Your title says combi, so..
With combi's the most important figure is the flowrate. 11 litres/min is
fine for showes and the odd slow filling bath. Here is a recent post of
For an even better flow rate and cheap too for what you get, assess using
two Worcester-Bosch Junior combi's.
For high flowrates it is cost effective to use two Juniors and combine the
DHW outlets. Worcester-Bosch will supply a drawing on how to do it, or ask
me here. Two Juniors are available for around £1000 to £1100 depending on
what sized units you buy. They have 24 and 28 kW models, you could use two
24kW or two 28 kW combi's or one of each. That is cheaper than the
Worcester HighFlow 18 litres/min floor mounted combi and can deliver about
21.5 litres/min and never run out of hot water. The highest flowrates of
any infinitely continuous combi is 22 litres/min, which is the ECO-Hometec
which costs near £2K.
Have one combi do the downstairs heating on its own programmer/timer
(Honeywell CM67 or equiv) and one do upstairs. Natural zoning, so you don't
have to heat upstairs when you are not up there saving fuel. The running
cost will be approx the same as a condensing boiler heating the whole house.
No external zone valves either, and simple wiring up too. The Juniors are
simple and don't even have internal 3-way valves.
Also if one goes down you will have another combi to give some heat in the
house and DHW too. Combine the outlets for the DHW bath pipes and all the
baths you want very quickly and no waiting. Best have the showers on
separate combi's. It will do two showers no problem at all.
The Juniors are not condensing combi's, yet overall heating costs will be
equivalent to a one condensing boiler as the upstairs will not be heated
most of the time.
A win, win, situation.
Its advantages are:
- space saving (releases an airing cupboard). Both can go in the loft, or
at the back of the existing airing cupboard.
- never without heat in the house,
- high flowrates (will do two showers and fill a bath in few minutes,
- No waiting for a cylinder to re-heat
- Natural zoning, one does upstairs and one does down
- hardly any electrical control work (running a wire to a programmers/stat
and power to each,
- simple no brainer installation,
- minimal components used.
- less piping used
- cheap to run overall as upstairs would be off most of the time
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.
Otherwise, you'll just need to set a high temperature and rely on the
underfloor thermostatic mixing valve to produce a low return temperature,
which may be enough for reasonable efficiency anyway. Should the Micromat's
price put you off, I'm very happy with my Worcester-Bosch Greenstar 28HE,
which is a mid-high end boiler at a much lower price.
For frequent baths and superlative multiple showers, install either an
unvented hot water cylinder (i.e. Heatrae-Sadia Megaflo) or a heat bank
system (i.e. DPS Pandora). If having it installed professionally, you'll
find more installers familiar with the unvented cylinders. However, for DIY
installation, the heat bank will be an easier job, although will require a
higher flow temperature than is optimal.
Your ion exchange water softener should be a high flow type, such as a
Kinetico 2020c HF. Whatever you buy should be metered, not timed.
(http://www.man-heiztechnik.de/_html/produkte/micromat.php ) but all the
information is in German.
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. Any way around this for a mixed
underfloor/radiator solution? (Other than doubling the size of the rads!)
There's an English section if you go to
Look for the Micromat, and yes it is condensing.
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
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
I replied to your email and basically I think you could do three
- 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
To email, substitute .nospam with .gl
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