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Author Topic: My Thermal Store Calculations Make For Depressing Reading...  (Read 7927 times)
dan_c
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« on: May 11, 2012, 09:04:42 AM »

Hi all,

I spent a while looking around for calculations or rules of thumb to help me size a thermal store to suit my needs but I couldn't find anything useful. So, I went back to a first principles approach and created a little spreadsheet to work things out for me. There are assumptions built in and things are simplified a little but I think as a comparative tool for store size and boiler output it is a useful comparison. I wanted to see what kind of store and boiler I would need to give me central heating in the morning (after burning wood the night before).

The initial inputs to the calcs are store size, store temp, heat input and time, as well as approximate heat loss per hour from store overnight. Then I have inputs for the amount of heat I want to draw off the next day, and the lowest temp that is useful for heating.

The output from this is the time my store can supply central heating. Unfortunately in my case I currently have radiators in my house which means I need high temp CH water from the store - I think this is what makes the figures look so bad. I'd really like someone to look over my numbers/spreadsheet and see what they think.

Here’s one example:

Volume: 1000litres
Heat into store: 18kW
Time: 4.5hrs
Store start temp: 18’C

In order to keep the post short I will just state that these inputs give me a store at 87’C by the following morning (2’C drop over 7 hours). I have assumed that my minimum useful temp from the store is 65’C for my radiators and so I get 2hrs worth of heat from 12kW worth of radiators. The whole house load is actually 17kW but I was working on the amount of radiators I would have on in a normal winter morning.

The depressing bit is if I compare the energy input (81kWh) to energy output (24kWh) then I am only getting 30% of that back as useful heating. Most is left as <65’C water in the store.

If I pull my useable temperature for CH down to 40’C then the heating would last 4.4hrs giving me more like 65% of my input. How I wish I had under floor heating!

Do these numbers sound anything like close to correct?

Thanks
Dan
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Brian-s
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« Reply #1 on: May 11, 2012, 09:15:26 AM »

Does that not mean that the store start temperature on the second night will be a lot higher and make it more efficient?
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dan_c
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« Reply #2 on: May 11, 2012, 10:20:01 AM »

Yes that is very true Brian.

I will extend the spreadsheet to cover a week and then see how it looks.
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dhaslam
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« Reply #3 on: May 11, 2012, 10:57:01 AM »

If a room is at 15C the rediction in output  from  a radiator at 45C  compared to 65C  is only 40% and  of course  with less output the stored heat lasts  longer as well.           
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DHW 250 litre cylinder  60 X 47mm tubes
Heating  180,000 litre straw insulated seasonal store, 90X58mm tubes + 7 sqm flat collectors, 1 kW VAWT, 3 kW heatpump plus Walltherm gasifying stove
billt
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« Reply #4 on: May 11, 2012, 11:04:10 AM »

Just because the radiator design temperature is 65C doesn't mean that you have to run them at that temperature, you'll still get useful heat into the house at 35C.

This is the advantage of weather compensation, it adjusts the flow temperature so that the heat output matches the heat requirement. In normal coolish UK weather that will give a flow temperature of 30-45C. The temperature will only get up to 50 or more in very cold temperatures, which aren't that common, in the southern half anyway. The other advantage with a store based heating system is that the mixing valve on the return reduces the flow back to the store and creates and maintains stratification, so the high temperatures for DHW are kept for as long as possible.

Your figures look about right to me. I have a 2500l store and only need to light the boiler every 4-5 days in cool weather; it's been light 6 times in the last month with an average temperature of about 8C. I do let the temp drop to 45C though but it generally only gets heated to 75-80C.

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dan_c
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« Reply #5 on: May 11, 2012, 12:43:21 PM »

Thanks for the replies and opinions.....


If a room is at 15C the rediction in output  from  a radiator at 45C  compared to 65C  is only 40% and  of course  with less output the stored heat lasts  longer as well.          

Agreed and I understand your point completely with regard to temperature and time from the store. However, if I need 12kW to keep the place sensibly warm in the morning then I need 12kW don’t I? Once the rads start dropping they won’t be transferring 12kW as the temperature difference is just not there to keep the heat transfer rate high enough. I’ll try turning the combi CH right down and see if I can get some cooler flow temperatures to compare to at the moment.

I suppose this is why people talk of having oversized radiators to deal with lower circulating temperatures.


Just because the radiator design temperature is 65C doesn't mean that you have to run them at that temperature, you'll still get useful heat into the house at 35C.

This is the advantage of weather compensation, it adjusts the flow temperature so that the heat output matches the heat requirement. In normal coolish UK weather that will give a flow temperature of 30-45C. The temperature will only get up to 50 or more in very cold temperatures, which aren't that common, in the southern half anyway. The other advantage with a store based heating system is that the mixing valve on the return reduces the flow back to the store and creates and maintains stratification, so the high temperatures for DHW are kept for as long as possible.

Your figures look about right to me. I have a 2500l store and only need to light the boiler every 4-5 days in cool weather; it's been light 6 times in the last month with an average temperature of about 8C. I do let the temp drop to 45C though but it generally only gets heated to 75-80C.



I agree with your first point completely. I guess my point is that if I need the amount of heat transferred from the rads that I think I do, I simply won’t get it if the radiator temperatures are lower than planned. That said, I think that the “whole house method” and even normal radiator sizing is based on quite bad weather – something like minus 5 outside? On the majority of days I could probably get away with less heat and therefore lower radiator temperatures and consequently more time from the whole system.

The sticking point here is that theoretically I actually need 17kW for the whole house. I already dropped that to 12kW for the sake of my comparison based on my estimation of what we radiators we do run on “normal” winter days.

At 17kW output I’m actually down to 84minutes of CH with 65’C flow temperature. If I go for 17kW output but drop the useable temp to 40'C then I get approx 3hrs of CH – which would be fine. But I’d need larger radiators wouldn’t I?

I will do some reading on weather compensators, sounds like an excellent idea to alter flow temp based on actual temperatures and therefore actual heat requirements. I can see how that would yield efficiency improvements on any days that are warmer than the situation the CH was designed for.

Do people fit weather compensators to the CH outputs from thermal stores?
« Last Edit: May 11, 2012, 12:45:43 PM by dan_c » Logged
Countrypaul
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« Reply #6 on: May 11, 2012, 02:26:38 PM »

Can I ask where does the figure of 17kW for the whole house heating come from? Is it a SAP calculation, or based on you current boiler, or have you put 17 x 1kW electric heaters aound the place (OK - maybe not).

If the 17kW requirement is based on it being -1C average outside, then it coudl make a signifiant difference to how well things work for the majority of the time.

I'm looking at putting a TS into a refurbishment for which we have eventully got PP. I have no idea yet of the heat requirements though, but will be going with UFH. I was hoping that there would be enough heat in the TS to allow 24 hrs heating of the place at 0C outside. Now it looks as though we will have a beam and block floor with screed downstairs (previously expected timber floor), so I will have to rethink my plans as that will also form aTS.

Paul
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billt
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« Reply #7 on: May 11, 2012, 02:30:23 PM »

Mine has one and the controllers are available so I'd expect that there are a few in use.

The americans seem to call them outdoor reset controllers, there's a decent article here http://www.pmengineer.com/CDA/Archives/cdd55d5472298010VgnVCM100000f932a8c0 .

You don't need to change the radiators, although making them bigger might be a good idea if you were intending to do it anyway. My system was designed for a standard oil boiler 20 years ago, so the radiators are normal sized. I fitted a Honeywell weather compensating controller then which controlled the boiler burn duration to modulate the flow temperature. As you say, the system output is designed to provide enough heat at low outdoor tempeartures with high radiator temperatures. If the outside temp is higher then you need much less heat, which can be provided by lowering the water temp rather than switching the pump on and off. The Honeywell controller  worked so well in terms of maintaining a comfortable temperature and minimising fuel use that I wanted to use a weather compensating controller for the log burning system.

There are a couple of temperature plots of my system showing the outside, flow and top and bottom store temperatures for the last week. The highest flow temperature was about 46C very briefly, generally it was in the low 30s.


* W-comp.jpg (102.53 KB, 1074x616 - viewed 2159 times.)

* W-comp-ts.jpg (96 KB, 1083x618 - viewed 2195 times.)
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dan_c
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« Reply #8 on: May 11, 2012, 02:52:34 PM »

Can I ask where does the figure of 17kW for the whole house heating come from? Is it a SAP calculation, or based on you current boiler, or have you put 17 x 1kW electric heaters aound the place (OK - maybe not).

If the 17kW requirement is based on it being -1C average outside, then it coudl make a signifiant difference to how well things work for the majority of the time.

Sure.

I did three different things actually.

1. Whole house boiler sizing method from Sedbuk.
2. Sized a radiator for each room based on normal radiator sizing approaches (with correction factors for outside walls, north facing upstairs vs. downstairs, wall insulation, loft insulation etc) and then added them all up to get a total figure for all rooms.
3. I added up the rating of all my existing radiators in the house.

All three methods gave me somewhere from 17kW-18kW which was very suprising but somehow reassuring. Smiley

My current boiler is a Worcester Heatslave 20/25.
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Justme
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« Reply #9 on: May 11, 2012, 03:15:36 PM »

Can I ask where does the figure of 17kW for the whole house heating come from? Is it a SAP calculation, or based on you current boiler, or have you put 17 x 1kW electric heaters aound the place (OK - maybe not).

If the 17kW requirement is based on it being -1C average outside, then it coudl make a signifiant difference to how well things work for the majority of the time.

Sure.

I did three different things actually.

1. Whole house boiler sizing method from Sedbuk.
2. Sized a radiator for each room based on normal radiator sizing approaches (with correction factors for outside walls, north facing upstairs vs. downstairs, wall insulation, loft insulation etc) and then added them all up to get a total figure for all rooms.
3. I added up the rating of all my existing radiators in the house.

All three methods gave me somewhere from 17kW-18kW which was very suprising but somehow reassuring. Smiley

My current boiler is a Worcester Heatslave 20/25.

But, the standard boiler would not be running constantly. So your 3 or 4 hours of running time would actually cover more like 6 to 10 hours of usage.

Try averaging the heat losses.
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Countrypaul
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« Reply #10 on: May 11, 2012, 04:03:00 PM »

As Justme has already pointed out for a normal boiler it constantly turns on and off if it sized in the usual way. This means that you actual heat requirement is often a lot less than the boiler can produce as the boiler and radiators etc. are there for a worst case situation. With a thermal store you don't need to look at the worst case, it is more releveant to look at say all but the worst 5% of days (in these casses you may want to use to boiler during the night sort of appraoch) but otherwise could rely on the previous evening wood burning.

You may also want to consider the heating running for longer at a lower temperature, rather than say the normal 60 mins before you get up in the morning.

Paul
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dan_c
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« Reply #11 on: May 11, 2012, 05:58:26 PM »

billt - I didn't reply earlier as I wanted to digest your post and the linked webpage properly (interesting reading - thank you).

I had given zero thought to any form of weather compensation device and to be honest, other than seeing the phrase mentioned on here a couple of times I knew nothing of it. When you think about it properly it seems stupid to rely on anything more basic as this actually does exactly what you wants in terms of a control system.

I've done a brief search and haven't found anyone else using one on a thermal store yet for CH output - something the americans seem to refer to as a "mixing reset" controller rather than boiler reset.

Lots of other points - thanks for kicking my grey matter into action a bit more....

Regarding the kW output and boiler sizing....I suppose one way I could get a feel for an averaged heat loss is to keep a record of the on vs. off time of my boiler on a typical cold morning and if it is, for example, 50:50 then I could half my 17kW to 8.5kW for a steady heat output from a store. Unfortunately it's a bit warm now for these sorts of exercises.

In reality, what I really do not want to do is make a real mess of this and undersize the store and then find I need the oil combi going in the morning as the store is depleted of useful heat. The numbers were to give me a feel for what is possible and it sounds like my numbers are pessimistic which is good.....I think. Are you guys thinking that my numbers are probably Ok and it's more a case of getting to grips with realistic "needs" from the store in terms of heat and time?

On one of your points Paul - a cheapish approach could be a simple manual mixer on the CH output from the store to drop my circulating temps as you suggest and then just run for longer to get the benefit. The real deal is probably the weather compensation and I will do some more reading on this.

I do have other questions about my planned store setup but they're more to do with direct vs. indirect and people's preferences for control and setup. Should I start a new thread or bundle things in one thread? On one hand I don't want to seen to be creating threads left right and centre but on the other hand, burying lots of mini subjects in one thread probably makes life harder for people searching at a later date.
« Last Edit: May 11, 2012, 06:11:56 PM by dan_c » Logged
Countrypaul
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« Reply #12 on: May 11, 2012, 06:26:21 PM »

Intresting point that I had not thought of, you suggest a manual mixing valve to keep the CH circulation temperature down, it is usual to use  a thermostatic mixer valve (especially with UFH), but I have no idea if those can be controlled by any weather compensation device - which would seem an obvious option (well to me anyway).

Paul
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dan_c
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« Reply #13 on: May 11, 2012, 07:07:15 PM »

Ah - I think that may have been a bit of a brain fart on my part, I was actually thinking "thermostatic" but somehow ended up with "manual".
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