On Fri, 16 Apr 2004 00:19:06 +0100, "John Aston"
wrote:


"Christian McArdle" wrote in message
. net...

Your main issue here is choosing a boiler that can maintain a low flow
temperature for the underfloor for efficiency reasons, whilst giving a
high
temperature to the hot water system and radiators. I believe the MAN
Micromat can be set up to do this, and Andy Hall may be able to advise on
such a system.

Is the Micromat a condensing boiler? I've traced the manufacturer's website
(http://www.man-heiztechnik.de/_html/...e/micromat.php) but all the
information is in German.

There's an English section if you go to

http://www.man-heiztechnik.de/index_e.php

Look for the Micromat, and yes it is condensing.



From what I read on this newsgroup, the condensing boiler will not operate
at its best efficiency if it has to heat water up to the temperature
required for conventional radiators.

Yes and no.

You size radiators to meet the worst case heat loss - ie. a cold
winter's night. With a non-condensing boiler, this is done at 82
flow, 70 return.

A condensing boiler's efficiency increases with reducing return
temperature. When the dew point around 54 degrees is reached, the
graph of efficiency vs. temperature shows a knee in the curve and the
efficiency increases at a greater rate below this as condensing
happens. It is not correct to say that there is a sudden step change
in condensing mode - purely a rate of change - so 53.5 is not hugely
better than 54.5

When the weather is warmer, the weather compensator sensor causes the
flow temperature and return temperature to be reduced by reducing the
boiler burn rate and pump speed. Thus it will be working at higher
efficiency anyway.

If you size the radiators larger, you can run the flow at a top
temperature of 70 (return 50) and the boiler will modulate down
further. Therefore it is true to say that it will generally be more
efficient.


Any way around this for a mixed
underfloor/radiator solution? (Other than doubling the size of the rads!)


I replied to your email and basically I think you could do three
scenarios.

- 85 degree radiator design and UFH connected using a blending valve

- 70 degree radiator design and UFH via blending valve

- 50 degree for both. However, the radiators would then be quite
large.


..andy

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