Please can you recommend which electrical heaters are most efficient - there
seem to be panel or tube for outhouses or convection or oil filled.
I am looking to heat a garden wooden office that I think is reasonably well
insulated so I want an efficient background heater for the winter for those
times when we are not in it.
I also want to avoid any damp - is there a minimum temperature that I should
aim for? The frost settings of 5oC seem a bit too low.
All electrical heaters are equally efficient in turning electrical power
into thermal energy. What varies is the output temperature and the
transfer mechanism to the surrounding - eg by convection, conduction or
radiation or a mixture of them. A mix of convection and a bit of
radiation is normal for free-standing background heaters.
The problem with background heating of a garden wooden office is that of
distributing the heat to all parts of it. It is the temperature of the
coldest part that is significan (see below). Using a relatvely large
number of small heaters will require less electrical power to achieve
the same minimum temperature throughout. Heating mats or heating cables
have the lowest running costs, for effective coverage. A number of
small tubular (greenhouse) heaters in the potential cold spots is better
than one central heater.
The temperature needed to avoid condensation and damp depends on the
A relative humidity of 100% means dew point is same as air temp.
Everything is going to get damp and rust. The only answer is air
conditioning to reduce the relative humidity.
For 90% RH dew point is 3 degrees Fahrenheit lower than air temp. For
every 10 percent lower, dew point drops 3 deg.
Let's take the case when there is a lot of movement into and out of the
office. The air in the office will, at times and in places, have the
same temperature and humidity as outside air. So, measure the relative
humidity or look it up on a weather site. Let's say that it is 90% and
40 degree F. The water in the air will condense out where-ever in the
room the temperature is less than 3 degrees F from the current outdoor
air temperature. So, if the current outside air temperature is, say, 40
degrees, you need to ensure that nowhere in the room is less than 37
Let's take the case when there is little movement into and out of the
office. Measure the room temperature and relative humidity. Let's say
the humidity is around 50%. The room air temperature is 60 degrees. You
simply have to ensure that no part of the room is less than 45 degrees.
So, the setting of you thermostat will depend on how you use your
office, the range of temperatures and relative humidity at your location
and the location of your heater(s)....
You can buy an (inexpensive) RH meter. And a thermometer. Fit a
thermostat (ideally to the coldest spot in the room). Set the thermostat
according to the RH and air temperature. You can set it to the worst
case value for the whole Winter, or something lower, depending on how
much you really don't want things getting damp.. Or vary it, week, by
week, as conditions change.
Thank you very much for a very comprehensive and helpful reply.
Presumably if the door isn't left open on a wet or damp day and the building
is reasonably waterproof, is it possible to predict RH? Or is the only
solution trial and error using the method you have suggested?
I'd assumed that it was a wooden building, with unsealed wood on the
inside and that it could be exposed to a lot of sunshine on a warm
Winter's day. I wouldn't even like to hazard a guess as to how the RH of
the air in the building will vary over a Winter - except that there
could be times when it is quite high but most of the time will be quite
low. But why guess or attempt to predict when a simple measurement will
tell you? Once you have a feel for how the conditions vary in your
building, you will be able to predict things pretty well.
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