When to run my boiler

[CCCS]

I’ve got a 90 litre (might be more) unvented cylinder in a four bedroom house. The boiler is set to heat the water for an hour from 4am each day. At the moment I have the central heating on, downstairs only, from 5 - 10 pm.

I now live on my own and have a shower each day. I have a night time electricity tariff.

Would I save money having the water heated for an hour or less in the evening instead?

I realise I’d be paying daytime electricity rate but does having the water heated at the same time as the central heating on save money?

[GasEngineer]

Your cylinder will be well insulated so even if you have it timed to come on all day the boiler will only fire up to heat it for a fraction of that. Even a 200 litre cylinder will only take about 30 minutes to heat from cold, and will stay hot for a couple of days if you don't use the hot water.

So just have the hot water set to come on at the same time as your heating.

Electricity cost is neglible as its only powering the pump and the controls.

[Dr Mike Oxgreen]

Just to be clear, is your boiler electric or gas?

I’m guessing it’s electric, otherwise your electricity tariff is largely irrelevant. Likewise your shower, is it electric or does it use stored water from the tank?

Assuming you’ve got an electric boiler, my instinct is that it makes no sense to make it do additional work heating water at daytime rate when you could heat the water at night rate.

Edited to add: As above, if your boiler is gas it probably makes sense to heat both at the same time. But then I don’t understand why you say “I realise I’d be paying daytime electricity rate”?

Edited by Dr Mike Oxgreen on Saturday 30th November 09:41

[CCCS]

said:

Thank you politely pointing out that I hadn’t switched on my brain. The idea came to me at 4am when the boiler woke me up.

It’s a gas boiler and 240 litre cylinder.

I’m a dumb ass.

[Richard-D]

The potential for benefit will depend on the layout of your system too. My house has a very long run of pipework between the boiler and hot water tank/central heating loop. As this pipework is large diameter (due to the run) there is a decent volume of water that gets heated/wasted with each cycle.

[Dr Mike Oxgreen]

Okay, so with a gas boiler it makes sense to heat both simultaneously, assuming your boiler is powerful enough when it’s a severely cold day. The fact that it wakes you up at 4am is another good reason for shifting the water heating to the evening.

[CCCS]

Richard-D said:

The potential for benefit will depend on the layout of your system too. My house has a very long run of pipework between the boiler and hot water tank/central heating loop. As this pipework is large diameter (due to the run) there is a decent volume of water that gets heated/wasted with each cycle.

It’s only 5 metres.

[CCCS]

Dr Mike Oxgreen said:

Okay, so with a gas boiler it makes sense to heat both simultaneously, assuming your boiler is powerful enough when it’s a severely cold day. The fact that it wakes you up at 4am is another good reason for shifting the water heating to the evening.

I’ll do this, thank you all.

[B'stard Child]

CCCS said:

Dr Mike Oxgreen said:

Okay, so with a gas boiler it makes sense to heat both simultaneously, assuming your boiler is powerful enough when it’s a severely cold day. The fact that it wakes you up at 4am is another good reason for shifting the water heating to the evening.

I’ll do this, thank you all.

There is a potential downside to heating HW at the same time as CH - water is lazy and takes the easiest path

In the case of my system if I tried to do both the HW circuit would take all the flow and during that period the CH would get little to none

I fitted a gate valve to balance the circuit - restrict the flow to the HW cyl but downside was in order to balance the circuits the HW would take much longer than if I separated the HW and CH cycles