Sizing thermal store

[James]

I am currently tying to work out what volume size of thermal stor

(thermflow) I need for domestic hot water and heating. Since space i limited for the thermal store, can anyone provide me with th calculations required for determining the minimum size of stor required to do this?

Water will be heated by a 10 kw wood burning stove, a solar pane (solar twin) and a backup immersion heater. The heating requirement i only 6.2 kw since the house is quite small and will be well insulated.

One of the main requirements is that enough water is stored at night t provide enough hot water for at least 1 hour heating in the morning enough water for showers for 3 people without having to put the stov back on. The mixing valve will reduce water temperature from 8 degrees down to 40 degrees and I have read that a shower can requir

15L of 40 degree water per 5 minutes.

Any help much appreciated

Jame

-- James

[Christian McArdle]

That water flow would be the equivalent of the piddliest of low flow electric showers.

A typical mixer shower attached to a mains pressure heat bank will use about

12 litres per minute.

A 120L cylinder will be close to providing this, although it may drop out early if stratification is bad.

Now, 6kW for an hour will need approximately 80L of water at 80C. Adding the two will give you around 200 litre heat bank, although there is little in the way of padding, so I'd go for a 250 litre. I may also have mucked up the calculations, so don't spend money on the basis of them without checking with the heat bank supplier.

Finally, it is based on 6kW for the heating. You may find that although the steady state requirement of your house is this, that the peak heating demand during the morning is many times this. If your rads are capable of 12kW, for example, then it will use the energy at this rate for the first hour, effectively exhausting 160L of the cylinder, so you would need more like a

300L cylinder.

Christian.

[James]

Thanks Christian

300 L should meet our needs with a bit to spare. Looking at a fe shower model specifications, I can see the initial flow rate I foun from a heating publication on the net is, as you say, piddly!

Out of interest, how do you calculate the L of water required for 6 k of heating

-- James

[Christian McArdle]

There is probably a better method, but I just used typical combi boiler equivalence to do it. A combi boiler 24kW boiler will provide about 10lpm of delta 35 water, so 6kW is equivalent to about 2.5lpm. The heatbank has about double this delta, so it will use about 1.25lpm. 60 minutes would then make

75litres used plus a bit for luck. It probably needs a bit more for luck, really, as it will be quite a destratifying load compared to water heating, where the return temp can be kept down due to the cold mains water.

Indeed, you may find that the effect of the rads on an unreheated heatbank is to almost entirely destratify it, which could drop the shower temperature too much towards the end of the hour. There would be enough energy in the cylinder, it just wouldn't be very usable.

Christian.

[Doctor Drivel]

James

As a guide the temperature of 17 litres of water can be raised by 50°C by a single kW/hr. You need 17 lites for every kW/h you need for house heating.

6 kW is 6 x 17 = 102 litres of stored water for heating. Now this is heated before the timer cuts-in in the morning, so all this will be pumped into the rads. You can increase this 102 litres for abigger buffer.

DHW usage is a little subjective, but as a guide around 80 litres at 60C tap temp per person per day. That 320 litres just for DHW for modern day usage. But the thermal store stores water hoter than this, so 25% can be deducted making 240 litres.

Add the 240 litres for DHW to your 102 litres for CH and you have a thermal store of 352 litres. So look for a store of 350 litres and you should be OK. The wood burning stove should mean you can store water at 80C plus in the store, which means more energy stored, which means a longer operating time.

[Doctor Drivel]

Correction - I rushed the figures.

As a guide the temperature of 17 litres of water can be raised by 50°C by a single kW/hr. You need 17 lites for every kW/h you need for house heating. 6 kW is 6 x 17 = 102 litres of stored water for heating. Now this is heated before the timer cuts-in in the morning, so all this will be pumped into the rads. You can increase this 102 litres for abigger buffer.

DHW usage is a little subjective, but as a guide around 80 litres at 60C tap temp per person per day. That 240 litres just for DHW for modern day usage. But the thermal store stores water hoter than this, so 25% can be deducted making 180 litres.

Add the 180 litres for DHW to your 102 litres for CH and you have a thermal store of 282 litres. So look for a store of 300 litres and you should be OK. The wood burning stove should mean you can store water at 80C plus in the store, which means more energy stored, which means a longer operating time.