New boiler, most efficient way to heat hot water cylinder?

mappy24

Hi,

We had a new condensing boiler fitted yesterday, it replaced our old back boiler. We have retained the hot water cylinder which has now been fitted with a thermostat that has been set by the installer to 65 degrees C. We also have a new programmable timer installed, replacing our old one.

On the old central heating system we used to heat the hot water cylinder for a couple of hours in the morning and this would give us enough hot water to last all day. The way I understand it works now with the thermostat is that the boiler will heat the cylinder until it reaches the desired temperature and then cut off the heat, it will then maintain that water temperature as long as the programmable timer has the hot water turned on.

My question is this:

Is it more efficient to keep the timer on 24/7 so that a constant temperature is maintained in the hot water cylinder?

Or, is it more efficient to revert back to what we had before - just heat the cylinder for a couple of hours in the morning? I'm thinking that if the water in the cylinder is cooler, then it will use more energy to heat it back up to 65 degrees again the following morning?

Any suggestions?

NowRetired

My water heater uses 5 kWh per day to keep my hot water hot when none is used.

3kWh of that is for the pilot light.

I found that out when I forgot to turn the gas off when I went on holiday.

All you can do is try a few days timed and the same days on all the time and of course take meter readings and compare the 2 periods

Cardew

It is cheaper to heat the water for as short a period as it takes to provide sufficient Hot water for your needs.

Having said that, if your tank is well insulated the heat loss is very small. Modern tanks lose about 2kWh per 24 hours when kept at 65C. and that heat isn't 'lost' most of the year as it warms the fabric of the house.

NowRetired

Modern tanks lose about 2kWh per 24 hours when kept at 65C.

Which is what mine loses.

My tank has a foam and cylinder jacket.

However I would prefer it not to heat the fabric of my house in this weather.

Cardew

NowRetired wrote: »

Which is what mine loses.

My tank has a foam and cylinder jacket.

However I would prefer it not to heat the fabric of my house in this weather.

Agreed;)

However it does put into perspective the small difference in savings between Hot Water on 24/7 and timed.

No matter how careful we are with timing HW periods we will still lose some heat from tank - say 1kWh in 24hrs??

rogerblack

Cardew wrote: »

No matter how careful we are with timing HW periods we will still lose some heat from tank - say 1kWh in 24hrs??

Not quite.
If you heat the cylinder to just the required temperature - then use it all - you don't lose any heat over the remaining time - when it's cold.
Needless to say - this is at best tricky.

In addition - if your hot water cylinder is separated from your boiler - then the pipes in between them are heated all the time the boiler is heating the tank - which can be a considerable extra loss.
If you have the tank 3m from the boiler, then that's possibly an extra 400ml/m*2*3m = 2.4l of water that needs heated every time the boiler goes to heat the cylinder up a bit.

This can easily add up to an extra 20 or 30 l of water heating need a day, comparing leaving the cylinder on the thermostat versus just having it on for a short burst to heat up in the morning.

matelodave

The hotter a tank is the more energy it loses, so you are better off just heating it when you need it.

We just heat our tank to 45 degrees in the mornings and use most of it up - we both have a shower in the mornings and the water is hot enough to wash stuff up in the evenings (most stuff goes into the dishwasher). By the next morning the tank temperature is down to about 35-38 degrees and it takes our heat-pump about an hour to reheat it - uses about 2.5kw of electricity a day

southcoastrgi

timed & turn the cylinder stat down to 60 which is what is should have been set at

Richie-from-the-Boro

matelodave wrote: »

The hotter a tank is the more energy it loses, so you are better off just heating it when you need it.

We just heat our tank to 45 degrees in the mornings and use most of it up - we both have a shower in the mornings and the water is hot enough to wash stuff up in the evenings (most stuff goes into the dishwasher). By the next morning the tank temperature is down to about 35-38 degrees and it takes our heat-pump about an hour to reheat it - uses about 2.5kw of electricity a day

45C ? .. .. risky me old M8

lstar337

matelodave wrote: »

The hotter a tank is the more energy it loses, so you are better off just heating it when you need it.

We just heat our tank to 45 degrees in the mornings and use most of it up - we both have a shower in the mornings and the water is hot enough to wash stuff up in the evenings (most stuff goes into the dishwasher). By the next morning the tank temperature is down to about 35-38 degrees and it takes our heat-pump about an hour to reheat it - uses about 2.5kw of electricity a day

I think you should at least up that to 55-60c, unless you like being very unwell.

Legionella optimum growth temperature is between 20c and 45c. Looks like your tank is bang in the danger zone.

You don't need to be drinking the water to get very ill, the vapour inhilation is enough, especially in a shower!

mappy24

The company that fitted the boiler suggested leaving the hot water on all day but after reading this thread I've decided to set the timer to heat the water for 1hr in the morning and 1hr in the afternoon before we get home.

I've also turned the cylinder thermostat down to 60, as suggested.

Thanks all.