Using a dehumidifier as a heater (efficiency of dessicant vs compressor)

Sefi

I am holding off from turning the gas boiler on as long as possible to try and help face down a certain Russian leader.

I noticed the relative humidity in the house is in the range 60-70% all the time and I started to wonder if I could kill two birds with one stone... reduce the humidity (which I worry is affecting musical instruments and heirloom photos), while getting heat from the latent heat of the condensed water.

Some web pages stated that the efficiency of dehumidifiers can be expressed in Litres/kWh and suggested 1.30 L/kWh is a less efficient dehumidifier and 1.7-1.9 L/kWh is a good one.

Now I think I am on firm ground if I assume 1 Litre of fresh water weighs 1kg.

I read that the specific latent heat of water is 2,264.705 kJ/kg

and the Internet also gave me the conversion factor 1 kJ = 0.000277778 kWh

so each 1 litre of water extracted should release 0.629 kWh of latent energy

I also assume all the energy consumed by the dehumidifier must be converted to heat (ok - noise and wind but ultimately they are going to be very small relative to heat and will probably not get out of the room before being converted to heat by friction etc.).

So I concluded that if you run a dehumidifier that does 1.7 L/kWh for long enough to use 1kWh of electricity then output heat is 1+1.7*0.629=2.0693 kWh

i.e. you have halved your electricity cost. That current energy prices for me are 38.14p kWh of electricity and 11.08p kWh gas. A ratio of 3.4.

So it still does not make heating by electricity cheaper than heating by gas... all things being equal (which they are not as the dehumidifier lets me heat a single room, while turning on the gas is going to be heating the bathroom, boiler, etc. so cannot be as efficient if you are just seeking to heat a single room).

So my conclusion was it was a good idea to get a dehumidifier - with visions of having all gas cut off mid-winter and hunkering down in a small room, with it getting humid due to our breath.

After some dithering, I bought the Meaco DD8L model (not the lesser DD8L Junior, nor the more fancy DD8L Zambezi variants). This is a desiccant dehumidifier so should work well in low temperatures (if I want to use it in the garage some time), and is lighter, so more portable between rooms.

I have now had a chance to evaluate it. Overall I am happy with it. If I put it on in the bathroom after a shower for an hour, I avoid condensation and avoid opening the windows (which is how the house loses a lot of heat). It will help when we dry clothes. I can use it to help protect the heirlooms and instruments as I originally intended. A large spider that was inhabiting the bathroom curled up into a ball and died when I ran it on the laundry setting for a couple of hours... a little side benefit.

However - I evaluated it's efficiency. I can weigh the water it extracts, and measure the power consumption using a Kasa Smartplug with the Watt App.

I was surprised to see the efficiency is mostly around 0.38 to 0.4 L/kWh. Far below the figures I had researched on the web and I mention above.

(It equates to putting out 1.25 kWh of heat for every 1kWh of electricity consumed)

I had read that desiccant dehumidifiers are less efficient that compressor ones and that is presumably the main explanation. I don't see specified efficiencies in the specs of the desiccant dehumidifiers I looked at. Perhaps the optimistic efficiency range given (1.3-1.8 or so) only applies to large size whole-house installations? Also, they are presumably for compressor type running in ideal conditions of high humidity and temperature.

Anybody see errors in my calculations? Other experiences?

Leon_W

I've no idea on your calculations but do think that desiccant dehumidifiers make very good single room heaters.  Our's has roughly two settings, 300w and 600w.  Not only does it heat a single room really well, but you also have the advantage of dry air, which feels warmer than a comparable damp room at the same temperature. 

grumbler

If the dehumidifier power is, say, 500W it heats a room like any 500W electric heater and with the same efficiency - 100%.

In fact, the efficiency is slightly higher as condensation is exothermal process. Condensing 1L of water releases about 0.7kWh of heat.

D924

Electricity comes from burning gas anyway. You just have to burn more of it because of transmission and generator efficiency losses. By using electric instead of your gas boiler you are increasing UK gas consumption and helping a certain short and balding man we don't like.

Sefi

grumbler - thanks for summarizing my rather long post. The main thing I was trying to get to was the multiplier you get by running a dehumidifier instead of an electric heater. In the case of desiccant type dehumidifiers it is about x1.25, though I would be interested to hear what other people get with other models and in other conditions.

D924 - That is a good point, almost all the time the marginal electricity in the UK is being generated from gas and I believe electricity generation from gas is only about 50% efficient... and then there are transmissions losses as well. Nevertheless, I think it is still useful to know what uplift on power consumption a dehumidifier provides. There might be times this winter when we have no gas supply. Some people are in apartments where gas is not an option. Also, I still suspect turning on a small dehumidifier in the room I am in, will probably result in less gas consumption (at the generator) than I would burn by turning on the boiler and having it pump heat to the bathroom radiator, the hot water tank, and likely, other rooms as I am probably too lazy to go around turning all the other thermostats down.

Apodemus

Sefi said:

There might be times this winter when we have no gas supply. 

There may be times when electricity generation is switched off to save on our gas supply, but it is very, very unlikely that gas supply to homes would be switched off.  The safety issues around turning it back on in thousands of domestic installations make domestic gas rationing very much a last resort.  When there is a gas outage, they literally have to make every property safe again when it is turned on - and that sometimes includes having to gain entry to houses where the owners might be away for a few days.  Domestic rationing just isn't going to happen under anything but the worst possible scenario and the electricity supply will have gone long before that.