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Author Topic: DIY Pellet Stove Homemade Backboiler Addition!  (Read 334 times)
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« on: November 20, 2020, 10:46:36 PM »

Hi People, thought i'd do a quick write up of my homemade backboiler and home made thermal store  I've installed in my Artel 12kW Pellet Stove.

It all started when our old wood burner has degraded to the point it needed tidying up, and I had the idea of a pellet stove to replace our heating requirements, with the hope that it would allow us to automate and regulate our heat source to match requirements better.

Earlier in the year, just as the virus hit I came across an Artel pellet stove in working order for 150 on ebay. I think I bid 153 and won it, only problem was it was a round trip of a few hundred miles up into Scotland. No worries, we took all our own food, the camping stove, and headed up keeping ourselves to ourselves. I had to bring my partner as there was no way i'd lift it on my own. We took our trailer, and headed up there. My partner was a little unhappy, as I often have these crazy ideas and it results in last minute frustration in how we'll execute the collection of such things!

Anyway, stove home, and it looked like a good one. There was a damaged tile on the outside of the stove (we knew about that), but that's it. It ran great outside for a test and produced intense heat. Winner. It also ran at around 1kW (12kW max) by altering settings.

I searched for flue components in the 3" pipe, and the cost was significant in the UK vs elsewhere by the time i'd added in 45degree bends and a cowl etc. So I cracked out the TIG and obtained some 3.5" stainless exhaust tube. I also got a V-Band connector to allow easy dismantling for cleaning, and 2 flexi joints to allow movement (the stove is being fitted on a double-decker bus.

I knocked up a bracket from scrap to bolt to the rear of the stove to take the weight of the lower flu pipe section, this was to ensure if the Bus is driven that the weight of the flue (or downward pressure) doesn't get exerted on the aluminium cast exhaust blower outlet. I also fitted a condensate drain port, as there was some confusion as to whether i'd need one (i haven't so far). Cleaning can be done when removing the exhaust blower with a vacuum).

Once fitted, and bolted to the metal stairs banister infill, i set on with making a tough cowl for the top. I get heat transfer is significant, and the brackets are only welded on the side. However the stove shuts down at 260c exhaust temp, and I can adjust that lower if needed. The panel is metal. I had 6" of stainless pipe that was 3mm thick, and some plate, so built an angled hat, in the hope that it would survive any tree branch strikes.

I then got the stove working temporarily, and work well it did!  But the plan was still to add a backboiler to the stove, but the hot water tank was undersized now that we had a bath (was about 75litres before). I still had a second stainless steel tank (ex crisp factory flavouring tanks, about 2mm wall thickness). I cut the original tank in half and added the middle from a second larger diameter tank to make a 155litre tank. I added some 3/4" stainless steel ports for an external immersion (future project, PV and Lister CS powered elements willis heater style), backboiler, DHW, and radiators (and fill and vent). I argon purged the tank, as I had done the flue pipe - this made for wonderful smooth backs of welds, essential on stainless! So as my partner says, the cost has been significantly more than nothing! haha.

The plan was to use the tank as a heat store / buffer. I have previously experimented with Arduino / ESP8266 to make a thermostat and controller for a webasto we had. I was confident that using flat plate exchangers I could add the following functions to suit our lifestyle:

- DHW via flat plate exchanger - allowing one pump (no mains water, just a 12v pump providing 2bar) to feed equal pressure cold and hot to shower, bath and taps.

- DHW return loop, to prevent water being wasted whilst waiting for hot. How  i will manager this function is yet to be decided, it may just be a timer after flow has been detected, allowing you  to briefly turn tap on then off and wait for it to get hot before turning it on again. Wasting water is an issue, as living off a tank.

- Backboiler via FPHE allowing fine control of the pump speed on the tank side of the exchanger to act like a budget ladomat, and maintain boiler temperature. It will also allow me to preheat stove possibly with a second reverse pump (don't think that will be necessary - but if condensation an issue during warmup, I may add later).

- Whilst i'm at it, and i have some used exhangers,  thought i'd also run the radiator circuit off one. Can easily PWM control a tiny 12v pump to limit the radiator temperature to match demand. The circuit is only run upstairs and mainly used to keep the two bedrooms warm overnight when the stove is off, or when the daughter insists on having her door closed. Heat distribution from air alone is pretty good.

All circulation pumps are the tiny 12v ones as found on ebay, the only exception is the backboiler circulation pump, this is a low energy CP50 set to minimum (4watt), and this will be set to be triggered once the pellet auger starts, and will switch off a set time after pellet feed stops.

I made a large frame to sit the tank on, which is over a metre tall, it is mounted by the skirt around the bottom only to prevent heat loss. I decided to make a 40mm angle iron box around the tank, to encase a 50mm kingspan box. I then mounted a black chequer plate  panel to the front allowing pipework to be fitted to it and pumps and heat exchangers. I decided to fit a glass cupboard door on it eventually and will polish up the pipes making it a bedroom feature!  I also intend to have a visual representation of available hot water using addressable LED strip mounted on the tank. I'll assign colours to temperature ranges. I fitted 10 evenly spaced 1-wire temperature sensors to the tank before insulating, they're only 1 each.

The backboiler circuit is a sealed system, run at 1bar with 3bar safety relief, and a small 2litre expansion vessel. As it happens I fitted a 3/4" relief on the boiler itself, and one on the tank as it gave a visual representation as to the pressure. The one on the stove will have its gauge swapped for a low pressure switch, which will prevent the stove from running wthout water pressure (use a boiler pressure switch).

Similarly I made the radiator a sealed system to match, also with a 2litre expansion vessel  (I got two unused on ebay cheap!).

The tank itself is open vented, with 20/25litre plastic expansion tank on top. It's got a ball float valve set to only fill a couple of inches and will be manually filled via a valve before it, and the lid is sealed on to prevent sloshing if the bus is driven. The vent and overflow for the tank is piped in 32mm waste pipe down and out of the floor.

The backboiler itself was made from 4mm steel plate. I got most of it cut from - who cut it to size. This reduced slitting disc consumption to a minimum (15 for the backboiler!) I welded outside corner joints, and confidently know i have good penetration, it's not the neatest as I've done multiple passes to maintain 4mm of weld thickness in all places. Probably totally unnecessary as it won't corrode that much! (inhibitor + paint on outside), but if a job is worth doing..! I'm 100% happy that it will operate safely for many years.

The only space for water connection to the boiler was via the bottom of the back boiler, and the ash pan compartment of the stove. Going through the side or rear would create problems as the side and rear of the stove is multilayer, the room air is passed via a chamber in the back of the stove, and cutting in to that is just asking for problems.

I decided to run a port each side (bottom) and have one with a pipe running internal to the back boiler up to the top of the back boiler, it extends out of the top into a little dome, thus minimising the amount of trapped air in the back boiler (should it rot, then it's just the hump to repair).

The initial plan was for the back boiler to push up above the door, meaning a bleed nipple on top wasn't possible, but it obviously had to all fit through the door opening. I later changed the plan to having it at door opening height, so if needed I could drill and tap a bleed screw.

It is mounted using bolts into  4x m10 connector nuts welded on the outer sides of the boiler from underneath, allowing expansion upwards.

The back boiler has 2 steel threaded 3/4" pipes extending out of the bottom of it, through the ash pan chamber, and out of the bottom of the stove where it changes to copper for the rest of the plumbing, the pump is mounted under the stove (it doesn't get warm under there).

There was some discussion with a friend as to whether the stove would thermosyphon, but I have yet to experiment. A 4 watt circulating pump seemed ideal, as keeping the water flowing (i imagine) should help prevent any localised boiling), and we are only drawing heat off the circuit when the temperature is above the minimum we set. It bled it up quickly, so that's worth it for that alone!

So this is where I am now - Tank installed, and plumbed hot/cold feed to taps. I have yet to add the return loop from the kitchen sink (need to get a pump for the return, going to experiment with 12v ones too, i have set the water pressure to 1.2bar with a regulator), stove is now running with backboiler (although i've damaged the temperature sensor on the stove and it only runs on minimum now). I'm awaiting a PCB from JLCPCB that i cobbled together for a ESP8266 (this will control the circulation pump on the stove, report temperature of backboiler in and out via MQTT over wifi and be able to start and shut down the stove via serial comms, and cut the feed auger via a relay. mechanical overheat stat will be added, and pressure switch in series with feed auger), and another board which has loads of mosfets and 1-wire temp sensors input for the tank control. (will have loads of data to send out over MQTT for a web based visual display like exchanger in/out temps, tank temps at 10 locations, etc) That board has an ESP32 which will allow PWM of many outputs for pump motor control.

It's been a long project, but it's made good use of lock down time whilst I  haven't been working.

For anyone wondering about the durability of the 10-15 chinese ebay 12v pumps - I have a few, and have used some for about 10years, i've never had one leak or fail. These are reused, and two of them were seized after drying out with rusty heating water flakes - twisting the impeller a few times resolved that. 1/2" flexible tap connectors fit lovely. The black pumps are 1/2" again but far more rigid. I daren't mount the beige ones rigid, but the black ones are solid enough to hang off one rigid connection. Noise isn't a worry, we're used to that!

Anyway, enough waffling! I'll update as I get more completed :-)
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« Reply #1 on: November 21, 2020, 08:52:15 AM »

I love a good project Grin

Kidwelly South Wales
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« Reply #2 on: November 22, 2020, 08:46:41 AM »

Sounds like only part (impressive part) of an interesting project - is that a static double-decker or a travelling double-decker?


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