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• FIRST POST
• WobblyDog
• 15th Jul 17, 12:43 PM
• 459Posts
• 292Thanks
WobblyDog
My hot water cylinder is currently heated once a day. In between those times, the temperature at one point on the cylinder drops from 53 to 45 Celcius. Hot water usage is very low in the house at present. It's a 140 litre tank, heated by a non-condensing gas boiler about 10 metres away.

Is that an acceptable rate of heat loss, or should I be trying to improve things?

This graph shows the cylinder temperature (Celcius) over 24 hours. It also shows that the temperature of the pipes going "in" and "out" of the boiler remain above ambient at all times, so presumably water heated by the hot water cylinder is convecting through the boiler in one direction or another while the pump is off.

https://i.imgur.com/ddxpctI.png
Last edited by WobblyDog; 15-07-2017 at 12:52 PM.
Page 2
• dggar
• By dggar 16th Jul 17, 9:23 AM
• 570 Posts
• 278 Thanks
dggar
My boiler is using about 8kWh of gas per day to put about 1.3kWh into the hot water cylinder. Even allowing for heating up the boiler and 20 metres of 22mm pipe, that seems inefficient. Maybe I ought to get it serviced.
Originally posted by WobblyDog

Have you estimated the input of kWh using the electric immersion heater (I assume it should still be approx 8kWh).

If you have how does the cost of 1kWH of gas compare with 1kWh of electricity for your energy tarriff.
• jack_pott
• 16th Jul 17, 11:08 AM
• 4,471 Posts
• 5,771 Thanks
jack_pott
My boiler is using about 8kWh of gas per day to put about 1.3kWh into the hot water cylinder. Even allowing for heating up the boiler and 20 metres of 22mm pipe, that seems inefficient. Maybe I ought to get it serviced.
Originally posted by WobblyDog
That figure of 1.3kWh you're quoting from coffeehound is just the heat lost through the lagging, you need to add on the energy required to heat the water you use too.
• getmore4less
• 16th Jul 17, 12:46 PM
• 31,643 Posts
• 18,956 Thanks
getmore4less
Where are you measuring the cylinder temp?

Any use of water sucking in cold will make a massive difference.

I have my temp gauge at the normal place(near the bottom) and a small draw off drops that quickly and it goes back up when you stop drawing.

Insulting all the pipework off the cylinder will help.
• jack_pott
• 16th Jul 17, 1:42 PM
• 4,471 Posts
• 5,771 Thanks
jack_pott
Insulting all the pipework off the cylinder will help.
Originally posted by getmore4less
Stop wasting so much energy, you f*cking useless piece of pipe.
• WobblyDog
• 16th Jul 17, 1:45 PM
• 459 Posts
• 292 Thanks
WobblyDog
Where are you measuring the cylinder temp?

Any use of water sucking in cold will make a massive difference.

I have my temp gauge at the normal place(near the bottom) and a small draw off drops that quickly and it goes back up when you stop drawing.

Insulting all the pipework off the cylinder will help.
Originally posted by getmore4less
I'm measuring temperature near the top of the tank. I'm assuming that convection and conduction within the tank over a period of hours will cause the temperature at the top to drop proportionally after cold water is introduced at the bottom, but I've never drawn enough hot water to test that.

In answer to another question, my electric kWh cost about 4 times as much as my gas kWh, so if I had an electric immersion heater it would be a viable option.
• Alex1983
• By Alex1983 16th Jul 17, 2:05 PM
• 791 Posts
• 461 Thanks
Alex1983
Think your worrying far to much about it. How much is your gas bill currently?
• coffeehound
• 16th Jul 17, 2:06 PM
• 1,253 Posts
• 2,142 Thanks
coffeehound
Thinking about it, the 1.3 kWh figure doesn't include heating of the copper tank itself so 1½ kWh is probably nearer the mark.
• jack_pott
• 16th Jul 17, 2:31 PM
• 4,471 Posts
• 5,771 Thanks
jack_pott
Thinking about it, the 1.3 kWh figure doesn't include heating of the copper tank itself so 1½ kWh is probably nearer the mark.
Originally posted by coffeehound
The heat capacity of the tank is only about 1% of the heat capacity of the water.
• Gloomendoom
• 16th Jul 17, 4:08 PM
• 13,668 Posts
• 17,950 Thanks
Gloomendoom
Have you estimated the input of kWh using the electric immersion heater (I assume it should still be approx 8kWh).

If you have how does the cost of 1kWH of gas compare with 1kWh of electricity for your energy tarriff.
Originally posted by dggar
The immersion heater will heat the water in the tank directly so should be more efficient than a gas boiler that heats itself up, then the water in it and then has to transport that water some distance to the tank with all the associated heat losses from the pipes along the way.
Never argue with stupid people, they will drag you down to their level and then beat you with experience.” - Mark Twain
• coffeehound
• 16th Jul 17, 6:50 PM
• 1,253 Posts
• 2,142 Thanks
coffeehound
The heat capacity of the tank is only about 1% of the heat capacity of the water.
Originally posted by jack_pott
Well given the pipework and water therein leading off the tank is taking some of the heat too, and that the calculation was only to one significant figure, it seems reasonable to round it up to the half kWh.
• getmore4less
• 17th Jul 17, 6:16 AM
• 31,643 Posts
• 18,956 Thanks
getmore4less
I'm measuring temperature near the top of the tank. I'm assuming that convection and conduction within the tank over a period of hours will cause the temperature at the top to drop proportionally after cold water is introduced at the bottom, but I've never drawn enough hot water to test that.
Originally posted by WobblyDog
I just checked my water temp from Sunday when we were out all day with small draw off around 12:00am

heating cycle at 15:20-15:40pm
peaked at 44c 16:44
dropped to 33c 7:31 about 15hr.

I insulated all the pipes in the airing cupboard, the ambiant temp in there dropped a lot and the tank heat loss reduced significantly(did not record the old profile)

25mm foam tank, just realised missed the important pipe the cold inlet.

if you are using very little hot water you don't need to heat a full tank,
may be worth trying a top emersion and only heat around demand time.
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