Domestic Hot Water

pip895

How much energy do you put in to heating your domestic hot water? 

I always assumed for us it was pretty low - I believe the UK average is <2KW/day/person in the household.

We have a solar thermal system so on a sunny day the sun provides all we need.  I have just done some analysis using the detailed gas use data from our smart meter and it looks like we are using about 12kWh a day averaged over the year.  This is about 3 times more than I was expecting - there are 4 adults in the household all have daily showers but not long ones - DD has perhaps 1 bath a week we use a condensing gas boiler with a 60deg flow temp. 

lohr500

That sounds high if your thermal solar is providing all you need on sunny days.

I use oil to heat our hot water tank and we have a large bath and a daughter who spends ages in the shower. Both the bath and the shower are used daily. Plus general washing up and hot water for hand washing.
Through the summer we have been using an average of 1.5 litres of oil per day, which with a 90% boiler efficiency should work out at around 13 to 14kW.

Veteransaver

Boilers aren't particularly efficient at heating hot water, the achieve nowhere near their claimed 90% efficiency. It's because they need to run at a higher temperature so don't achieve condensing for much of the heating cycle. Also significant losses in the pipe work to and from the cylinder.

QrizB

pip895 said:

How much energy do you put in to heating your domestic hot water? 
I always assumed for us it was pretty low - I believe the UK average is <2KW/day/person in the household.

Our gas for hot water use was about 4-5 kWh/day in the summer when the heating was off. One bath and one or two showers per day.
It'll be a bit higher now as the incoming cold water is colder, but I can't separate it out from the central heating.

Spoonie_Turtle

Our monthly average across the year is 51kWh input for 145kWh output (heat pump); 4.82kWh daily average output across the year.
170l tank, 4 adults but we have an electric shower and no bath.  It would be slightly lower if we didn't do the weekly legionella cycle which uses the immersion heater for the last 5-10℃.

[Obviously seasonal variations due to incoming water being colder, also colder months means running water longer for it to get warm for washing hands.]

markin

Do you hand wash dishes with the tap running? long pipe runs, leaving many Lt of hot water in the pipes? 

A tank can have heat loss from 0.6 to 2-3kwh depending on the size and insulation and location.

I think my immersion tank now that its set at 45c uses around 2.5 per person plus 1kwh heat loss, Difficult to measure its use directly.

MultiFuelBurner

ASHP 1.5kwh every other day heated to 45-50oC

pip895

Spoonie_Turtle said:

Our monthly average across the year is 51kWh input for 145kWh output (heat pump); 4.82kWh daily average output across the year.
170l tank, 4 adults but we have an electric shower and no bath.  It would be slightly lower if we didn't do the weekly legionella cycle which uses the immersion heater for the last 5-10℃.

[Obviously seasonal variations due to incoming water being colder, also colder months means running water longer for it to get warm for washing hands.]

My reason for doing the analysis is that I am thinking of getting an ASHP - there are complications with getting the correct pipe work to the boiler so I was trying to find out how significant a part of the system the hot water is.  Turns out it is more significant than I thought and I’m not sure why.

pip895

Veteransaver said:

Boilers aren't particularly efficient at heating hot water, the achieve nowhere near their claimed 90% efficiency. It's because they need to run at a higher temperature so don't achieve condensing for much of the heating cycle. Also significant losses in the pipe work to and from the cylinder.

Ours has been running at 60deg for the last 15 years.  I have recently put the flow down to 55deg but the data on that isn’t in the analysis can’t see it making much difference..  

Qyburn

Veteransaver said:

Boilers aren't particularly efficient at heating hot water, the achieve nowhere near their claimed 90% efficiency. It's because they need to run at a higher temperature so don't achieve condensing for much of the heating cycle. Also significant losses in the pipe work to and from the cylinder.

Normally DHW needs a lower flow temperature than space heating.

FreeBear

pip895 said:

Veteransaver said:

Boilers aren't particularly efficient at heating hot water, the achieve nowhere near their claimed 90% efficiency. It's because they need to run at a higher temperature so don't achieve condensing for much of the heating cycle. Also significant losses in the pipe work to and from the cylinder.

Ours has been running at 60deg for the last 15 years.  I have recently put the flow down to 55deg but the data on that isn’t in the analysis can’t see it making much difference..  

I had been using an ancient Baxi back boiler with a gravity fed DHW system. Reducing the water temperature to ~45°C and turning the gas off when water was not required cut my gas consumption by half - At one point, down to 3500KWh per year.