Heat bank and UFH

[Jeff Mowatt]

First, thanks to Christian for introducing me to this very interesting device a few days ago.

Here's what I'm now thinking of doing:-

Installing a heat bank capable of providing heating in the form of UFH for an area of 40 m2 ground levels and 24 m2 upstairs

Installing woodburner with boiler and economy 7 electricity supply (There is no mains gas and I want to avoid an oil boiler and tank on top of wood storage space)

So, as I understand it, my hot water will be thermostatically controlled and pumped regardless of heat source, allowing me to run a shower unit without fear of scalding at any time of year.

During the colder months my woodburner boiler will make a day/evening deposit in the heatbank and the heat exchanger will distribute to both radiators and hot water supply as required.

At the end of the day if I allow the fire to go out, my pumped heating will be making a withdrawal from the bank which will be topped up by electricity at economy 7 rates overnight. I can use this hot water the following morning, relighting my fire to avoid drawing on peak rate electricity.

Therefore I must calculate what capacity my heat bank needs to be and what heat transfer rate my heat exchanger needs to have. I need to heat a total space volume of around 150 m3 which should be viable with about

10kw energy from what I've read elsewhere.

[John Rumm]

Even small water to water heat exchanges tend to have fairly prodigious capacity.

The space heating requirements will be dictated by the surface area of the surfaces (i.e. walls, windows, floor etc), the temperature differential across them, and the quality of the thermal insulation of those surfaces. The volume as such matters less. The number of air changes in the room per hour should also be factored in. Google back on u values and heat loss calculations for more information.

[Doctor Drivel]

Sizing the thermal store.

Assumptions:

- Average day temp of +4C. The worst case example should be calculated in for your area.

- Ballpark of 312 Btu/h-F. "A single floor house of approx 2000 squ foot has say a total thermal conductance of approx 312 Btu/h-F. This can be done using normal heat loss calcs for a specific house to be more accurate."

- 16 hours of heating. Some may want 24 hours.

- Minimum temp to heat the house 60C (140F). Rads have Max 82C (180F) and min 60C (140F). UFH min of say 40C.

Sizing for a house of 2000 squ foot heated over 16 hours:

A Btu is the amount of energy needed to raise the temperature of a pound of water 1 degree F. When a pound of water cools 1 F, it releases 1 Btu.

A single floor house of approx 2000 squ foot has say a total thermal conductance of approx 312 Btu/h-F. This can be done using normal heat loss calcs for a specific house to be more accurate.

January is the coldest month with say an average outdoor temperature of 4C (41F). To stay warm on an average winters day with an internal temp of 71F (21.66C) over 16 hours, the house needs 149,760 Btu (43.89 kWh).

16 hours x (71F - 41F) x 312Btu/h-F = 149,760 Btu (43.89 kWh).

Thermal store temperature = 82C (180F). Min temperature to keep the house warm on a rad system is say 60C (140F).

149,760 / (180F - 140F) = 3744 pounds of 180 F water cooling to 140F. 3744 pounds of water = 1,698 litres. So, a store volume for CH of 1,698 litres of water.

Boiler rating (Kw/hr) = (litres x Temp rise in deg C)/(14.3 x minutes) (Swap minutes and kW/hr to find the fastest reheat possible from a known boiler size)

(1,698 x 40C) / (14.3 * 60) = 79.16 kW

79.16 kW to heat the store from approx 40C to 80C in one hour.

Over six hours that will be 79.16 / 6 = 13.19 kW per hour. So, a 1,698 litre store can be heated from 40C to 80C over 6 hours by just over four 3 kW immersion heaters on cheap rate electricity.

Realistically a thermal store of 2,000 litres combining the CH and DHW will need 15.55 kW ( 5 3 kW immersion heaters) to raise it temperature 40C over 6 hours.

It is a matter of doing your own calcs and reaching the thermals conductance figure. Then replace your own figures with mine in the worked example. You may only want Economy 7 to do a part of the daytime heating and solid fuel the rest during the day. Adjust the calcs accordingly.

Should be easy enough to slot into a spread sheet and see what different permutations bring up.

[Jeff Mowatt]

DD, Thanks for all those calculations!

I guess that also means that too large a thermal store would create a significant lag between boiler input and heat output.

[Tony Bryer]

Two tons of water and 2m3 of space taken. For a place the size of a decent 2-bed flat. How realistic is this?

[Doctor Drivel]

Water is heavy, so don't put it in the loft.

About the size of an average sized airing cupboard.

2000 squ metres of floor space is the size of a flat?

Realistically if the coldest day in winter is to be heated by overnight Economy 7 then a larger store is probably required. The more the insulation in the house the smaller the store.

[Tony Bryer]

Unless I've got the messages screwed up the OP said "Installing a heat bank capable of providing heating in the form of UFH for an area of 40 m2 ground levels and 24 m2 upstairs"

which makes 64m2

[Doctor Drivel]

I gave an e.g, of 2000 squ feet which concluded a £2,000 litre store. That is what you were commenting on, not the OP case.

[Tony Bryer]

So you offered the answer to a question that wasn't being asked ...

[Jeff Mowatt]

Tony, I realised this was an illustrative volume which needed to be scaled down significantly. All the same, even a 1000 litre bank is going to take some time to warm up.

Really its a the convenience I need, being able to direct energy to a common pool taking advantage of the lowest cost forms. The convenience of being able to keep warm without worrying if the fire goes out overnight or if I'm away for more than an hour.

I think a smaller heat bank with an apprpriate heat exchanges will be fine.

Jeff

[robgraham]

Just a caveat based on personal experience of wood burners. Be aware of two things - you are going to commit yourself to locating and processing wood for the stove for your lifetime, and secondly the amount of heat that these stoves put into the back boiler is about the same as is 'leaked' into the room the stove is in. If your room is large then that is fine but 10kW into the water and 10kW into the room makes for a very hot room. There is the third warning and that is like Wind Generators, wood burning stoves are notoriously over-specced for their heating capability.

Having discovered all these things I now have an oil burner which does the regular 'keep everything warm' with the wood burner running in parallel and taking over when it is hot - and that is a far less stressful set up as far as I'm concerned as I don't have to work hard keep the fire alight and I don't have to work hard to make sure I have a large stockpile of dry wood.

Rob

[Jeff Mowatt]

Rob, Thanks for the warning. I had another warnimg last week from an uncle who used to make a living selling logs, he told me "There be only two people as'da get warm from a log fire, him who's a'cutting and him who's a'carrying"

Anyway, this is what I had in mind. 4kw to room, 2.5kw to water and 6kw to radiators.

have no idea how it apportions these figures

Jeff

[Doctor Drivel]

He didn't ask a specific question on it. If you understood the calc, you wouldn't be making such a fool of yourself.

[Tony Bryer]

If pointing out that an answer that refers to a 2000ft2 property may well be of doubtful relevance to a property a third of that size I plead guilty. Next you'll be telling us that you'd put one combi in a 50m2 property and two in one twice that size!

[Doctor Drivel]

It was clearly a worked example which you clearly didn't understand. I do not know the heat loss of his house so gave an example of a round figure of

2000 squ foot, and assumptions with one being the heat loss. All he has to do is calculate his heat loss and subsitute his own figures.

I will? A poor attempt at redirecting the focus of your foolishnes.

[Christian McArdle]

To store enough energy to run for long without any heat input would require an enormous tank. What would help is to ensure that the timer is set to heat the house up before the Economy 7 runs out. A 250 litre tank should then keep it going for a couple of hours until the solid fuel can be started. You will certainly need to specify multiple immersion heaters, especially if you want to keep the house up to temp overnight.

What is essential is to ensure that the insulation is seriously up to spec. If possible, double the required building regulations insulation.

Christian.

[Christian McArdle]

Remember that one of the major advantages of the heat bank is that the oil burner can be retrofitted to the system. Quite frankly, I'd run the pipes for it during the install...

Christian.

[Doctor Drivel]

With an existing house there is only;by so much you can do. Cavities, triple glazing and insulate and sealed front doors. 400mm in the loft. But make sure it is air-tight.

[Andy Hall]

When did it go up to 400mm? That's new, even for you........

[Doctor Drivel]

The Centre of Alternative Energy have a graph and 400mm is the optimum on cost heat saved. But that was when energy was cheaper. So, the more the better.