Combi-boiler hot water - pls settle argument!

Lozenger

Is it hugely expensive to leave the water on all day?

We have a fairly new combi (Highflow 440) and my other half is used to the old tank system and goes crazy at me if I dont immediately switch the hot water off as soon as I have had a bath... I thought the whole point of it was instant hot water but he insists on it being off til we need it (then I forget to switch off).

It's causing arguments - please help!!:D

stokefan

nope, you can leave the combi boiler on all day, when you switch the water taps on, the boiler lights when you switch the water off the boiler stops, you just pay a bit more electric because the boiler is on standby all day

jb66

combis are designed to be left on all day/night

aelitaman

I have one of these boilers and it actually uses a hot water reservoir to get a high flow rate and instant hot water. It keeps this small tank of water hot 24 hours if the hot water control in the bolier is on.

Like the sad person I am I measure the gas used to keep the tank hot for three weeks and then measured the gas used if I switched the hot water on just for my showers over the another 3 week period.

I was using 32Kwh pr week with the hot water turned off after shower and 65 KWh per week with the hot water control on 24/7.

So by my calcs it was using 33Kwh per week to keep the internal hot water tank hot. 4.7 KwH per day which on my gas rate costs 10p per day or 35 pounds per year.

Lord_Blue

aelitaman wrote: »

I have one of these boilers and it actually uses a hot water reservoir to get a high flow rate and instant hot water. It keeps this small tank of water hot 24 hours if the hot water control in the bolier is on.

Like the sad person I am I measure the gas used to keep the tank hot for three weeks and then measured the gas used if I switched the hot water on just for my showers over the another 3 week period.

I was using 32Kwh pr week with the hot water turned off after shower and 65 KWh per week with the hot water control on 24/7.

So by my calcs it was using 33Kwh per week to keep the internal hot water tank hot. 4.7 KwH per day which on my gas rate costs 10p per day or 35 pounds per year.

Wow! That's some major working out there.

mech_2

aelitaman wrote: »

I have one of these boilers and it actually uses a hot water reservoir to get a high flow rate and instant hot water. It keeps this small tank of water hot 24 hours if the hot water control in the bolier is on.

Like the sad person I am I measure the gas used to keep the tank hot for three weeks and then measured the gas used if I switched the hot water on just for my showers over the another 3 week period.

I was using 32Kwh pr week with the hot water turned off after shower and 65 KWh per week with the hot water control on 24/7.

So by my calcs it was using 33Kwh per week to keep the internal hot water tank hot. 4.7 KwH per day which on my gas rate costs 10p per day or 35 pounds per year.

Interesting. I tried this and only seemed to save about 7kWh a week. I suspect mine doesn't store as much water. However I also saved 2.2 kWh of electricity a week because the boiler electronics were no longer on standby. This is worth about the same amount in addition, so I saved about 40p a week on my current tariff.

Don't forget that for approximately 6 months of the year the boiler will be left on for heating anyway, so the saving is likely to be only half the yearly figure.

I decided it just didn't save enough to be worth the hassle in the end. But it was interesting putting a figure to it (or maybe I'm just odd that way).

Cardew

aelitaman wrote: »

I have one of these boilers and it actually uses a hot water reservoir to get a high flow rate and instant hot water. It keeps this small tank of water hot 24 hours if the hot water control in the bolier is on.

Like the sad person I am I measure the gas used to keep the tank hot for three weeks and then measured the gas used if I switched the hot water on just for my showers over the another 3 week period.

I was using 32Kwh pr week with the hot water turned off after shower and 65 KWh per week with the hot water control on 24/7.

So by my calcs it was using 33Kwh per week to keep the internal hot water tank hot. 4.7 KwH per day which on my gas rate costs 10p per day or 35 pounds per year.

Whilst I accept these figures are posted in good faith, I really find those figures hard to believe.

The specification for Hot water tanks in conventional(non-combi) CH systems usually give their heat loss measured to a BSI standard.

Esentially this is the loss of heat over a period of 24 houirs for a full tank of water kept at 65C with the ambient temperature at 20C(not certain of this figure)
A well lagged tank of approx 150 litres will lose between 2kWh and 3 kWh in 24 hours(some even less)
In practice as the water is never at 65C for 24/7 the losses are much lower than this.

So I really do find it difficult to believe that the small reservoir in a combi could lose 4.7kWh a day.

It would negate the main advantage a combi has over a conventional CH system with a Hot Water tank.

Does not the combi boiler spec give the tank losses?

aelitaman

Cardew wrote: »

Whilst I accept these figures are posted in good faith, I really find those figures hard to believe.

The specification for Hot water tanks in conventional(non-combi) CH systems usually give their heat loss measured to a BSI standard.

Esentially this is the loss of heat over a period of 24 houirs for a full tank of water kept at 65C with the ambient temperature at 20C(not certain of this figure)
A well lagged tank of approx 150 litres will lose between 2kWh and 3 kWh in 24 hours(some even less)
In practice as the water is never at 65C for 24/7 the losses are much lower than this.

So I really do find it difficult to believe that the small reservoir in a combi could lose 4.7kWh a day.

It would negate the main advantage a combi has over a conventional CH system with a Hot Water tank.

Does not the combi boiler spec give the tank losses?

As you know there is considerable debate on these boards between your mathematical calculations and what people measure in practive with regards to whether they have have the immersian on all day or only to heat water for a certain time.

First point is that it is not just the heat loss from the tank to take into account of as instead of boiling a kettle for hot water I would turn the hot tap on when I was running the DHW 24/7. So this will use extra gas.

Also my results may be sqewed because my boiler is in the old coal celler outside my house and it is a big house so I have long runs to my taps in the kitchen and bathroom which means that the intial hot water goes through a long period of being cooled before it hits the taps and when finished there will be the hot water left in the pipes to go cold.

I have just timed as to how long it takes for hot water to hit my kitchen tap and it is 60 seconds. So with a flow rate of 12 litres per minute (measured as well) this means that just to get hot water every time I turn the tap on I waste the energy to heat 12 litres of water as this is just left in the pipes hot at the end.

I accept that if the boiler is in the kitchen or close to the bathroom and only has 1 meter run to the taps the energy difference would be lower than mine.

But these are my measured results over a decent time period.

Cardew

aelitaman wrote: »

As you know there is considerable debate on these boards between your mathematical calculations and what people measure in practive with regards to whether they have have the immersian on all day or only to heat water for a certain time.

First point is that it is not just the heat loss from the tank to take into account of as instead of boiling a kettle for hot water I would turn the hot tap on when I was running the DHW 24/7. So this will use extra gas.

Also my results may be sqewed because my boiler is in the old coal celler outside my house and it is a big house so I have long runs to my taps in the kitchen and bathroom which means that the intial hot water goes through a long period of being cooled before it hits the taps and when finished there will be the hot water left in the pipes to go cold.

I have just timed as to how long it takes for hot water to hit my kitchen tap and it is 60 seconds. So with a flow rate of 12 litres per minute (measured as well) this means that just to get hot water every time I turn the tap on I waste the energy to heat 12 litres of water as this is just left in the pipes hot at the end.

I accept that if the boiler is in the kitchen or close to the bathroom and only has 1 meter run to the taps the energy difference would be lower than mine.

But these are my measured results over a decent time period.

Firstly, they are not my 'mathematical calculations'. I am merely quoting the heat loss specifications that in most cases is printed on the tank.

However your 'back to back' comparison test illustrates perfectly why your figures are not representative.
You cannot compare consumption where for one set of results you turn on a hot tap, especially when you waste '12 litres' of hot water, to get hot water, and in the other set of results you boil a kettle to get hot water.

It is rather like saying I use 6 gallons of petrol in a week when I drive to work, and only 3 gallons when I catch the bus to work.

aelitaman

I answered the question that the OP asked "Is it hugely expensive to leave the hot water on all day with a combi."

Possetjohn

Hi all,

Followed this discussion with interest!

Cardew while I agree with your comments about the gas consumptions with in aelitaman's post surely the heat losses in the small storage tank in some combi boilers is not the only thing you have to consider. Every time the water in this tank drops below the preset temperature the boiler has to fire again to reheat it, and therefore reheat the water in the primary and direct hot water heat exchangers.
Although the natural convection in these forced air primary heat exchangers is low there will be some and because of the inherent efficiency of the condensing heat exchangers. I think this is the part which will give the higher gas consumption/heat losses.

My first year's experience of these combi boilers shows the production of hot water is quite quick even after a cold start in the summer so I would not bother with it.

My next task will be to improve the pipe runs in the downstairs part of the house to minimise the dead volumes in the hot water pipes