Most efficient electrical heaters


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.
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Broooz wrote:

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 relative humidity.
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 degrees.
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.
--
Sue

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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?
Regards
Bruce
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Broooz wrote:

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.
--
Sue












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OK thank you
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