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Vanessa

Thermal stores

My head is spinning on this! I THINK I've got my final answer, but would like to run it past you lot to see if you've got any further thoughts.

I did my initial research before leaving the UK, and so know the basic principles behind the thermal store. I've had many-a fruitless battle with French suppliers of hot water tanks, who either tell me what I'm asking for "does not exist" or try to sell me something differen!! bandhead

So, I've decided "sod 'em" and am planning to get THIStank from the UK. It supplies mains pressure hot water (so satisfying the French desire for this).

I've chosen the version without the coil for the boiler (which will be a back-boiler on a wood-burning stove) so that we'll have some early morning heat to rads in the coldest months even if the fire goes out overnight. This version also allows some solar heat for the rads too.

So ... thoughts anyone? Does this look OK? Does anyone know of a better one for similar money (this one, the size we want which is 200l, will cost just short of 1100 sterling)?

TIA Very Happy
vegplot

You could do worse than asking Dulas if they can advise. I've no idea if they will be able to help you but they are experts.

http://www.renewable-resources.com/index.htm
RichardW

200L looks small. We have been looking at 1500L aprox.


Justme
Vanessa

Their website says the 200l is for a household with 1 bathroom and 2 shower rooms - we have just 2 shower rooms.

Are you suggesting the larger capacity because it's wood-fired?
RichardW

You want to look at how much heat can be stored in that amount of water (cant find it at the min). What your heat requirment is for 24 hours. How much heat output the stove is & how long it will be on for. How long you want to go between fireings of the stove.

Then you will have

a, heat produced
b time produced over
c, heat needed
d, time needed over
e, temp of water
f, heat store size required at said temp



Justme
vegplot

To add to justme last post you will need to consider what type of heat inputs you have. If you are wood firing the most efficient way to burn is very, very hot but this is not a good match for heating demands. Wood fired heat stores normally have just one or two burns a day with heat stored in large hot water tanks.

However, if you're on gas then that burns efficienctly in a modern boiler and therefore can be used more on demand requiring a smaller hot water volume.

If you're unsure get some professional independant advice.
Vanessa

Lummy gov'nor, that's a bit technical Shocked Confused and I haven't a clue about how much heat we need!!

We currently have a 25kw oil fired boiler, but that's only alight occasionally as we can't afford to run it!! Maximum of 4 showers a day ... that's when we've got visitors. The house has 10 rads. But as to how much energy we use, I haven't the foggiest idea!!
vegplot

vanessa wrote:
Lummy gov'nor, that's a bit technical Shocked Confused and I haven't a clue about how much heat we need!!

We currently have a 25kw oil fired boiler, but that's only alight occasionally as we can't afford to run it!! Maximum of 4 showers a day ... that's when we've got visitors. The house has 10 rads. But as to how much energy we use, I haven't the foggiest idea!!


Sorry Smile

Okay start off conducting an audit of your needs. To do this you'll need to know the thermal performance of your house and what temperature you like to have rooms heated and for how long. Then you'll need to determine your hot water needs. Once you have this info you'll know what your energy needs are.

Next you need to specifiy how you wish to provide that heat, just ideas thrown into the air is a good starting point.

Do not. however, go to manufactuer to gain this info they will sell you what they think you need.
Chez

You could do worse than talk to Navitron (.org.uk). We got our solar stuff via them, including a vented hotwater cylinder which contains three coils (oil, solar, DHW) and an immersion, plus a spigotty thing at top and bottom in case we want to also attach a woodburner. It's about 250l I think and was about 700, including VAT.

They were pretty helpful with the calculations, which was good, because I went all blond Sad.
Vanessa

I think I like Chez's approach best! Sorry guys!! There's just too much "how do I do that" with what you've suggested Confused
RichardW

How do you intend to heat the water in the tank? You said the back boiler wont be connected so you can get morning heat, I dont understand that.

The specs on the tank you have linked to needs a boiler that can replenish the total heat store from cold in 30 mins (20kw) but you dont want to use the oil boiler & are not connecting the wood stove but are connecting the solar.

I dont think that solar is a good option for heating a heating system tank as peak demand is at the worst time of year. Ok for domestic hot water unless you have a very well insulated property.


Justme
Vanessa

Sorry, I obviously wasn't clear in what I wrote!!

The wood fire with back boiler WILL be connected, but not via a coil. It'll feed "direct" into the thermal store, so that solar can contribute a bit towards heating the house "on a good day".

We haven't got solar yet, but plan to in the near future ... hence planning for that at this stage so we get it right.

So, our main source of heat in winter will be wood, secondary will be solar, emergency will be immersion heater (but only for dire emergencies!!). In the summer, it will be solar (although probably not for this year).

Hope that's a bit clearer?
vegplot

vanessa wrote:
Sorry, I obviously wasn't clear in what I wrote!!

The wood fire with back boiler WILL be connected, but not via a coil. It'll feed "direct" into the thermal store, so that solar can contribute a bit towards heating the house "on a good day".

We haven't got solar yet, but plan to in the near future ... hence planning for that at this stage so we get it right.

So, our main source of heat in winter will be wood, secondary will be solar, emergency will be immersion heater (but only for dire emergencies!!). In the summer, it will be solar (although probably not for this year).

Hope that's a bit clearer?


Keep your solar for hot water.
Vanessa

... how would we separate it? One thermal store ... Confused Maybe I'm just tired today!!
vegplot

vanessa wrote:
... how would we separate it? One thermal store ... Confused Maybe I'm just tired today!!


If you're using a fuel and solar you can use a double coil tank specially made for the purpose. Or a twin tank system. The solar and wood heat the hot water only.

There is virtually no point in using solar to heat heating water unless you have a large water store and a large solar thermal array. That's not to say it couldn't be done but it's not economic.
RichardW

Ah right. Sorry more questions.

Whats the output of the stove?
How long will you keep it a light for?
Will that be ticking over or full speed?

Justme
Vanessa

OK, stove will be 14Kw (Hunter Herald). I expect it will be burning all day most days in winter, and perhaps for a "short blast" each day in slightly warmer weather in autumn and spring.

Whether it will ever be "ticking over" or not, I don't know ... the weather has been so odd this year with one day 14 degrees and the next not scraping above zero, who can tell?!!
RichardW

Is that how you currently use it?
14kw is that to water or room or both combined?
(just checked thats 45000 btu or 13kw to water) thats slightly below the 20kw recomemneded but a sit will be on for long periods should be ok.

Thats a lot of wood each day. We run ours for about 6 hours each night & that uses about 15-20kg.

With that amount of burning you would prob get away with a small tank as the tank will be getting a top up continuosly. The tank will only need to provide heat from its store for a short period whilst the fire is out IE over night & first thing in the morning.

Justme
Dee J

After much searching... the 200l Gledhill Torrent is what we finally settled for. A partial success - when heated by the immersion everything is fine - but our woodstove - which is small and has the same theorhetical rating as the immersion barely manages to get the tank above 40C - OK it's only a 3KW rated back boiler - but it's no match for a 3kw immersion! We're planning on installing a Rayburn with uprated boiler as a replacement. Couple of comments - tank needs to be around 70C at the thermometer (about 18" from the top) to ensure the occasional un-green deep hot bath. Also the supplied immersion tripped it's overheat on first use. Replaced the stat and has been OK since. We went for the RE model with solar coil in preparation for some navitron solar panels (when we can afford them). Couldn't find any other tanks at a better price (even navitrons own by the time you've added in all the extra coils etc).

Dee
RichardW

You need to have a serious fire going to get the full rate out of a stove. Was speaking to the local shop about it & they often have calls about lack of heat & when they visit they see a fire that is just to small & not roaring away enough to get any heat into the boiler.

Would not want to be using a 3kw electric heater to heat 200L of water eek high bills.

Also remember when looking at stoves you need 3 heat numbers
1, to room
2, to water jacket
3, combined (wont be simply adding the two together as you would think in all cases some imply that adding the boiler will take some of the room heat & add more to the water).

The combined figure will allow you to work out fuel usage. Heat out divided by efficiency % times 100 will give you input kw then see what the kw / kg your fuel is.

Justme

Justme
Vanessa

Justme wrote:
Is that how you currently use it?
14kw is that to water or room or both combined?
(just checked thats 45000 btu or 13kw to water) thats slightly below the 20kw recomemneded but a sit will be on for long periods should be ok.


At present, we have just the ancient, inefficient oil boiler that we want to "do away with"; we also have a Deville wood fired cooker in the kitchen (but not one capable of running a back boiler) As we want a fire in the lounge anyway (and to have that involves installing a flue pipe as there's no chimney in that room), it seemed like sense to go for a fire with back boiler to do the heating.

So, the Hunter isn't installed yet. The Hunter says it has a 24,000 BTU back boiler. The people we've found who could install it for us say it could easily run the rads we want AND do the hot water ... are they lying to us?!!!

Quote:
Thats a lot of wood each day. We run ours for about 6 hours each night & that uses about 15-20kg.


The quantity of wood isn't a problem, as we have our own woodlands. We know how much work is involved bringing it in, but we don't go out to work, so this isn't a problem. When we're too old to chop the wood, we'll have to move!!! Wink
RichardW

Hunter web site says 24 to 45k btu. So must come with option of a better boiler. If you are going for the 24k one I see troubles ahead.

Justme
Vanessa

I'll have to check!! Surprised

I really, really appreciate this help, BTW. I understand the systems etc (having studied them as part of my "Home Economics" course when I left school ... HONESTLY!! We even had to do stuff like working out the calorific value of coal!! Shocked ), but not how to work out "what we need" in terms of water storage and heat output. I might have studied that, as well, but it's WAY too long ago Embarassed Laughing

Soooooooooooo, it's starting to look like we might be better-off changing the kitchen range to one with a back boiler, and having a small "decorative" stove for the colder evenings in the lounge? Last thing I want is to have a COLD house Wink and have to start all-over again Rolling Eyes

What BTU rating would you recommend then, JustMe?
vegplot

vanessa wrote:
What BTU rating would you recommend then, JustMe?


This is where you need to know what your heating loads are likely to be and to calculate these you need to know, or have a got estimate, of the thermal perfomance of you house. If your house needs 24KW to heat then you bolier need to be able to provide that heat plus a bit more as a margin.

You could work it out in one of two ways.

The first requires knowledge of the material your house is built with, their area and U value and then some maths. Surprised

Or, you could simply heat the house over a period of few days or a a week and measure the fuel consumption. A simple calculation will convert that to kWH used, divide by the hours of heating and you have a good approximation of your desired heat input to heat the house.
Vanessa

Laughing I can tell you what the house is built of!! The main part is stone, 3ft thick walls. The extensions on both ends are the multi-chambered red bricks that the French seem to adore (I think it's a total of 12 chambers, so like having 4 mini cavity walls). The roof is modern cement tiles, and there's a goodly layer of fibreglass insulation in the loft (200mm).

U values ... don't think they were even "invented" when I was at college Embarassed Laughing Laughing ... I know what they are, but not how they're calculated.

As for doing a "test heating" ... I daren't!! We're running the central heating only every other day at the moment, and then it's only on for 2 hours in the morning and the same at night ... so the "ambient" temperature is, er, rather cool to say the least!! It's OK for working in, but not good for sitting around (which is what I'm doing at the mo as MOH is making a dreadful mess in the kitchen so I'm avoiding the dust!!). As I'm one for "craft projects" when I have the time, being able to "sit around comfortably" is a priority.

I'd guess that 25kw is OK though, as that's the rating on the existing boiler.

The musing about the kitchen range is because the French do lots of those with quite big back boilers, but mostly look at us like we're mad when we talk of a fire with a back boiler!!
Chez

Rolling back a bit here past the technical stuff (see earlier 'blonde' comment), could you run the stove through the DHW tank on a coil instead of having a direct feed and then on round the central heating?

I don't think you actually need a tank to run heating, do you? Just the back boiler. And that would save the solar just for the hot water, which I also think is the best idea. I don't think you'd get enough out of it to make it worthwhile. I know we're in the Frozen Plains Of Wales compared to you - but the ambient temp provided by our solar panels has been around 25 to 30 degrees c over the winter. This is good in terms of only needing the Evil Oil to top it up, but not really worth it in terms of heating.

I am sure that there was an online calculator about how big-a stove/boiler combo one needed in relation to your space / no. of rads ... I will see if I can remember where it was.
Vanessa

Thanks Chez. The techie stuff, whilst I appreciate is necessary, is really doing my head in!! Rolling Eyes
Chez

This isn't the one I used - but it looks pretty straight forward ...

http://www.flickeringflame.co.uk/tech_detail/tech.htm

Is that any use? Do you need a btu / radiator calculator thingy, too?
Vanessa

Yep, that's useful, Chez.

That reckons I need 18Kw for the heating, and 3Kw for the room ... plus, of course, whatever it takes for the hot water.

Deffo starting to think that maybe the Hunter won't be up to the job?

It's quite a major investment (although will pay for itself within 3 years on "oil saved"), so I want to get it right!!
Chez

That's quite a small amount for the room, isn't it, thinking back to when we were specc-ing ours? I think there is a hunter, or a herald, or something like that, has a small output to the room and large output to the boiler ...

*goes away to google*
vegplot

Chez wrote:
That's quite a small amount for the room, isn't it, thinking back to when we were specc-ing ours? I think there is a hunter, or a herald, or something like that, has a small output to the room and large output to the boiler ...

*goes away to google*


This is the reason why you need to have a good idea about the heating loads of your house. If you're only guessing then you could guess wrong and that could be expensive.
Vanessa

Until I started this thread, I'd gone on what the heating guy told me ... Embarassed

The site you posted, Chez, that's the outputs it recommends.

Now, I don't have the thermostat set to 20 or 21 degrees, it's 18 for "getting up" and for "sitting in the evenings", and 12 for the rest of the time (our timer is also the thermostat, so quite handy for dual-temp programming) - so could possibly get away with a little bit less than they're recommending - BUT if we have another "normal" year here, it'll be minus 5 during the day and a helluva lot colder at night ...

There are too many variables for my frazzled brain!! Crying or Very sad
vegplot

vanessa wrote:
Until I started this thread, I'd gone on what the heating guy told me ... Embarassed

The site you posted, Chez, that's the outputs it recommends.

Now, I don't have the thermostat set to 20 or 21 degrees, it's 18 for "getting up" and for "sitting in the evenings", and 12 for the rest of the time (our timer is also the thermostat, so quite handy for dual-temp programming) - so could possibly get away with a little bit less than they're recommending - BUT if we have another "normal" year here, it'll be minus 5 during the day and a helluva lot colder at night ...

There are too many variables for my frazzled brain!! Crying or Very sad


Yes, it can be mind blowing Sad However, don't get too worried.

The simple thing to remember is that your boilers needs to match (and slightly exceed) your total heating requirements. The first step is to work out how much heat your house loses, in KW, this will give you a very good idea of how big your boiler needs to be.

Your stone house will need far more heat than the straw bale house I will be hopefully building this year, its heat requirement will be only 3-5KW peak. If you can get a heating engineer to come round and do the calcs for you, you may be able to have this done free Smile

Solar hot water heating will only be really useful in the summer to heat hot water. You will get some benefit in winter but not as great.
Chez

Ours is a two story stone house with two foot thick walls and about four inches of loft insulation (we'd like more) and we keep the thermostat at 14 degrees c. It takes ages to heat up and ages to cool down.
vegplot

Chez wrote:
Ours is a two story stone house with two foot thick walls and about four inches of loft insulation (we'd like more) and we keep the thermostat at 14 degrees c. It takes ages to heat up and ages to cool down.


High thermal mass. Ours is a stone house as well although we don't have central heating as such, just a multifuel stove.
Vanessa

I've checked, and it's the 45,000 BTUs-boiler model, so that's "good news"?

The house is approx 200 square metres with standard 2.4m ceilings and will have 10 rads (currently has 12, but won't need the 2 in the lounge with the fire in here).
vegplot

vanessa wrote:
I've checked, and it's the 45,000 BTUs-boiler model, so that's "good news"?

The house is approx 200 square metres with standard 2.4m ceilings and will have 10 rads (currently has 12, but won't need the 2 in the lounge with the fire in here).


I've lost the plot about what you actually want to do.
RichardW

I take a day off & it all goes mad. lol

Justme
Vanessa

I'm sorry!

Plan is to heat house with wood, solar to provide DHW in summer and take chill off tank in winter (although solar will not be for a year or two yet).

Questions are - 1. What size Thermal Store. 2. Is 45 000 BTU back boiler sufficient?

Ta very much!! Wink
RichardW

You cant answer those questions without knowing the heat load / demand of the property / ocupation.

Once you have the daily load in Kwh you can calc a boiler size using the running hours to work out daily output in kwh.

A 45k btu boiler runjning 24 7 produces loads more heat than a 110k btu boiler runing for 2 hours per day. Thats what the thermal store is for. Produce it store it use it. Unlike traditional boilers that produce & use at the same time so need higher outputs.

Justme
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