Storage heaters - actual specifications of heat loss during the day.

rogerblack

Is anyone aware of any storage heater that actually specifies the amount of heat lost per hour, with the dampers closed?
This surely is a key measure, if you're not needing heat all day, yet I've been unable to find it specified anywhere.

Cardew

Agreed, it is perhaps the most important factor in the performance of a storage heater.

However it will be very hard to specify as you presumably can only measure heat loss in BTU(or equiv) an hour. That will depend on the starting level of stored energy and the temperature of the room in which the heater is situated.

Richie-from-the-Boro

A 16 brick tin is designed to take about ± 18kWh in ± 7 hours. The answer to your question on how slow the slowest possible release of that ± 18kWh is whatever heater uses the lowest possible thermal conductivity material as insulation.

Dimplex already use a nano-insulator, see #5, in their case if I REM correct it was microporous insulation. In general then nano is better than the old mineral wool or even older asbestos, again from memory Dimplex claim to use it in some of their 'fan assisted' range and state its 2.5 times more insulating value than standard storage heaters, and a linear reduction [25%] in running costs.

I've always wished I could find a cheap supplier of these nano-blankets, it wouldn't take long to join the mi££ionaire class.

£70 a heater sized sheet here

rogerblack

Cardew wrote: »

Agreed, it is perhaps the most important factor in the performance of a storage heater.

However it will be very hard to specify as you presumably can only measure heat loss in BTU(or equiv) an hour. That will depend on the starting level of stored energy and the temperature of the room in which the heater is situated.

Agreed, the delta-t is important - but the initial maximum charge level is such that at least initially, room temperature should have a very limited effect.
Clearly one can attempt to calculate from first principles, but this is problematic.

grahamc2003

rogerblack wrote: »

Is anyone aware of any storage heater that actually specifies the amount of heat lost per hour, with the dampers closed?
This surely is a key measure, if you're not needing heat all day, yet I've been unable to find it specified anywhere.

Yeah, it's strange it's never quoted. I suggested some months ago on one thread or another that there could be a '17:00 efficiency', defined as the charge left at 17:00 from a full charge at 07:30 with the output damper closed.

Not sure why manufacturers don't push this - the numbers would show the major step forward from the very thick old storage heaters compared to the newer slimline ones.

My estimate would be the number would be typically something like 25% for the old uncontrollable types, and 75% for modern slimline storage heaters. Things like ronte ones, advertised as 'storing the heat' would be around 10% imv.

The actual heat loss per hour would have to specify which hour, since the heat loss would be an exponential decay.

Cardew

grahamc2003 wrote: »

The actual heat loss per hour would have to specify which hour, since the heat loss would be an exponential decay.

Being pedantic, wouldn't it be exponential only if the room temperature remained constant?

Richie-from-the-Boro

Premature death in this thread. The question posed in the title "Storage heaters - actual specifications of heat loss during the day ?" could have a academic theoretical answer or a 'real world' day use answer. As far as I'm aware no night store maker gives out such figures, a few wet systems makers however do, get their figures the do the below. Here is the 'wet' contribution.

Most modern condensing boilers are highly efficient and both flow & return are liable to be around 75°C on the outboard and will lose about 10°C on the inboard return. by the time they return - that's a delta of T50, [multiply by 1.2675 to get T60] So given that a double convector has twice the surface of a single get any manufacturers specs and work the math from there. If the system is losing more than 10°C then the pipework and jointing are on their pipe size load limits and need to be reconfigured.

So if you add 4.7 inches to the height, multiply that by the width then factor it for Delta-T you will get the answer for an existing radiator without any fancy algebra. Reverse the calc for an answer to what size rad do I need [heat loss calc] for a given output.

Pipe size - max loads
6mm - 750W / 8mm - 1.5kW / 10mm - 2.5kW / 12mm - 4.1kW 15mm - 6kW / 22mm - 13.4kW / 28mm - 22.5kW - 35mm - 35kW

T50 °C factor for the above
0.09 for SP single panel [no convector fins] / 0.13 for SC single convector / 0.19 for DX double: 1 panel with convector's + 1 without / 0.24 for DC double convector

grahamc2003

Cardew wrote: »

Being pedantic, wouldn't it be exponential only if the room temperature remained constant?

Yeah, for a perfectly exponential heat loss, everything would have to remain constant except for the storage heater temperature itself. Even then, you could say it wouldn't be perfectly exponential because the area of the storage heater would shrink a little as it cooled, therefore affecting the area radiating the heat. The effects would be really tiny though.

I don't see how looking at gas fired radiators is anything to do with the question posed. The performance of radiators assumes a constant radiator temperature, with the heat lost being replaced constantly. Storage heaters gradually cool down, so have max heat loss around 07:30, and min heat loss at 00:30, or zero heat loss if/when the storage heater reaches the room temperature.

rogerblack

While clearly it would have issues in some properties - this might almost work in some cases.
http://www.dimplex.co.uk/products/domestic_heating/installed_heating/electricaire/technical_specification.htm
Is useful.
http://www.dimplex.co.uk/assets/kb/brochure/Electricaire.pdf
Claims these are from 2.5K.
Presumably this is the 'R10' smallest model.

http://www.dimplex.co.uk/products/domestic_heating/installed_heating/electricaire/technical_specification.htm
The R10 model specifies 'case emission' as 750W.
It stores 73kWh.

What is the worst case heat loss?
Assuming it's charged fully at midnight-7AM, and used at 5PM-midnight.

It loses 14*.7, or about 10kWh - or 15%.
Clearly some of this loss will be useful - perhaps the last hour or two will contribute to the heating when it's actually needed.

At current prices, where I am, I would pay about 6.8p/kWh for night rate electricity to charge this.
Assuming for the moment that this would work for my heating - and that I need it 60 nights a year - so a total of 5100 units a year. (including 500 units of non storage use)
Total bill including 1200 units of peak rate is £537
Swapping round to gas - using 1700 units of electricity and 4600 of gas. is £486.

Hmm.
Gas is cheaper - sure - but £50 a year is going to be easily eaten up in the annual service, and it's unlikely to install even a simple gas system for 2.5K.

(Clearly, the above unit would only work for domestic heating in a limited number of properties. Lots of dubious assumptions made above).

sm0038

v useful thread.

I have a largish living/dining room/kitchen space fitted with two storage heaters.

Am i right to assume that - if i want to use the heat in the evening - i can minimise heat lost during the day by putting both heaters on at 50% as opposed to one at 100%?

Since heat loss occurs faster with a greater temperature difference, it seems intuitive that two low grade heat sources will result in less heat loss than one higher grade, or am i missing something important?

many thanks.