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

Cardew

kcs940 wrote: »

A heating engineer once told me it would be cheaper to leave the heating on when it is cold.
Think of it like a car engine, when the engine is cold the car uses a lot more fuel than when the engine is hot.
If your heating is switched off the enviroment gets cold, and when the heating is switched on it uses a lot of fuel to heat the enviroment back up.
Everything in your house absorbs heat i.e furniture,carpets etc so all these things have to be heated back up and these things contribute to the heat in a house. A house that is kept at a steady temperature the heating just switches on every so often to keep the temperature steady & so does not use that much when the house is already warm.

Welcome to the Forum.

I wish 'heating engineers' wouldn't spout such nonsense. Actually the mostly don't!

I take it you haven't read the hundreds of posts on this subject? Or looked at the laws of thermodynamics.

Do you keep your kettle simmering all day at a steady temperature?

If you went away for a couple of years and left the property empty would you keep the house at a nice 'steady temperature' so 'the heating just switches on every so often to keep the temperature steady & so does not use that much when the house is already warm'

How about a year? A month? A week? A day? 12 hours? 6 hours?

At what point do the laws of thermodynamics not apply to your property?

Let us put it another way.

If you keep the house at, say, 20C and it is 0C outside you will lose heat at a certain rate and the CH has to replace that heat to maintain the house at 20C.

If the house temp is at, say, 18C, you will lose less heat and so the CH has less heat to produce to maintain that 18C.

The lower the temperature in your house, the less heat it loses, and the less heat needs to be provided to maintain that temperature.

By having the heating timed, as opposed to constant, the house temperature will drop a little and less heat will be lost.

If your heating engineer's theory was correct, why would there be a facility for timed heating on a CH system?

orrery

bestyman wrote: »

Your room thermostat only switches your heating off when the set temperature has been achieved, so turning your thermostatic radiator valves down will mean your radiators will not get as hot and will gently heat your home to the set temperature.

This is incorrect. The radiators get as hot as they would without a thermostatic valve fitted, they simply shut off when the ROOM is at the correct temperature.

It is partly correct. The radiator valves restrict the flow of water gradually as they approach the set temperature - they don't just "shut off". Once the room reaches the target temperature for a period of time, the valves are likely to find a mid position (you may well hear a slight hissing noise from the valves) where the radiator is held at an appropriately warm temperature so as to maintain the correct room temperature.

However, their answer is a little simplistic.

brianben

My house was built in 1984 with cavity wall insulation and loft insulation. I increased the thickness of the loft insulation and have part double glazing. I leave all my doors open and control my heating by the wall thermostat. I don't know if it is cheaper to set the timer for the heating, but don't really care as I would rather be warm and comfortable. When I was young I lived in a house with none of the above and used to have ice on the inside of my bedroom windows. Good old days!!!

oldskoo1

brianben wrote: »

My house was built in 1984 with cavity wall insulation and loft insulation. I increased the thickness of the loft insulation and have part double glazing. I leave all my doors open and control my heating by the wall thermostat. I don't know if it is cheaper to set the timer for the heating, but don't really care as I would rather be warm and comfortable. When I was young I lived in a house with none of the above and used to have ice on the inside of my bedroom windows. Good old days!!!

Whilst this thread is about what's cheaper, what's comfortable is also important.

Like you i like to be comfortable and we have it on from 6am to 10pm. Even though i have a programmable stat that could vary the temperature through the day i don't bother as you end up with cold periods as the stat adjusts downwards. I work from home 3 days and the wife is here the other 2 so for us we need day heating. If we all worked 9-5 away from home i'd defiantly have it on timed.

I would recommend turning it off overnight, there really isn't anything gained by having it on overnight. Its the overnight temperatures which are the lowest in the 24 hour period so heat loss is at its highest. No need to have the heating on for the 8 hours you are in bed. It really is a waste, in my case between 12% and 15% waste / increase in cost.

bestyman

orrery wrote: »

It is partly correct. The radiator valves restrict the flow of water gradually as they approach the set temperature - they don't just "shut off". Once the room reaches the target temperature for a period of time, the valves are likely to find a mid position (you may well hear a slight hissing noise from the valves) where the radiator is held at an appropriately warm temperature so as to maintain the correct room temperature.

However, their answer is a little simplistic.

If you set a rad valve to number 3 which is say 20 C, the rad valve will be fully open until the room air temp gets to 20c, at which point it shuts off.
I have been fitting TRVs for 29 years, I've fitted thousands of them. Please don't correct me for the sake of it.

The original MSE statement which said
Your room thermostat only switches your heating off when the set temperature has been achieved, so turning your thermostatic radiator valves down will mean your radiators will not get as hot and will gently heat your home to the set temperature.

is 100% incorrect.

DragonQ

I have my heating set to come on from 17:00-19:00 right now. Tis probably longer than necessary but on some days I get in at 17:00, and on others I might not get home until 21:00 so having it on for those 2 hours is more convenient than changing it every day.

It's been 6 hours since the heating turned off and, with a little help from my desktop PC, the temperature in my bedroom is the same as the thermostat temperature. Looks like this flat holds heat pretty well, to my surprise.

oldskoo1

bestyman wrote: »

If you set a rad valve to number 3 which is say 20 C, the rad valve will be fully open until the room air temp gets to 20c, at which point it shuts off.
I have been fitting TRVs for 29 years, I've fitted thousands of them. Please don't correct me for the sake of it.

Yes the bit about the radiator temperature is rubbish, the TRV has nothing to do with how hot it gets. Temperature is determined by the boiler temp + losses on route.

However TRV's aren't fully open until 20c for example. The aperture gradually narrows as it nears 20c.

You can hear some TRVs whistle as the aperture narrows as it reaches temperature. Sometimes because the pump speed is too high and the pump isn't resistance sensitive. Sometimes because the TRV is fitted backwards if they only accept flow from one direction.

Either way the whistle is caused by water forced through a gradually narrowing valve.

malc_b

bestyman wrote: »

If you set a rad valve to number 3 which is say 20 C, the rad valve will be fully open until the room air temp gets to 20c, at which point it shuts off.
I have been fitting TRVs for 29 years, I've fitted thousands of them. Please don't correct me for the sake of it.

Well Danfoss say

"Also, TRV control action is modulating as opposed to the on/off cycling of conventional room thermostats. When a wall mounted room thermostat switches off, it must then cycle through its switching differential – temperature difference between switch-on and switch-off points – before heating can be restored. TRV control action is gradual and in proportion to room temperature changes, without any On or Off switching points."

And they should know since they make them :-).

malc_b

oldskoo1 wrote: »

Yes the bit about the radiator temperature is rubbish, the TRV has nothing to do with how hot it gets. Temperature is determined by the boiler temp + losses on route.

That's not strictly true. The water leaves the boiler at say 85C and returns at 75C, if the flow rate has been set correctly. It should not lose much heat along the pipe work so arrives at the rad at ~85C and leaves at 75C. But if the TRV is partially closed then the flow is less so it will arrive at 85C and leave at say 65C. Average rad temperature will not be 80C ((85+75)/2) but 75C ((85+65)/2) so rad will be bit cooler. Note this will also drop the rad efficiency since difference to 20C room is now 55C (90%) not 60C (100%).

The way you balance rads is to measure the temperature drop across them which tells you the flow. Hence the reverse applies, less flow, more drop, lower average.

bestyman

Ok, Keep it simple.
I go into a cold house (say 10c) and in one room set rad valve to 3, in another room set rad valve to 4 and in another room set to 5.
Turn on heating.
All radiators will get EXACTLY as hot until the desired temperature is reached then they will shut off.

By "shut off ", I do not mean suddenly shut off instantly. if set to 20, they may start shutting off gradually at 18, 19 or even 17, but this is irrelevant. The original statement that I highlighted in red is 100% incorrect and the point of my post (as a qualified plumber with 30 years experience) was to point this out to people on the forum so they do not think that setting the TRV to 2 or 3 will result in a lukewarm radiator, which is clearly not the case.

Anyhow gotta go now, someone just rang up with a job, but if I get stuck Ill post back here.