Hot Cylinder Droping Temperature!

Kevincoholan

Hi, I live in my newly built house for almost 2 years now. I have noticed after I get the hot cylinder up to 65 degrees, it looses tempreature quickly.

My cylinder is a 300L triple coil (Solar, Stove & Oil) - 2 years old
I have tested the system over night and it drops 4.5 degrees in 7.5 hours

0.6 degree drop every hour!!!

I have a digital temperature guage in the middle and top of tank.
The tank was at 69 degrees (Middle) and 74 degrees (Top)

My tests were carried out with valves into cylinder closed (No tap was turned on)
The Stove was not lighting.
The solar was switched off.

Is this an acceptable temperature drop or is there something wrong?

Any advise, greatly appreciated.

freddyuk

Not very surprising especially if all the valves and pipework is uninsulated.?
I have a foam insulated cylinder with a cylinder jacket over that and all the exposed parts insulated and still there is a natural loss of around 5-6 degrees overnight. Just try to reduce it as much as you can. Check there is no loss via the solar pipes. If the hot press is hot then you are losing heat somewhere.
Of course don't boost the cylinder at night when you are not going to use it unless it is "free" heat from the stove.
The manufacturer should offer the heat loss calculations for the cylinder.You will need a computer to work out the losses from degrees to kWh to make sense of the numbers.

Dtp1979

Kevincoholan wrote: »

Hi, I live in my newly built house for almost 2 years now. I have noticed after I get the hot cylinder up to 65 degrees, it looses tempreature quickly.

My cylinder is a 300L triple coil (Solar, Stove & Oil) - 2 years old
I have tested the system over night and it drops 4.5 degrees in 7.5 hours

0.6 degree drop every hour!!!

I have a digital temperature guage in the middle and top of tank.
The tank was at 69 degrees (Middle) and 74 degrees (Top)

My tests were carried out with valves into cylinder closed (No tap was turned on)
The Stove was not lighting.
The solar was switched off.

Is this an acceptable temperature drop or is there something wrong?

Any advise, greatly appreciated.

A pic of the cylinder please. On the outlet you must have 457mm of horizontal pipe before the vent or one pipe circulation may occur. What is your solid fuel thermostat set to? Either way, as Freddie said, that heat loss isn't very high.

John T Carroll

Kevincoholan wrote: »

Hi, I live in my newly built house for almost 2 years now. I have noticed after I get the hot cylinder up to 65 degrees, it looses tempreature quickly.

My cylinder is a 300L triple coil (Solar, Stove & Oil) - 2 years old
I have tested the system over night and it drops 4.5 degrees in 7.5 hours

0.6 degree drop every hour!!!

I have a digital temperature guage in the middle and top of tank.
The tank was at 69 degrees (Middle) and 74 degrees (Top)

My tests were carried out with valves into cylinder closed (No tap was turned on)
The Stove was not lighting.
The solar was switched off.

Is this an acceptable temperature drop or is there something wrong?

Any advise, greatly appreciated.

One "typical" supplier of Hot Water Cylinders quote "A standing heat loss of 2.31 Kwh/24 Hrs" for a 300 Litre cylinder. This equates to 0.276C loss per hour. (2.31*860/300/24). I presume the "standing loss" is calculated by maintaining the 300 Litres at 60C and then measuring the energy consumption over a set period.
In your case it is unlikely that the whole 300 Litres is at +60C, it depends on whether the stove or oil or a combination of both were on during the day, if both were on then a very high % of the cylinder could be at +60C. Some of the apparent loss will be transferred to the water layers below so this is not really a heat loss. 4.7C drop in 7.5hrs equates to a (worst case) loss of 5.02 Kwh/24 hrs. (300*0.6*24/860). If you assume a true loss of say 75% then you are talking of a loss of 3.77 Kwh/24 hrs or 0.45C/hr (3.77*860/300/24) which in a real world probably isn't really outrageous bearing in mind that initially the Cylinder Top was at 74C and the Middle at 69C.

Kevincoholan

Dtp1979 wrote: »

A pic of the cylinder please. On the outlet you must have 457mm of horizontal pipe before the vent or one pipe circulation may occur. What is your solid fuel thermostat set to? Either way, as Freddie said, that heat loss isn't very high.

Hi DTP1979,

Im not sure how to add a picture, but the tank is only 2 years old and is a normal type 3 coil.

My tank is on the same level as the stove.

The outlet does not go horizontal, it goes vertical almost straight away. But I have checked this pipe and it doesnt seam to be hot (when not hot used).

Kevincoholan

freddyuk wrote: »

Not very surprising especially if all the valves and pipework is uninsulated.?
I have a foam insulated cylinder with a cylinder jacket over that and all the exposed parts insulated and still there is a natural loss of around 5-6 degrees overnight. Just try to reduce it as much as you can. Check there is no loss via the solar pipes. If the hot press is hot then you are losing heat somewhere.
Of course don't boost the cylinder at night when you are not going to use it unless it is "free" heat from the stove.
The manufacturer should offer the heat loss calculations for the cylinder.You will need a computer to work out the losses from degrees to kWh to make sense of the numbers.

John T Carroll wrote: »

One "typical" supplier of Hot Water Cylinders quote "A standing heat loss of 2.31 Kwh/24 Hrs" for a 300 Litre cylinder. This equates to 0.276C loss per hour. (2.31*860/300/24). I presume the "standing loss" is calculated by maintaining the 300 Litres at 60C and then measuring the energy consumption over a set period.
In your case it is unlikely that the whole 300 Litres is at +60C, it depends on whether the stove or oil or a combination of both were on during the day, if both were on then a very high % of the cylinder could be at +60C. Some of the apparent loss will be transferred to the water layers below so this is not really a heat loss. 4.7C drop in 7.5hrs equates to a (worst case) loss of 5.02 Kwh/24 hrs. (300*0.6*24/860). If you assume a true loss of say 75% then you are talking of a loss of 3.77 Kwh/24 hrs or 0.45C/hr (3.77*860/300/24) which in a real world probably isn't really outrageous bearing in mind that initially the Cylinder Top was at 74C and the Middle at 69C.

Thanks Freddyuk andJohn T Carroll for your advise.

The tank is not fully insulated at the moment, I better start off with that.

Thanks again

Dtp1979

Kevincoholan wrote: »

Hi DTP1979,

Im not sure how to add a picture, but the tank is only 2 years old and is a normal type 3 coil.

My tank is on the same level as the stove.

The outlet does not go horizontal, it goes vertical almost straight away. But I have checked this pipe and it doesnt seam to be hot (when not hot used).

Your cylinder must be higher than your stove. Check the boiler in your stove to see if it's becoming warm when the cylinder is hot. Obviously don't light the fire!

John T Carroll

Kevincoholan wrote: »

Thanks Freddyuk andJohn T Carroll for your advise.

The tank is not fully insulated at the moment, I better start off with that.

Thanks again

Had a very good Solar day in Cork last Tuesday so the whole 130 ltrs (150 Ltr nominal cylinder) was at 63C, I deliberately shut off the cold water feed into it for a period of 8 hrs (overnight) to ensure that nobody could steal any hot water, the cylinder temperature fell at an average of 0.63C/hr (averaged from 3 sensors) over that 8 hour period.

A UK friend of mine has recently installed a Dimplex Quantum Hot Water Cylinder and he said his temperature drop overnight is only 0.25C/hour, however this allegedly has "superior" insulation.

Wearb

John T Carroll wrote: »

Had a very good Solar day in Cork last Tuesday so the whole 130 ltrs (150 Ltr nominal cylinder) was at 63C, I deliberately shut off the cold water feed into it for a period of 8 hrs (overnight) to ensure that nobody could steal any hot water, the cylinder temperature fell at an average of 0.63C/hr (averaged from 3 sensors) over that 8 hour period.

A UK friend of mine has recently installed a Dimplex Quantum Hot Water Cylinder and he said his temperature drop overnight is only 0.25C/hour, however this allegedly has "superior" insulation.

That is interesting. In your previous post you mentioned a manufacturer quoting . 0.276C/h. That is a large difference and also difference for UK friend seems closer to manufacturer quote.

Any ideas why shch a difference?

TheChizler

I discovered recently that once the sun went down the cylinder was losing heat to the solar, after midnight the flow pipe at the roof was hot. Think there's a one way valve gone. Try feeling the temperature of the pipes going to your tubes at a time that they should be cold.

freddyuk

That looks about right to me. There is a cost/benefit issue to buying a new super insulated cylinder so probably not worth an upgrade just insulate everything you can see. I have to think the larger the cylinder the better performance. My 200L buffer certainly performs better than my standard cylinder.
Do you set your stat higher than 63c to make the most of the energy? May as well store it hotter and then use mixing valve to protect life and limb.

John T Carroll

Wearb wrote: »

That is interesting. In your previous post you mentioned a manufacturer quoting . 0.276C/h. That is a large difference and also difference for UK friend seems closer to manufacturer quote.

Any ideas why shch a difference?

In my case I'm happy enough because I told the manufacturer to be a bit "mean" deliberately with the insulation so as that I could get the biggest volume cylinder into my compact hot press, I think my foam spray is approx. 35 MM. The figures below are for the Tribune range of cylinders and is straight from their brochure. The calculation for the temperature drop for any of them is: (Kwh*860/cyl.vol/24) in Deg.C/hour.

Model (Capacity)
Standing Heat Loss (kWh/24hrs) 120ltr 1.05, 150ltr 1.23, 180ltr 1.32, 210ltr 1.58, 250ltr 1.84, 300ltr 2.1.

Should have added that the standing heat loss is, I think, carried out with the cylinder contents at 65C and a 20C ambient.

John T Carroll

freddyuk wrote: »

That looks about right to me. There is a cost/benefit issue to buying a new super insulated cylinder so probably not worth an upgrade just insulate everything you can see. I have to think the larger the cylinder the better performance. My 200L buffer certainly performs better than my standard cylinder.
Do you set your stat higher than 63c to make the most of the energy? May as well store it hotter and then use mixing valve to protect life and limb.

Yes, I have a antiscald valve. I have the "solar stat" set at 80C, I have a flat plate array (no heat dump) and the highest cylinder temperature I have ever seen is 76C/77C even with little or no usage so the panels have never stagnated. I also control the boiler coil motorised valve with a second output from the solar controller, setpoint 62C with 5C hysteresis. I use a sensor which monitors the cylinder temperature about 3/4 way up the boiler coil and only use the cylinder top sensor for monitoring, this ensures that the motorised valve will open well before the top sensor detects a drop in temperature and helps to give a relatively large hot water capacity when there is a big draw off.

John T Carroll

Kevincoholan wrote: »

Thanks Freddyuk andJohn T Carroll for your advise.

The tank is not fully insulated at the moment, I better start off with that.

Thanks again

A few months late but better later than never as I found a formula some time ago, I think in some Part L regulations which states that the heat loss should not be greater than....Kwh/Day based on a maintained water temperature of 60C. You can do your own calcs below.

Cyl Cap.Litres.. Heat Loss.Kwh/day.. Heat Loss.DegC/hr
100 2.08 0.74
150 2.62 0.63
200 3.11 0.56
250 3.56 0.51
300 3.98 0.48