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Author Topic: 5 Yr report of living with a 60kW Log Batch Boiler and 5,000L Thermal Store  (Read 3269 times)
kristen
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« on: January 29, 2014, 10:27:58 PM »

In January 2009 we replaced 2 x oil boilers with a Log Batch Boiler and 2 x 2,500L (linked) thermal stores. At the time our central heating oil boiler was dying, and we were faced with the choice of replacing it with oil again, and then probably 10+ years before having the chance to change again, or making what seemed at the time a bold decision to switch to a Log boiler.

Lady Luck was on our side, the price of oil doubled and we are now saving about 4,000 p.a. on fuel - logs are costing me around 1,000 p.a. and based on the amount of oil we used to use that would be about 5,000 now.

Details of the install are in this thread:
60kW Log Batch Boiler and 5,000L Thermal Store Project Report

Some observations:

In Winter we light the boiler around 3-4pm. It burns for up to 6 hours (depending on amount and type of wood). We time lighting it to coincide with central heating requirement for the evening, and the aim (some skill / experience is required to achieve this!!) is that the boiler goes off as we go to bed and with the thermal stores fully charged  angel (And of course all my pigs are fed, and ready to fly ... Smiley )

If the thermal stores over-charge then the boiler damps-down, and we waste fuel as it then just smoulders (this happens when the temperature of the boiler water gets to something like 90C). I expect it is not very good for the boiler either.

Even with 5 year's practice this was hard to get right! Some days the wood being used might be more softwood / less hardwood, whatever. The solutions I have come up with are:

Central Heating forced-on timer.  On lighting the boiler I select N-hours for the heating to be on. This is independent of the thermostat. What we aim to do is to heat the house with whatever heat the boiler produces, which the thermal store doesn't need (to get to fully-charged). This means that as we go to bed the house may be 1C or 2C hotter than thermostat, but I figure that that extra heat will mean it is 1C-ish warmer in the morning too, so less heat will be required to get the house to Morning Temperature. I set the number-of-hours according to how much wood I have put in the boiler.

Then there also a tank temperature sensor, in conjunction with the boiler-fan being on, which is used by the Resol control to force the central heating on when the store temperature comes close to maximum.  (However, the central heating cannot dissipate the heat as fast as the boiler generates it, so this is set with some margin of error before the tank will become fully charged. We seem to have it about right though)

Thermostat Settings and Household Heat Demand
=====================================
In the morning (5am-ish) the thermostat will demand heat to bring the house up to waking-up temperature and the thermal stores will fall from about 80C (that's what they settle to, top to bottom, when "fully charged") down to about 60C.  At about 9am the thermostat falls back 3C - so heating goes off.  On a reasonable proportion of winter days there is some solar gain to the house, and no daytime heating is needed; then sometime after lunch the house starts to feel cold, and we then manually press the "boost" button on the thermostat (which gives an hours return to normal thermostat temperature).  That tends to be enough until lighting-up time mid afternoon.

Depending on how much heat demand there has been during the day at 3pm lighting-up time the store will either be close to 60C (no additional heat used since breakfast) or nearer to 40C (below 40C the thermostat is disabled as not enough heat to make running radiators worthwhile).

If tank is at 60C, and the weather is not cold (strong wind blowing means we need more heat, as does a bitterly cold evening / night) then I half fill the burner.

At 40C, and mild weather, I 3/4 fill it, and at 40C and cold weather than I cram it full (and check at 9pm to see how hot the tanks are, and consider putting another armful of logs in)

On bitterly cold days (night -5C or lower, day not getting much above 0C, no solar gain) we have used up the heat in the thermal store by lunchtime-ish, and we light the boiler then, and reload at 4pm-ish.  Even in the bad winters than has only been necessary on a handful of days during the winter.

Logs
====
We buy logs direct from forestry. They come on forestry vehicles, and the "logs" are 4-5M long.  Our boiler will take logs 1M in length, but in practice it is impossible to load it "tightly" with logs that long. So we came to the conclusion that 50cm logs were best, we load the back half of the boiler first - full to the top - and then light a fire in front of that; the fan draws the fire through the stack of logs behind and it is alight within a couple of minutes.  We then load the front, on top of the starter-fire, if we need any more than a half-load burn.

(We are fussy when cutting up the logs - I have a measuring stick for 50cm logs, its no good having an assortment from 40cm to 60cm, and in particular anything over 50cm gets in the way when filling the boiler with a full load)

Logs are stored for 2 full years. I have a moisture meter, and we get down to about 17%. The first couple of years we didn't have the benefit of having wood that was that dry, and when we did it made a HUGE difference to the heat we get out of the logs, as does having hardwood. Mostly we have been buying Chestnut, although on occasion we burn pine and although it is 25% cheaper we use quite a bit more like-for-like, so it just isn't cost effective - twice as much double-handling stacking and carting is way more "costly" for me than wood which burns for longer.

All the odds & sods from bits and pieces felled in the garden (that are definitely not uniform 50cm lengths!) are used in Autumn and Spring. At that time we just "chuck some in" to the boiler and light it; quite often in November a casual burn like that lasts 2 days. Come mid December to end February we are pretty much on a burn every day - rare are the exceptions, even in a mild year like this has been so far (but this year my proportion of half-load burns is significant) - and it is much easier to predict burn time etc. with uniform logs

Lifestyle changes
=============

Coming home late is a pain. House cold, no heat in tanks.  Assuming boiler is laid then light match and its only a few minutes until the boiler is going, but it is a good 30 minutes until there is much heat in the Rads. In practice if we are not at home during the day then there is no day time "top up" usage, so at 6pm the store is still 60C, and the thermostat clicks in to Evening Mode and warms the house, so I still have to light the boiler when I get home, but the house isn't cold.  On a really cold day then coming home late is not so good.

DHW / Thermal Store
================

We had part of the primary thermal store dedicated to DHW (take-off for CH was below that point). We never had a problem with DHW on days when the boiler fired up, but in Summer with Solar it was less than perfect.

The DHW was via a pre-heat coil low down in Store-1, and a final-heat coil at the top of Store-1 (if the pre-heat got all the way up to 60C there was no cooling, and no stratification, of the tank by the top coil). This meant that we needed the Solar Thermal to get Store-1 to 64C at the top - that's the top 1/3rd of a 2,500L tank ...it was usually "close" but that could still mean that a bath was not really HOT hot (the girls in this household are not the showering type).

As part of moving the boiler and thermal stores FROM the garage INTO the house we have added a DHW tank.  That has coils for Solar and Boiler and a pair of immersions (Boost at the top and E7 at the bottom).  Have not yet reconnected the Solar Thermal, but my expectation is that the Solar will heat the DHW first (and not take very long, its only a hundred litres or so, rather than a few thousand!) and then the solar can switch to heating the Thermal Store.

The original pre-heat and final-heat coils for DHW now pre-heat the mains water going into the DHW tank, so in practice the feed for the DHW tank will always be 40C-60C, so the solar make-up on the DHW tank will be quite small. We also have the option to use E7 immersion on any Summer days when Solar has not been "enough".

DHW Pre-heat Loop
==============

We have very long pipe runs to bathrooms, and oversized pipes (someone's bright idea back in the 60's). So we would have to run 20-30L to get hot water at a tap.  As originally installed there was a gravity loop (driven by the towel rails), so we installed a pump on that (the gravity loop wasted an incredible amount of energy keeping the circuit hot all day, and night, long Sad ).  Originally the pump was triggered by a pressure-drop - so turn the hot tap on, discover it is cold, turn it off (by which time the pressure drop had triggered the circulation pump to come on), count to about 5 minutes, and then hot water will be available at the tap.

What we found, in practice, was that there were odd occasions when the pressure dropped enough to trigger the DHW circulation pump. I don't know what might have caused it at 3am - someone flushing a loo perhaps? or someone at the water board backflushing the treatment plant perhaps? Smiley but either way the DHW came on much more frequently than was needed. It also came on when someone used the hot tap in the kitchen (which was close enough to the tank that hot water arrived quickly, and the tap was used often and didn't need the pump to run at all).

Our first solution was to change the 5-minute timer (which came on when the pressure switch triggered) to add a "hold off" timer for an hour or two (I forget exactly). So once it had come on it would not then come on again for another hour or so. We had also, originally, installed a pipe-stat on the return that countermanded the pump when the return was 40C.

Still wasn't really a very good solution.

I then replaced the pressure-switch with an X10 remote switch in the bathrooms (tried wireless, it wouldn't get through the walls, PITA to try to run a cable, even bell-wire, from bathrooms to DHW pump, so decided to use X10 which "signals" over the mains wiring). So the solution was then Push-button, count to 5-minutes, then hot water available at tap.

The programming ability of the X10 controller that I got was disappointingly basic Sad However, it could do a Dusk & Dawn calculation for my latitude using its internal date, and cold also handle events based on timers, so we were able to add a "Run at 05:30am so we can have 5 minutes longer in bed and not need to press the hot-water-please button". I think this is the best solution, only available in bathrooms far enough from DHW tank to warrant it, not needed in kitchen. I still have the 40C countermand pipe-stat on the loop-return.

Enhancements I would like: Have the DHW pump come on when the central heating pump is on.  Might as well heat the DHW circuit, any escaping heat is only into the building space, same as the central heating.

Put temperature sensors on feed-pipe to hot tap in each bathroom and use them to stop the Pump when suitable temperature is reached at the bathroom that made the request.  A Red/Blue LED in the bathroom would be handy too - otherwise instead of counting to 5 minutes the temptation is to keep turning the tap on/off to check!

A better, more sophisticated, home automation system could be used to make everything work a bit smarter. The wiring is complex enough, and using the Resol to control it is pretty obtuse, a more grown-up home automation system should allow much more fine tuning - and "if I try this I wonder if it would be better" experiments (without having to spend 500 on a Sparky to make physical wiring changes)

Summary
=======

Dinner party invitations fallen dramatically as people no longer want to invite me only to be told they aren't doing enough to be green  Cool Cool Cool

Saving us money on heating fuel.  

Lifestyle changes have been OK for us, I work from home, we have an OddJob man who splits wood during the winter when gardening work is rained off. It might suit others better to have a backup boiler to just get enough heat into the thermal stores when going away for a weekend, or a holiday, during the winter. Particularly if your OddJob would not be prepared to come in and light the boiler on those occasional weekends. But what fuel source?  For us that would be Oil, we'd have needed a new oil tank (original was inside the house, and supplier was getting Iffy about filling it), and so on.

We do have 2 x 18KW immersions in the thermal store - designed to be "Okay" for heating cover if the boiler broke and servicing was going to be a while - but we'd have to turn off everything else electrical in the house.

For anyone with a supply of wood its probably a no-brainer.  Ditto for straw probably (with a suitable straw boiler).

If you are still with me at this point thanks very much for your patience Smiley and I hope my experience can be of some help to anyone else thinking of doing it.

Added 30Jan2014 15:48 :

I was never happy with boiler and thermal store being in the garage.  No "usage" of the waste heat. Despite what people told me that posh thermal stores are well insulated and don't lose heat ... mine did (forgotten the figure, but it was not 1C per day or anything remotely as small as that, their might be details of standing losses in my original installation thread).  Also, in the garage the dT between thermal store water and external temperature was huge - garage was typically similar temperature to outside, whereas inside the house its nearer to 20C.  The room inside the house where boiler / store lives now is like an oven Smiley and the bedroom above is toasty (just checking the logging temperature sensor in the boiler room at it varies between 24C and 32C depending on time and how near/far from the boiler; rest of house is 18.5-19.5C).  Looked at another way: that's the heat that we were throwing away before ...

Dust in the house is an issue though, so having the boiler room under positive pressure from house (i.e. air flows from house into boiler room, not the other way) would help.

I should have hooked up a ducted air feed to the boiler with a electrically operated louvre door, wired to the boiler's fan.  Having a hole in the wall 24/7 is nuts.

We have a Carbon Monoxide detector in the boiler room, and a mains-powered heat detector (hooked up to the house alarm system, not just a battery operated solo-job that we might not hear)

Our immediate-use log store is adjacent to the boiler room, for ease of loading, but fireproofed. My insurance company had some stipulations. It can be filled external. We have about 3-4 weeks worth of log stored in the house so that if we have foul weather / heavy snow we will be able to burn for at least 3 weeks before we have to refill from barn. In normal weather we top up each week - so we don't get caught with it empty when bad weather arrives Smiley
« Last Edit: January 30, 2014, 03:48:50 PM by kristen » Logged
Greenbeast
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« Reply #1 on: January 30, 2014, 08:31:58 AM »

Good post!
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todthedog
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« Reply #2 on: January 30, 2014, 08:38:27 AM »

Great post.
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chasfromnorfolk
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« Reply #3 on: January 30, 2014, 11:33:58 AM »

Useful, practical info... thanks for taking the time to share it, invaluable to anyone considering a similar route.

Chas
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texasholden
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« Reply #4 on: January 30, 2014, 02:40:48 PM »

Great post. I'm pretty much decided on replicating this on a much smaller version (400 ltr) store. How big is your CH system (total KW of rads)? - might help me decide if 400 ltr is big enough.

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kristen
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« Reply #5 on: January 30, 2014, 03:42:40 PM »

Thanks for your kind encouragement chaps.

I live in a big house. We have skirting radiators, so bit hard to quantify them in more conventional terms. We had a heat-loss calculation done as part of sizing the thermal store and boiler, but my "sanity check" was that our old boiler (old, and thus some loss of efficiency to be expected) was on for 2 hours in the morning and 4 hours in the evening, or something like that, and it was originally rated at 70kW. Our pool boiler was a similar size. After the heat loss calculation came back the kW x Burn Time of the new log burner turned out to be similar.

We have good thermal mass, so we "overheat" the house on the Morning and Evening runs, and the structure absorbs that, and then transmits it back out. That, for us, probably works better than trying to evenly maintain a temperature (as without that boosting we would have to do that 24/7 I suspect). Probably needs a different strategy in a house with less thermal mass.

Our thermal store cannot hold anything like the whole output from a burn, but it seems to work for us to combine the daily burn with household evening heating requirement - we sit around in the house, and use more of it, of an evening than a morning (which is breakfast-and-go for Mrs K and breakfast-and-home-office for me).  Another way of looking at it would be that the thermal store, cooling from 80C to 40C needs to provide all the heat from morning to when the boiler is hot again (say 4pm), so I would suggest if a calculation of the energy / kWh of a 40C gain/drop on 400L (size of your proposed store) matches the amount of heat you think you need from early morning to tea-time then that should be about right.
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kristen
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« Reply #6 on: January 30, 2014, 03:47:50 PM »

Some other thoughts (which I will now add to my earlier post retrospectively, duplicated here for anyone who has already read the original post - skip this if you are reading the thread for the first time!).

I was never happy with boiler and thermal store being in the garage.  No "usage" of the waste heat. Despite what people told me that posh thermal stores are well insulated and don't lose heat ... mine did (forgotten the figure, but it was not 1C per day or anything remotely as small as that, their might be details of standing losses in my original installation thread).  Also, in the garage the dT between thermal store water and external temperature was huge - garage was typically similar temperature to outside, whereas inside the house its nearer to 20C.  The room inside the house where boiler / store lives now is like an oven Smiley and the bedroom above is toasty (just checking the logging temperature sensor in the boiler room at it varies between 24C and 32C depending on time and how near/far from the boiler; rest of house is 18.5-19.5C).  Looked at another way: that's the heat that we were throwing away before ...

Dust in the house is an issue though, so having the boiler room under positive pressure from house (i.e. air flows from house into boiler room, not the other way) would help.

I should have hooked up a ducted air feed to the boiler with a electrically operated louvre door, wired to the boiler's fan.  Having a hole in the wall 24/7 is nuts.

We have a Carbon Monoxide detector in the boiler room, and a mains-powered heat detector (hooked up to the house alarm system, not just a battery operated solo-job that we might not hear)

Our immediate-use log store is adjacent to the boiler room, for ease of loading, but fireproofed. My insurance company had some stipulations. It can be filled external. We have about 3-4 weeks worth of log stored in the house so that if we have foul weather / heavy snow we will be able to burn for at least 3 weeks before we have to refill from barn. In normal weather we top up each week - so we don't get caught with it empty when bad weather arrives Smiley
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Bodidly
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« Reply #7 on: January 30, 2014, 04:17:54 PM »

Can I ask how many tons you are getting through a season I am guessing somewhere around 20 ton mark wet weight?
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kristen
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« Reply #8 on: January 30, 2014, 04:46:10 PM »

Can I ask how many tons you are getting through a season I am guessing somewhere around 20 ton mark wet weight?

Yes, its a bit less than that, 18 tonnes (green weight) I think.
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Brandon
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« Reply #9 on: February 20, 2014, 08:44:59 PM »

kristen.

May I just say that it is good to see you again.
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« Reply #10 on: March 08, 2014, 08:12:03 PM »

Thanks for sharing Kristen! Your original post was a huge inspiration for my own 25kW/2000l install. I've been considering a follow up post myself so this may just be the kick I needed Smiley
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kristen
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« Reply #11 on: December 05, 2018, 12:30:08 PM »

Just adding housekeeping : links to followup thread:

10 yr report of living with a 60kW Log Batch Boiler and 5,000L Thermal Store
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