New install condensing combi system

Mark Begbie · 2004-09-07T15:37:09+00:00

I have just bought a 2 bed lower flat into which I am intending toinstall a condensing combi system. Based on the on-line radiatorcalculators I need about 8.5kW of output in total for the sixradiators and hot water, so my questions to the group are a few:1) I had though I could use 15mm (plastic) pipe for the feed andreturn to the radiators, but it has been suggested to me that thisshould be upped to 22mm - is this good advice or would 15mm sufficefor this sort of installation?2) Being a condensing boiler I understand it will run more efficientlywith a lower circulation temperature - does this mean I have to deratethe radiators below their quoted capacity?3) Since all the boilers I have seen are substantially more powerfulthan I require for this building, I will have to put in an overpowerful model. I am guessing that I should also over spec theradiators, to an extent, in order to provide a decent load for theboiler and use a room stat as an additional step to prevent shortcycling (the other rads will all have TRVs on them). Is thissensible?4) I have collated a chunk of info / FAQs on this, but if anyone canpoint me in the direction of substantial / useful resources, againthis would be greatfully received.Many thanks in advance for any help,Mark

Sep 07, 2004 24 Replies

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John 21 years ago

As far as I'm "happy" with any modern (accountant engineered) product they are well built, at least they use stainless steel and plastics properly rather than aluminium in contact with acidic condensate. I'd not classify them as a two man lift by any stretch of the imagination but I suppose I've been around long enough to expect bags of cement to weigh a hundredweight (approx 50kG) not these namby pamby 25kG bags that are the norm nowadays. I have installed an Isar in the past so by way of comparison I can say it was much heavier and not as easy to install

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Pete C 21 years ago

Oops, meant to say burner/fan, the pump would best be left running.

Bit unnecessary for a non modulating boiler I would have thought.

cheers, Pete.

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Andy Hall 21 years ago

Except that the Keston Celsius 25 is a modulating boiler........

.andy

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Pete C 21 years ago

Ooops, I'm not a boiler spotter :))) Does the temperature control affect CH as well as DHW? I'd have thought it only affects DHW, and the rest of the time the boiler operates in condensing mode where possible.

For DHW it could be useful to have a lower output temp for bath filling, this could be done with a preset and changeover relay.

For more control something could be done with the flow and return thermistors.

cheers, Pete.

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Andy Hall 21 years ago

It isn't a combi either.

I have this functionality on my (MAN Micromat) boiler.

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has an outside temperature sensor as well as an indoor one, and programmable curves for CH operating temperature vs. environmental temperatures. It will thus modulate the CH temperature down to around 40 degrees and 3kW if there is not a great heat demand.

For DHW, there is a temperature sensor in a pocket in the cylinder which starts a hot water reheat cycle when a rapid drop in temperature is detected. In this mode, the boiler is run up to full output to reheat the cylinder rapidly.

.andy

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