On 30/11/2017 11:45, T i m wrote:
On Wed, 29 Nov 2017 12:31:42 +0000, Martin Brown
wrote:

snip

All we need is a heating engineer that knows stuff, both theory and
in practice
Â*Â* It was 12C @ 9am, now 16C @ 13:45.

This is with 5 of 2KW heaters and 2 ofÂ* 1.5KW heaters.

So if yuo wanted the demp to be 16C before the 10am class how many
heaters would we need in total. ?

You need to do the test with two different total power levels to
determine unambiguously what the thermal capacity and other losses are.

John gave him all the help for calculating the room losses but I'm
pretty sure it was more than he could be bothered with (and would
rather bite the hand that feeds him here in the big gaps between
leaning the labs between classes that do that).

Some much for engineering education in the UK.

The simplest way out would be to use a couple of 2kW air curtain heaters
over the lab doors which will rapidly warm the *air* inside the lab to a
comfortable temperature

He has said fan heaters are not allowed (as it might melt the
snowflakes) but I'm not sure if an air curtain (that I think I also
suggested) would count as such.

It would be by far the simplest solution for a largish laboratory a bit
noisy when heating things up from cold but once the air is warm a
relatively modest heat input from radiators can hold it there
(especially once the room is occupied by a decent number of people).

Our village hall has essentially the same problem of wanting to get it
to working temperature in the shortest time with the least energy usage
and only having electric heating available.

instead of these useless convection heaters
heating up the wall behind them and air that immediately rises up to the
ceiling.The room fills with warm air slowly from the top down.

But if they have low ceilings and people moving about?

Unless they are walking round on the ceiling their feet will freeze.


Any kind of fan heater would be way better than oil filled radiators if
you want to get the room habitable in the shortest possible time.

They do but can't (not allowed).

OK then a cold fan pointed at the useless convection heater(s).
Strange that an organisation that prohibits fan heaters permits
overloading of the lab ring main without a second thought.

Those propane powered fan flame things for heating big garage spaces are
pretty good and fast if you don't mind the smell of combustion products.
(I find them a bit scary YMMV)

If fan / convectors heaters aren't allowed ... !

I would still be interested to hear the outcome of the test of leaving
*just* the 1300W element on, to see how long it takes before it
overtemps (if it does at all).

Chances are that upping the power for some of the time will always
result in more heat output overall. The problem is that they are losing
heat too fast and heating up the wrong things mostly the air against the
ceiling and walls behind the oil filled radiators. Some fans on the
ceiling to push the warm air down would help a bit.

I've ordered a couple of electronic power controllers to experiment
with my own rads [1]. The aim is to limit the power to that that they
can actually dissipate as they are only going to be used to provide
'background' heating in the bedroom and I'd rather not hear either of
the stats clicking in and out all night. ;-)

If you don't mind being dirty about it you could do what the electric
blanket people do with a diode in the supply to halve output on low.

--
Regards,
Martin Brown