On Thu, 08 Sep 2005 11:48:25 +0100, Dave Fawthrop
wrote:

On Wed, 07 Sep 2005 18:39:33 +0100, Andy Hall wrote:

| The second point is that the existing radiator sizings will have been
| designed on the basis of 82 flow, 70 return and a worst case outside
| temperature of -1 or -3 degrees. However, these days are relatively
| rare, and most of the time, the output needed to keep the place warm
| is a lot less. So for at least 9 months plus of a typical year, the
| boiler will modulate down as will temperatures to more efficient
| ranges. When needed, the boiler will be able to deliver 82/70. Even
| though it won't be condensing, because it will have a better heat
| exchanger anyway, it will be more efficient boiler than the old one.

My house is so well insulated that it stays nice and warm on radiators
rated 3kw downstairs to which I switch another 1.5 kw upstairs in very cold
weather. These are similar to Stelrad Elite P1
http://www.stelrad.com/UK/elite.html from which I cribbed the ratings. I
did the heating calcs for these 25 years ago, since which I have improved
the insulation, so I believe that I could now use smaller radiators. I
have uses a 10 kw conventional boiler, for Central heating and domestic hot
water, without problems for 25 years.

My problem is that the smallest combi boiler I can find is the eco Hometec
EC16S which is rated by SEBUK at 22.0kw regulating down to 3kw. The
companies literature rates it at 16kw ish modulating down to 2.5 or 3.1kw.
Clearly much too big for my house.

Surely through most of the year, on minimum modulation this will cycle on
and off and loose most of its condensing advantages. DHW is OK

This is actually the MAN Heiztechnik Micromat. I have one of the
larger versions, but of course not a combi.

The control system is more sophisticated than most boilers in that
there is also weather compensation and slope control.

This means that it will modulate down such that the flow temperature
is at around 40 degrees. The pump also reduces in power to as low as
20% of full run rate.

Once you fall below the minimum 3kW modulation level, the boiler will
cycle, but at this low temperature on a very long time constant.

Since your existing boiler is a 10kW one, it will be cycling on the
thermostat at full power and 82 degree operation. I would suspect
that there is quite an overshoot on flow temperature as well.

Running the burner at 3kW and 40 degrees means that the boiler is
operating well into the high efficiency slope anyway and spare heat is
distributed within the building. Much of the efficiency loss through
cycling with conventional boilers cycling is flue loss when the boiler
no longer should be providing heat.



--

..andy

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