Running gas heating and hot water together?

max1000000

Hi there

I have a standard gas boiler (not a combi) which heats my radiators and hot water cylinder.

If the system is heating the radiators, will it use more gas if I tell it to heat the water cylinder at the same time?

In other words, does heating radiators and hot water cylinder together use more gas than just heating radiators alone?

Many thanks

Max

MattMattMattUK

max1000000 said:

Hi there

I have a standard gas boiler (not a combi) which heats my radiators and hot water cylinder.

If the system is heating the radiators, will it use more gas if I tell it to heat the water cylinder at the same time?

In other words, does heating radiators and hot water cylinder together use more gas than just heating radiators alone?

Many thanks

Max

Yes, it uses more energy to generate more heat. 

Depending on your exact setup it may be more efficient to run water and space heating at the same time, but there is no free energy. 

Ultrasonic

Overall, yes it will take more gas to heat up the additional water in the hot water tank.

(Instantaneous gas use may not increase if your boiler is already heating water for your central heating but if so the boiler will need to run for longer and so use more gas in total.)

Ebe_Scrooge

max1000000 said:

Hi there

I have a standard gas boiler (not a combi) which heats my radiators and hot water cylinder.

If the system is heating the radiators, will it use more gas if I tell it to heat the water cylinder at the same time?

In other words, does heating radiators and hot water cylinder together use more gas than just heating radiators alone?

Many thanks

Max

Yes - as Montgomery Scott famously said, ye cannae change the laws of physics.  The heat can only come from one place, and that's from burning fuel.

In terms of how it actually works, it can vary depending on your particular setup.  Commonly, the programmer will prioritise the central heating over the hot water - so it'll heat the radiators first, then when they're up to temperature it'll divert the heat to the hot water tank to heat that up.  If you had the hot water switched off, then the boiler would stop once the rads were up to temperature, and fire up again when needed.

But whether it prioritises the CH, the hot water, or does them both in parallel, yes it'll use more fuel.

Having said that, if you're going to need the hot water later in the day anyway, then it's not wasted heat.  Assuming your hot water tank is insulated properly (modern tanks are manufactured with decent insulation pre-fitted), then it'll stay hot for many hours, with only a negligible amount of heat loss.  So, if you need the water to be heated anyway, it makes no difference whether you do it at the same time as the CH, or set it to come on at a different time.  OK, there may be miniscule differences in terms of mechanical efficiency and heat loss from the pipes etc., but that difference will be so tiny as to be insignificant.

max1000000

That's a great answer, thanks very much. 

So let's say the boiler is on for exactly 1 hour: What you're saying is that it will use more gas in that hour if it attempts to heat both the radiators and the hot water, than it would just heating the radiators alone?

So the cost of exactly one hour of gas varies depending on what the gas is being used for? 

max1000000

To clarify: I set the boiler to go on for exactly one hour. During that hour, you're saying it would cost more to heat the radiators and hot water at the same time than to just heat the radiators? Even though it's still just one hour of gas?

Astria

If your system can heat both at once it can be marginally better to heat them both together than doing them separately but it's definitely going to use more gas as more heat is required.

I find that if I only do the hot water then the boiler will short-cycle as it gets up to maximum temperature very quickly, but combining heating means sufficient heat is drawn away from the boiler so it can run continuously meaning it's more efficient.

Apodemus

It is no more sensible to talk of "one hour of gas" than one hour of electricity or one hour of water.  If you turn all your electrical appliances on at once, you use more electricity.  If you open all your taps, you use more water.  If you run your CH and heat your water (and possibly light a gas fire and cook your tea on a gas cooker) you will use more gas.

MattMattMattUK

max1000000 said:

That's a great answer, thanks very much. 

So let's say the boiler is on for exactly 1 hour: What you're saying is that it will use more gas in that hour if it attempts to heat both the radiators and the hot water, than it would just heating the radiators alone?

So the cost of exactly one hour of gas varies depending on what the gas is being used for? 

max1000000 said:

To clarify: I set the boiler to go on for exactly one hour. During that hour, you're saying it would cost more to heat the radiators and hot water at the same time than to just heat the radiators? Even though it's still just one hour of gas?

Yes, in most cases it would consume more gas/cost you more during that one hour of operation if you were heating both the heating circuit and the hot water circuit, the only way it might not is if the weather was very cold and/or your system was undersized and was already operating at maximum output whilst heating only one of the two. 

There is no "one hour of gas", if can be burnt at different rates. As an example you can drive your car on a motorway for an hour, if you drive at 60mph for an hour you might use £10 of petrol moving the car. If you drive the car for an hour at 60mph pulling a caravan you are moving more weight and increasing drag, you might well find you use £20 of petrol. In both cases you will have had "an hour of driving", but one involved doing twice as much work. 

max1000000

Apodemus said:

It is no more sensible to talk of "one hour of gas" than one hour of electricity or one hour of water.  If you turn all your electrical appliances on at once, you use more electricity.  If you open all your taps, you use more water.  If you run your CH and heat your water (and possibly light a gas fire and cook your tea on a gas cooker) you will use more gas.

Ah but that's the question: are electricity and gas analogous? If a boiler is on for one hour, will it consume more gas the more load you place on it, or does a boiler deliver the same energy in an hour no matter what load is placed on it? 

max1000000

Astria said:

If your system can heat both at once it can be marginally better to heat them both together than doing them separately but it's definitely going to use more gas as more heat is required.

I find that if I only do the hot water then the boiler will short-cycle as it gets up to maximum temperature very quickly, but combining heating means sufficient heat is drawn away from the boiler so it can run continuously meaning it's more efficient.

MattMattMattUK said:

max1000000 said:

That's a great answer, thanks very much. 

So let's say the boiler is on for exactly 1 hour: What you're saying is that it will use more gas in that hour if it attempts to heat both the radiators and the hot water, than it would just heating the radiators alone?

So the cost of exactly one hour of gas varies depending on what the gas is being used for? 

max1000000 said:

To clarify: I set the boiler to go on for exactly one hour. During that hour, you're saying it would cost more to heat the radiators and hot water at the same time than to just heat the radiators? Even though it's still just one hour of gas?

Yes, in most cases it would consume more gas/cost you more during that one hour of operation if you were heating both the heating circuit and the hot water circuit, the only way it might not is if the weather was very cold and/or your system was undersized and was already operating at maximum output whilst heating only one of the two. 

There is no "one hour of gas", if can be burnt at different rates. As an example you can drive your car on a motorway for an hour, if you drive at 60mph for an hour you might use £10 of petrol moving the car. If you drive the car for an hour at 60mph pulling a caravan you are moving more weight and increasing drag, you might well find you use £20 of petrol. In both cases you will have had "an hour of driving", but one involved doing twice as much work. 

So domestic boilers are capable of outputting variable energy? I thought that it heated to one temperature and that the water flowing through it was constant, implying that a boiler hour is a boiler hour no matter what devices hang off it. But you're suggesting it is not.