Insulating hot cylinder and immersion cap

Fred_Bear_2

I have a small (60L) indirect hot water cylinder heated by a gas boiler. There is top mounted immersion heater for use if the boiler fails (not needed so far). Last week I spent a few days away and left the hot water on continuously. I was surprised to use 9.5 kWh per day.

[Allowing for a boiler efficiency of say 60%, this is equivalent to a heat loss of about 6kWh from the cylinder. I also calculated that this is equivalent to a constant temperature loss of about 3 deg C per hour.
I switched the programmer off for 4 hours and the temperature reduced from 60 deg to 50 deg i.e. 2.5 deg per hour, roughly confirming my calculation.]

The cylinder is insulated with polyurethane foam, and the connecting pipes are plastic although not insulated.

One place the heat will be escaping is through the metal immersion heater cap. I have often read that this should not be covered but no reason is ever given. My question is: Is it unsafe to cover the immersion heater?

Also any tips on reducing the heat loss would be appreciated.

Fred

C_Mababejive

I guess you left it on in error?

I'd start by checking the stat setting on the immersion heater and after that,insulate all the connecting pipes.

Canucklehead

Hi,


Part L requires a cylinder to have it's pipe connections insulated for a least a metre from the cylinder.

The cable to the immersion will overheat and the insulation will crack.

'Do not cover'.

That's about as good as it gets for heatloss.

GSR.

ormus

for a 3kw element, thats about 3/4 hrs of actual heating.
at a cost of about 1.30 per day. seems about right for an immersion heater, all told.
its probably as efficient as most houses.

Fred_Bear_2

ormus wrote: »

for a 3kw element, thats about 3/4 hrs of actual heating.
at a cost of about 1.30 per day. seems about right for an immersion heater, all told.
its probably as efficient as most houses.

Sorry ormus I didn't explain things properly: I am heating the water with the gas boiler. I mentioned the immersion heater because it is effectively a large hole in the insulation at the top of the cylinder.

Fred_Bear_2

C_Mababejive wrote: »

I guess you left it on in error?

I'd start by checking the stat setting on the immersion heater and after that,insulate all the connecting pipes.

No, I left the boiler on deliberately while I was away so I could check how much it is costing me to keep the cylinder hot. I've had the cylinder for about 10 years: It is a small 'fast recovery' cylinder which is supposed to be highly efficient.

For the past 5 years I have concentrated on reducing my electricity consumption (I've gone down from 18kWh/day to less than 6kWh/day). Now I want to do the same for my gas bill. First I need to find out where the heat is being lost.

I will insulate the pipes as you advise.

aboard_epsilon

how is it possible to leave the hot water on continuously ..

the cylinder should have a stat ..that turns the boiler off ..at predetermined temp ..suposed to be 60 i believe ..
if this were to malfunction ..and your boiler continues to roar away ..then you would get the tank boiling ..and hot water would flow out of the expansion pipe into the header tank .....then you will have loads of steam in your loft..that would be where all the heat is going .

btw i built a frame around my tank ...there is 18 inches of loft insulation between tank and outside world ..my tank is still luke warm after a week away

all the best.markj

Fred_Bear_2

Canucklehead wrote: »

Part L requires a cylinder to have it's pipe connections insulated for a least a metre from the cylinder.

The cable to the immersion will overheat and the insulation will crack.

'Do not cover'.

That's about as good as it gets for heatloss.

GSR.

Thanks for the Part L info Canucklehead.

Re the immersion: The cable is 2.5mm butyl cable rated at 85degC so I can't understand why it would be damaged by the 60degC temperature under insulation. Anyway, even without any covering, it will still be touching metal at 60degC.

Fred

aboard_epsilon

Fred_Bear wrote: »

Thanks for the Part L info Canucklehead.

Re the immersion: The cable is 2.5mm butyl cable rated at 85degC so I can't understand why it would be damaged by the 60degC temperature under insulation. Anyway, even without any covering, it will still be touching metal at 60degC.

Fred

it can be damaged by overheating from within.

meaning the the electricty flowing through it produces heat ..that needs to escape..

all the best.markj

Fred_Bear_2

aboard_epsilon wrote: »

how is it possible to leave the hot water on continuously ..

the cylinder should have a stat ..that turns the boiler off ..at predetermined temp ..suposed to be 60 i believe ..
if this were to malfunction ..and your boiler continues to roar away ..then you would get the tank boiling ..and hot water would flow out of the expansion pipe into the header tank .....then you will have loads of steam in your loft..that would be where all the heat is going .

btw i built a frame around my tank ...there is 18 inches of loft insulation between tank and outside world ..my tank is still luke warm after a week away

all the best.markj

Oh dear! I really did explain myself badly! What I meant to say was that I left the programmer timer switched to continuous hot water. There is a thermostat on the side of the cylinder which maintains the water temperature at 60deg. Just as well otherwise, as you say, I would have a very steamy loft.

I'm very impressed with your tank insulation. Many years ago I had a 50 gal cylinder built into a cupboard and I packed the cupboard with dense mineral wool insulation. (I used to work for an insulation manufacturer.) The thickness was about 3 inches and much more in the corners. Like your setup the water stayed warm for a week. I tried the same idea a few years later in another house using loose-fill polystyrene but it was useless. I guess the air was able to convect through the gaps.

Fred
p.s. I'm just popping up to the loft to check something....

brig001

Do you ever use the immersion heater? If not, you could disconnect it and remove the cable (switch off power and make safe first), then it doesn't matter if it is covered or not. You would have to remove the insulation to re-connect it anyway if the boiler broke down, so again it would be OK.

We put old sweaters and an old quilt round the back of our foam insulated cylinder which seem to have helped a bit, well the airing cupboard is cooler anyway.
HTH, Brian