the 22mm hot water pipework that would be in place with a copper cylinder install, which they would connect the combi too, so no difference in time of hot water to outlet other than the initial fire and warm up. I think 'that' screwfix community should be in padded cells . Combi boiler for hot return? NO. Link to comment Share on other sites More sharing options...
Nickfromwales Posted March 9, 2018 Share Posted March 9, 2018 @epsilonGreedy I take it you know a combi cannot provide heating and hot water at the same time? Eg they divert between either but never do both. Link to comment Share on other sites More sharing options...
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