On 08/03/2016 09:56, Tim Watts wrote:

First thing: do you have a cavity and is it insulated - if not, do that
first.

Then big efficient radiators. Bigger than you think, subject to space.

I've found in both the victorian flat over our shop and our 1960/70
detached house with non insulated floors, when using the "radiator
sizing charts" they've always been wildly over sized. When I sized up
the radiators for the bedrooms at home to work with UFH flow temp. I
needed massive rads. So in the end based on previous experiences I
simply took the biggest single panel finned radiator that would fit in
the space where space wasn't a variable i.e. under windows (single so it
didn't stick right out into the room) and used radiators that "looked"
the right size in the hall and master bedroom.

All chuck out more than enough heat even with flow never exceeding 55
degrees MAX. to bring bedroom up to temp quickly.

Not a very scientific approach but one based on previous results of a
"by the rule book" sizing.

1) Keep the boiler happy avoiding cycling;

Wouldn't an over-sized radiator system make the boiler cycle more than a
system where the radiators are perfectly sized for all but the rare week
or 2 every 10 years or more when temperatures go into minus double-digits?

Easier to add an additional heat source in those rare and exceptional
conditions than to design a system with huge over-capacity that will
rarely be required.


2) Give you a faster warm up from cold so less unnecessary heating;

It's all energy consumed. Bring it up to temp really quick with a high
input or slower with a lower input. A steadier and more constant lower
heat input would be preferable from a comfort perspective.
Weather compensated Boiler keeps temp nice all the time. With UFH it
means the floor is always just right. If I push 55 degrees around all
the time, the floor goes off and cools rapidly (between joists so no
thermal mass) which means a floor that's cold long before the room stat
has dropped to it's low set-point to fire up the zone and re-heat the floor.


3) Allow the heating to run at a lower water temperature which is more
efficient on the boiler (upto a point) and more comfortable to humans
(usually - opinions vary).

Definitely. And weather compensation is an absolute must-have for both
economy and comfort.


For example - a house I have with 2" cavity wall blown fibre (old) and
3" celotex in the roof requires a total of about 3-4kW electric heaters
to maintain steady state 21C downstairs when outside is 5C ish.

However, it takes a day to 2 days to come up from stone cold with 4kW.

So I will be aiming to put in a total about 5 times that at least in
terms of max output power of radiators at the usual delta-T, then I can
drop the water temperature (and thus the delta-T) a bit too.

When would you ever need to heat from stone-cold? I'd always leave the
heating set to perhaps 15 degrees if I could be arsed to re-program all
the room stats which I wouldn't.