Low cost effective heating for bedrooms

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SepticSceptic

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#1273544 30-Mar-2015 17:08

Aredwood:

Because of your condensation problems a dehumidifier is absolutely essential for you.
-snip
Here is a picture that explains the relationship between relative humidity. (which varies with temperature) And the actual amount of water that is in the air

In this example, if the air is cooled to less than 10deg (or things like the surface of windows are colder than 10 deg) then condensation will start to form. Even though the humidity level was pretty much ideal at 20deg. This is why ventilating the house during the day often doesn't help to reduce condensation at night.

Also heatpumps can only dehumidify when used for aircon (cooling). So the heatpump won't be any help in removing moisture during winter. (but will help only by keeping the house warmer)

So a good quality dehumidifier should be top of your shopping list. The heaters will then be used to just "top up" the heat from the dehumidifier.

And although it doesn't seem right - place the heaters in the coldest part of the room. Which for most rooms will be directly underneath the windows. To understand why, first imagine a room with no heater. The cold window glass cools down the air next to it. Which sinks to the floor. More air moves down to take it's place. And you then have a circulating air current going: Window to floor, along floor to opposite wall. Up wall to ceiling, along to ceiling to outside wall, And then back to window. Now place a heater against the wall opposite the window. The heater will cause this air current to speed up. The moving air will feel colder. The temperature will vary in different parts of the room. And the thermostat in the heater won't be able to respond as well to different heat loads (how cold it is outside).

Move the heater so it is now under the window. It's convection current cancels out the one from the window. The thermostat can now maintain a more accurate air temperature. A stable air temp is more comfortable than one that is constantly changing. This allows you to "get away" with heating the room to a lower temp than you otherwise would. And still consider the heatermate and "fan on top of the heater" as well.

Thanks for this - quite insightful.

TonyR1973

199 posts

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#1273864 31-Mar-2015 00:10

A dehumidifier is a must simply because it's almost a zero-sum game as far as the cost of running it goes. Typically they're only a 30W fan and a 150W compressor, but the duty cycle of the compressor is usually only 25% max. so the power consumption is only really (30W + (150W x 25%)) = ~100W or 3c/h. I've measured our ancient (25 years old) over winter Creda dehumidifier and that's about right. And as it's effectively operating in a closed system because the heating and cooling cycles are taking place in an enclosed space (just like your fridge which if you leave the door open actually overall warms the room, given time to reach a balance), the heat given off from the fan and compressor motors warms the room and the mechanical work done by the compressing of the refrigerant is released and recovered (mostly) to the room (compressed refrigerant loses its heat through the condensor, it expands and cools and that chills the air to condense the moisture to the evaporator which is below dew point which then collects in the water storage). The only direct loss is the energy to move the air - the mechanical work - and is minimal. But in dehumidifying the air, the specific heat (the energy required to effect a temperature change in the air of 1°C) of the air drops which means it requires less heat to warm it so they all but pay for themselves. Then there's the added benefit that the air FEELS warmer with lower humidity simply because the rate of loss of heat from your skin is lower due to the lower specific heat of the air - you'll feel colder in an 18°C room at 80% RH than you will at 18°C and 40% RH because your skin temperature drops faster as it's trying to heat the moist air with a higher specific heat.

So having dehumidified the air, then you just need to heat it.

Firstly, all electric resistance heaters (bar, oil column, fan - if you exclude the cost ot run the fan itself) are, energy wise, the same efficiency. That is to say for every 1W of power they "use" you'll get the same amount of heat from any of them and it's near enough 100%. Forget all the marketing hype about Econoheaters being cheap to run - it's only because they put out less heat! I could make one that uses half the power of their 400W model. It'd be 200W and it'd deliver just 200W of heating so it'd take twice as long to heat the room by 1°C. But I'd rather have a 2200W oil column heater with the thermostat set low because it can heat a room from cold much quicker if needed and it's ¼ of the price to buy. It also cost no more to run when performing the same heating duty.

The only real difference in types of heaters is in how that heat is felt by you. Someone raised the point earlier regarding the difference between radiant heating and convection heating. Radiant is good for localised heating. But for a habitable space you want to circulate the air so you're not huddled around the heater trying to stay warm, fearing moving to another part of the room where the Polar Bears are. For that you need convection. And radiant heaters do have some convection but it's better to give it a helping hand.

Within convection heating there are two ways this works. Firstly, there's natural convection. We all probably know heat rises (warmer air is lower density so the heavier air falls). But it also circulates and mixes to a degree simply due to the inertia in the air as it rises and falls. Adding a means of forcing the air movement helps this process and is termed forced convection. It'll give a more even temperature spread than natural convection alone. Some manufacturers have convection heaters with fans built-in.

But given you already have a fan in the dehumidifier which is moving air, if you have the right dehumidifier with the outlet pointing the right way being horizontally, you can just direct the outlet of the dehumidifier across the fins of a cheap oil column heater. That's what I do.

richms

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#1273869 31-Mar-2015 01:13

I prefer the mica heaters now that I have had one for a while. It radiates a crapload of heat with no glow, and convects as well. So it beams across the room to me and makes me start to feel warm quickly, and also convects the room warm over time but has no fan. Whereas an oil heater you feel nothing till the oil is hot and it starts to convect.

Richard rich.ms

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