Running gas heating and hot water together?

MattMattMattUK

max1000000 said:

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?

In this comparison, yes.

max1000000 said:
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? 

Gas consumption is load dependant. 

Astria

max1000000 said:

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.

Most gas boilers are capable of outputting variable energy depending on the output temperature and return temperature. For example, if the requested output temperature is 60c and the return temperature is 40c and it can't raise the water by 20c, it will burn more gas to heat the water faster, once the house is up to temperature and the return is more like 50c, then it will burn less gas as the difference is less so has less work to do.

MattMattMattUK

max1000000 said:

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.

Boilers are capable of operating at different levels, they tend to have a fixed output temperature (often set by the user, ideally to hit condensing), but will vary the burn depending on the input temperature of the water, the ambient temperature etc. If the unput flow is 5c that will require a higher burn than 15c to reach an output temperature of 55c.

max1000000

Brilliant, thanks very much. A lot of education in a short time - this forum is fantastic.

oldagetraveller1

Most modern gas boilers "modulate". That is, fancy electronics control their output.

If you listen to the boiler when starting from a cold system it will be noisy i.e. the fan runs faster in tandem to the increased gas supplied to the burner. As the sytem water reaches the desired temperature, regulated by the boiler thermostat, it will "modulate" down. The fan will run slower and the gas demand reduces - that makes the noise from the boiler almost undetectable.

 

Lorian

My system boiler modulates down well(10:1) It's on Y plan wiring  though so no hot water priority. I keep the flow temp only just high enough to heat the tank properly. The tank is old and could do with a bigger coil so if the system was heating just the water then deltaT is pretty low and the boiler wouldn't condense so I've set the water to be heated at the same time as the house is getting towards being up to temperature in the mornings. This means the flow is being split between water and heating around the time the heating demand is reducing due to TRVs switching off, somewhat optimising the return temp.

Glum

max1000000 said:

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

If my 'regular' non-condensing boiler is running constantly for one hour, it doesn't matter if it's heating hot water or radiators. As you suggest, it just uses gas for one hour at the same rate. It doesn't modulate or do anything clever like reducing the amount of gas depending on the load. So, without knowing the model of your boiler, it's hard to know your mileage.

max1000000

Since this group seems to have well above average knowledge, may I please ask for views on the old debate about whether I should turn down the heating to 16 degrees when I go to sleep and then setting it back to 21 degrees before I wake up? 

Thanks

Max

Lorian

Generally sensible as long as you have warm quilt/bedclothes. I imagine there aren't many MSE'ers who have their heating on while sleeping. 21c would be a bit warm for me but that really is personal choice influenced by age and activity levels.

max1000000

Lorian said:

Generally sensible as long as you have warm quilt/bedclothes. I imagine there aren't many MSE'ers who have their heating on while sleeping. 21c would be a bit warm for me but that really is personal choice influenced by age and activity levels.

Thanks. I guess a question is whether it is possible to drop it too far during sleep because it has to work too hard in the morning to get the temperature back up.

For example, if the temperature is 4 degrees overnight, is it better to drop the thermostat to, say, 16 degrees or to 13 degrees before raising it to 21 degrees in the morning? 13 to 21 is a lot more heating than 16 to 21.