Energy myth-busting: Is it cheaper to have heating on all day?

Cardew

50Twuncle wrote: »

Do dehumidifiers warm up the air in a house or not ?
I think that less humid air FEELS warmer than damp air - but is this a myth or not ?

Dehumidifiers produce heat when working. How much heat depends on the rating of the appliance and how long it is running.

In hot conditions, humid air feels much warmer than dry air. If you have ever been to a tropical climate(e.g. Hong Kong, Singapore etc) a temperature of, say, 32C(90F) with 90% humidity is almost unbearable with sweat pouring off most people.

Yet in desert areas with very low humidity(e.g. Arizona) 32C is comfortable.


Much has be written about the perception of warmth/humidity. You need to Google 'heat index' or the simple explanation is here:;)

Formula[edit]

The formula below approximates the heat index in degrees Fahrenheit, to within ±1.3 °F. It is the result of a multivariate fit (temperature equal to or greater than 80°F and relative humidity equal to or greater than 40%) to a model of the human body.[8][9] This equation reproduces the above NOAA National Weather Service table (except the values at 90°F & 45%/70% relative humidity vary unrounded by less than -1/+1, respectively).
\mathrm{HI} = c_1 + c_2 T + c_3 R + c_4 T R + c_5 T^2 + c_6 R^2 + c_7 T^2R + c_8 T R^2 + c_9 T^2 R^2\ \,
where
\mathrm{HI}\,\! = heat index (in degrees Fahrenheit)T\,\! = ambient dry-bulb temperature (in degrees Fahrenheit)R\,\! = relative humidity (percentage value between 0 and 100)c_1 = -42.379, \,\! c_2 = 2.04901523, \,\! c_3 = 10.14333127,\,\! c_4 = -0.22475541, \,\! c_5 = -6.83783 \times 10^{-3},\,\! c_6 = -5.481717 \times 10^{-2},\,\! c_7 = 1.22874 \times 10^{-3}, \,\! c_8 = 8.5282 \times 10^{-4}, \,\! c_9 = -1.99 \times 10^{-6}.\,\!
An alternative set of constants for this equation that is within 3 degrees of the NWS master table for all humidities from 0 to 80% and all temperatures between 70 and 115 °F and all heat indexes < 150 °F is: c_1 = 0.363445176, \,\! c_2 = 0.988622465, \,\! c_3 = 4.777114035, \,\! c_4 = -0.114037667, \,\! c_5 = -0.000850208, \,\! c_6 = -0.020716198, \,\! c_7 = 0.000687678, \,\! c_8 = 0.000274954, \,\! c_9 = 0 \,\! (c_9 \,\! unused).

A further alternate is this:[10]
\mathrm{HI} = c_1 + c_2 T + c_3 R + c_4 T R + c_5 T^2 + c_6 R^2 + c_7 T^2 R + c_8 T R^2 + c_9 T^2 R^2 + c_{10} T^3 + c_{11} R^3 + c_{12} T^3 R +
c_{13} T R^3 + c_{14} T^3 R^2 + c_{15} T^2 R^3 + c_{16} T^3 R^3\ \,

where
c_1 = 16.923, \,\! c_2 = 0.185212, \,\! c_3 = 5.37941,\,\! c_4 = -0.100254, \,\! c_5 = 9.41695 \times 10^{-3},\,\! c_6 = 7.28898 \times 10^{-3},\,\! c_7 = 3.45372\times 10^{-4}, \,\! c_8 = -8.14971 \times 10^{-4}, \,\! c_9 = 1.02102 \times 10^{-5},\,\! c_{10} = -3.8646 \times 10^{-5},\,\! c_{11} = 2.91583 \times 10^{-5},\,\! c_{12} = 1.42721 \times 10^{-6},\,\! c_{13} = 1.97483 \times 10^{-7},\,\! c_{14} = -2.18429 \times 10^{-8},\,\! c_{15} = 8.43296 \times 10^{-10},\,\! c_{16} = -4.81975 \times 10^{-11}.\,\!
For example, using this last formula, with temperature 90 °F (32 °C) and relative humidity (RH) of 85%, the result would be: Heat index for 90 °F, RH 85% = 114.9

Swipe

50Twuncle wrote: »

Do dehumidifiers warm up the air in a house or not ?

Yes, they do. Especially the desiccant ones which contain a small heating element which is why they cost a little more to run than the ones that use compressors.

inicholson

The heating engineers who installed my heating insisted that I didn't need a room thermostat because all the radiator valves are thermostats and there's a timer on the boiler so it only comes on when I want it.

I gave up arguing and let them install it. The system was fine all winter until I woke up on a warm spring morning to find the boiler firing and the radiators hot. I switched the boiler off and set the timer not to come on in the mornings. 2 days later the weather changed and I woke up in a freezing cold house.

I bought a combined timer/thermostat so now my heating comes on before I get up, goes off before I got to work, comes on again before I get home and goes off again shortly before bed time. But, it only comes on if it's cold!
In theory it comes on during the night or while I'm out if the temperature in the house falls below 15 Celsius - but this never happens.

Heating engineers are idiots.

inicholson

Also, a disturbing number of people don't understand thermostats. My mother in law turns her up to maximum until the house it warm enough and then turns it down. She's adamant that this makes the house heat up faster. It doesn't - the thermostat just switches the boiler on or off, it doesn't affect the temperature of the water in the radiators (there's a setting on the boiler to do this - I turned mine down).

I know she's not alone in believing this.

orrery

50Twuncle wrote: »

Do dehumidifiers warm up the air in a house or not ? I think that less humid air FEELS warmer than damp air - but is this a myth or not ?

A proper dehumidifier - one with a 'fridge' mechanism - does produce a significant amount of heat. The first heat exchanger panel cools the air (and the condensation either freezes on or runs off at this stage) and the other panel returns this heat to the air before it is expelled. This is all driven by a compressor, motor and fan which produces heat. Add to that the fact that the condensing process gives up heat too.

I'm ignoring other types of dehumidifier mechanisms here - the Peltier Effect ones are sold quite cheap, but I've found them completely ineffective.

Cardew

inicholson wrote: »

Also, a disturbing number of people don't understand thermostats. My mother in law turns her up to maximum until the house it warm enough and then turns it down. She's adamant that this makes the house heat up faster. It doesn't - the thermostat just switches the boiler on or off, it doesn't affect the temperature of the water in the radiators (there's a setting on the boiler to do this - I turned mine down).

I know she's not alone in believing this.

Whilst you are quite correct about it not affecting the water temperature, there might be some validity for your Mother in law's reasoning.


Depending on the siting of the wall thermostat, when that room reaches the set temperature it(as you say) turns off the CH. Thus other rooms which might not be warm enough for your Mother in law will get no warmer.


By turning up the thermostat at least she can ensure she gets the whole house warm to her satisfaction.


It is the age old problem of a fixed thermostat. Most people have it in a living room or the hall and that location determines the heating for the whole house.


P.S.
Please note 'Might' have some validity.

Cardew

inicholson wrote: »

The heating engineers who installed my heating insisted that I didn't need a room thermostat because all the radiator valves are thermostats and there's a timer on the boiler so it only comes on when I want it.

I gave up arguing and let them install it.


Heating engineers are idiots.

When was this CH system installed?


The system you describe was common 30 years ago - no wall thermostat, TRV's on all radiators - which required a bypass valve.


However for many years Building Regulations have laid down that a wall thermostat must be installed.


It was normal practice for a radiator in the same location as that thermostat not to have a TRV(not sure if that was mandatory). In any case the popularity of wireless remote thermostats, that can be moved from room to room, make that 'no TRV on one radiator' practice meaningless.

fredthefish

... combi boilers tend to be less efficient at heating water in the first place, so if you’re a large household using lots of water it might be cheaper to have a standard boiler with a well-insulated tank.

It's also worth thinking about practicality though too. If you are a large household using lots of water, you don't want to constantly be in a situation where the last person in the shower in the morning is stuck with a cold shower once the hot water tank has emptied! Even worse if they regularly have visitors (both my partner's parents and mine have this problem any time anyone visits).

Combi boiler all the way personally.

Cardew

fredthefish wrote: »

It's also worth thinking about practicality though too. If you are a large household using lots of water, you don't want to constantly be in a situation where the last person in the shower in the morning is stuck with a cold shower once the hot water tank has emptied! Even worse if they regularly have visitors (both my partner's parents and mine have this problem any time anyone visits).

Combi boiler all the way personally.

Well conventional wisdom is that large households using plenty of water are better off with a HW tank.


The problem with a combi is that in winter they struggle to produce a good flow for a shower even if that is the only HW demand.


They certainly cannot cope with two bathrooms, which will be the case in many 'large households'. Even with a single shower, if someone turns on a HW tap for a sink, the flow of HW to the shower is reduced.


I have a HW tank in my house and a combi in a separate annex and without doubt the HW tank system is my preferred option.


Mind you this debate has gone on endlessly.

HappyMJ

fredthefish wrote: »

It's also worth thinking about practicality though too. If you are a large household using lots of water, you don't want to constantly be in a situation where the last person in the shower in the morning is stuck with a cold shower once the hot water tank has emptied! Even worse if they regularly have visitors (both my partner's parents and mine have this problem any time anyone visits).

Combi boiler all the way personally.

The reheat times are so quick that hot water here in my house never runs out. Leave the boiler on all the time and when the cold water in the bottom of the cylinder reaches the thermostat the boiler will come on reheating the cylinder. It takes about 30 minutes to an hour from a cylinder of cold water to reheat which unless the occupants have a shower straight after each other without a break the hot water will never run out. I'm usually in the bathroom for more than 10 minutes after I've had my shower so there's plenty of time for it to reheat.

The other problem I've had is when the boiler has had a problem and is not working. There is not hot water. However, I have an immersion heater and can just turn that on getting hot water again until I can get an engineer in.