Home-Made Hot Water Collector Finally in Operation

Last night, late, around 2:00 AM I flipped the switch on my home-made radiant hot water collector. The collector sits on top of my hopper-fed Saey Hanover I coal stove and feeds my electric hot water heater. This has been a long process. I bought the 80 gallon electric water heater over a year ago and just plumbed it into my system in late October, running only the upper 4500 watt element to heat the top 30 gallons. At today's electric rates, I can't imagine using the electric elements for all my hot water. Over the last hear I've collected all the odds and ends to make this system pull heat off my stove. Details of the system below:

The tank originally came with 3 ports on the top, one of which was plugged. I removed the plug and made my own dip-tube where the heated water from the collector enters about mid-way down into the tank. I hope to keep the incoming warm water from disturbing the stratified layer of hot water above it by plugging the end of the tube and allowing the water to enter the tank through (8) 1/4 inch holes drilled around the bottom of the tube. We'll see.

I use a Grundfos UP15-10 bronze-body hot water recirculation pump because I'm circulating potable water. It's not a good idea to use cast-iron pumps designed for hydronic heating with potable water due to the highly oxygenated water constantly being replenished from your water supply line. The pump is operated by a Stiebel Eltron SOM-7si differential controller which monitors the collector temperature, tank temperature, and cold water outlet (to collector) temperature to determine when the pump should run to add heat to the tank.

The collector itself is a marvel of creative scrounging. I had picked up a junk window air-conditioner with the intent of using its thermostat to control the pump, but that was apparently the part that was bad on this particular air conditioner. Rather than find another one, I bought the (fairly expensive) SOM-7si differential controller and disassembled the air conditioner for more parts. I salvaged both the evaporator and condenser off the defunct A/C unit and junked most of the rest.

The condenser is a 3-layer unit about 18 inches long by 13 inches wide and contains 67 feet of 3/8 inch tubing with aluminum fins and galvanized steel end plates. The end-plates keep the collector's fins about 1/2 inch above the stovetop which means I'm not cooling the surface of the stove by conduction - only radiation and convection. This should prevent any problems with possible regulatory compliance (as if that was ever an issue with this group).

The condenser's tubes are joined with silver-solder which requires a lot of heat to soften. Both inlet and outlet tubes needed to be removed so they could be replaced with 5/16 inch soft copper tubing and compression couplings. I ended up using an acetylene plumber's torch but MAPP gas would probably work too. I stubbed out the inlet and outlet and connected stainless steel braid flexible sink-connecting hoses to it so the collector could be lifted off the stove and swung down to the floor for coal-hopper loading. I also installed a handle and a protective grate (also salvaged from the A/C unit) to allow easy and safe handling of the heavy, water-filled collector. I painted the collector black with high temperature gas-grill paint to help it collect radiant heat.

Before using this A/C condenser with potable water, I cleaned it out with detergent and a mild acid to remove any refrigerant or lubricant residue. I ran water and cleaning agents through it using a pressure-washer to assure I got every last bit out. I probably ran over a hundred gallons of clean water through it to make sure it was properly flushed out afterward. A/C systems run pretty high pressures and temperatures so I'm pretty confident this collector will work out well and hold any pressure I'm likely to encounter.

I installed a 150 PSI pressure relief right at the outlet of the collector, before any valves, to be on the safe side. The tank, of course, still has its 190 degree/150 PSI temperature/pressure relief valve but I wanted to be sure that I wouldn't have any steam explosions should I accidentally forget I'd closed both valves and left the collector on the hot stove.

So after hours and hours of wiring, plumbing and insulating pipes etc. I finally got the air bled out and turned on the controller at 2:00 AM this morning (December 1). The pump kept cycling on and off for a while due to the stove not being all that hot and the 56 degree inlet water I suppose. I revved up the stove and set it for the night before crawling off to bed. By 7:00 this morning I'd achieved about 104 degrees at the collector and the pump was running constantly. By 9:00 AM I was seeing 118 degrees at the collector and 103 degrees at the bottom of the tank. Wa-hooo!!! Looks like it's gonna work!!!

I took a long shower and giggled about how much less I was going to be spending on electric hot water. By the time I was done, the tank temperature had dropped to 95 degrees at the top (must be some mixing going on or a bad sensor)and the bottom was around 63 degrees. The collector was showing 13 degrees between the inlet and outlet as I left to run some errands at 10:00 AM. By the time I got back at 12:30, the temperatures were 113 at the collector and 104 at the bottom of the tank. That upper sensor still showed lower than expected temperatures though so I'm going to do some troubleshooting with another sensor probe.

Update 4:00 PM 12/1/07: The upper tank sensor was not really making good thermal contact with the tank. I dug out some insulation from the top of the tank underneath the AC power box and scraped the surface clean. I installed the DT1000 thermal sensor with conductive silicone grease and aluminum sticky-tape then covered with poly-foam. Reads fine now at 113 degrees. Collector is 118 degrees. Return line at tank is 114 degrees. Guess I've got to work on better pipe insulation.

I think this collector is going to work out well for me but I'm a little disappointed at the cooler temperatures. I was hoping to reach 160 degrees and be able to use the second pump relay to trigger a valve that dumps excess heat to a kick-space heater in the bathroom. I've still got the smaller evaporator (2 layers, about 36 feet of tubing) which could be added to the side of the stove in series with the bigger collector. We'll see.

I'll keep you guys posted.

Please post pictures of this set-up when you can.

Wood'nCoal wrote:Please post pictures of this set-up when you can.

I second that.

OK, here's some pictures:

The stove with collector and plumbing: Close up of the collector made from a window air-conditioner condenser: The DT-1000 temperature sensor attached to the outlet tube with conductive grease and aluminum tape: Valves and pressure relief: Tank, pump and controller: Tonight it's supposed to drop into the 'teens so I opened up the thermostatic control on the stove to setting 6 (of 10) and am getting about 125 degrees at the collector which drops to 120 at the tank entry point. Running the dish-washer dropped the temperature at the bottom of the tank to the mid 90's and lowered the collector temp to around 104 degrees till it heats that blast of cold (57 degree) water from the main. I expect tomorrow morning's tank temperature to be around 125 or 130 degrees.

I'd still like to see 150 or so, but that will take another stage of the collector. Still, if it peaks out at 130 degrees, I'll never have to worry about overheating the tank.

Very nice work, looks great.

The worst part about a project like this for me is not having it work the way I had hoped when I thought of it, but you have gotten results already.
Heating water with electricity is very costly, and the recovery rate of an electric water heater is very slow. I switched from a 40 gal. electric to a 30 gal. oil-fired water heater in 2002, what a difference. My electric bill dropped $50 per month (I had the water heater on a timer as well), and we stopped running out of hot water. Now I plan to add a coal burner w/ a hot water coil to the mix, so I won't be burning as much oil. The Aero water heater is great (it's a low boy side vent), but I'd rather heat with coal.

Please keep us updated regarding your results.

You got my vote for homemade engineer of the year Bob! That's a cool setup. We should start a TV show like "junkyard wars" but it will be about saving money on heating/hot water via unusual contraptions.

Thanks for the compliments. It's currently 14 degrees and windy outside. I've got the stove cranking at about level 7 or 8 (of 10) and am keeping the small 1200 square foot house in the high 60's to mid 70's in the living-room where the stove is installed. The pump is running and I've got a whole 80 gallon tank of 120 degree water without using any electricity other than the 85 watt pump motor. I've told the oil-man goodbye for good and am happily burning domestically mined coal instead of foreign oil. Since most of Massachusetts' electricity is provided by natural gas, I'd still be burning domestic fuels anyway, but at a much higher price.

Next year's plan is to install a 10-element evacuated tube solar collector on the south-facing side of my house. The existing SOM-7si controller can handle dual sources of heat with 2 pumps or a switching valve so I'm well on my way toward my goal of becoming more efficient and not relying on foreign oil. There's big tax incentives for alternative energy (solar) equipment purchases too. Massachusetts gives a tax credit of up to $1000 on 15% of the cost of equipment and installation. The feds give a tax credit of up to $2000 on 30% of the expenditures. Between the two, that's 45% of the first $1000 dollars and 30% of the next $1000. And none of the equipment or installation is taxable nor does it raise the property values for local property tax (in Mass anyway).

With my sweat-equity I can probably put in the solar collector, plumbing, and pump for under $2000. Of that $2000, I'll get $150 back from the state and $600 from the fed reducing my net cost by $750. I'll need to change out my water tank or install a Solar-Wand heat exchanger to utilize the anti-freeze filled solar loop. Stay tuned (even though it's not strictly-speaking coal-burning).

Bob, you did a hell of a nice clean job....congrats.

I like the set-up, but you must not be married. That would never fly here. I showed my wife the pictures as an illustration of someone who is (almost) as bad as I am with these ideas, trying to win her over to the "dark side".

I guess I should have mentioned that I'm a "function over form" kind of guy. No I'm not married - I guess a typical wife wouldn't like the looks of the collector. That's why the internal loops are probably popular. You'd still have the exposed piping to some extent in my situation due to the concrete base below the slate hearth floor.

As for performance, the tank was at 121 degrees this morning but the house was at 62 degrees The larger-than-I-remember nut coal got jammed in the hopper and the fire was nearly dead at 5:00 this morning. I may have to start mixing nut and peal coal so the smaller pieces help the bigger ones drop down onto the grates. I don't remamber having this problem last year, but apparently I've got it now. And me with 2 tons of nut and one ton of pea on pallets...

When I finally got the stove going again after unjamming the coal, I went downstairs to monitor the system. The pump was off most of the time and when it did cycle on for 30-60 seconds, the temperature at the tank feed line from the collector jumped to almost 150 degrees before the pump shut off again due to low collector temperature. I may start fiddling with the Delta-T on the controller to allow more of this cycling. It can slow the recovery rate but should raise the maximum temperature.

I really wish Stiebel Eltron had imported the original Resol DeltaSol controllers with their solid-state relays instead of the mechanical relays. The Resol DeltaSol BSpro is nearly identical to the SOM-7si but can control the pump speed to maximize temperatures. It duty-cycle modulates the AC to the pumps from 30% to 100% based on some extra parameters in its programming. I wonder why Stiebel Eltron doesn't buy that model and relabel it as their own? Must be some reason (like maybe Germany uses 240 volts which allows lower current switching circuits). But then again, there's a Canadian Resol distributor that has 120 volt versions. Must be an OEM product built that way for a reason (not that I'd agree with the reason).

Still shiverring in Massachusetts.

I've been running pea coal, I seem to get better results with it. I'm trying out pea/nut mix right now.
I think the pea coal ashes up the stove faster due to it's size, but the nut will burn longer but then I have problems like you just had.
I tried stove size, wasn't happy at all.
I've noticed some large pieces of nut that are almost stove size in the bin as well. I guess it's a matter of where it comes from.
I'm not familiar with your stove, it has a hopper, but it's not a stoker, am I right?

I burned pea coal the first season but found I couldn't get enough heat out of it on really cold days. I bought a bag of nut coal and tried it and like the boost in performance but it did go through the coal faster (of course). So this season I bought 2 pallets of nut and one of pea, hoping I'd guessed the right combination of cold to moderate temperature days. I really can't keep the nut coal burning when the temps get above 45 degrees in the fall and spring, but it usually burns fine in cold weather. This year's batch must have come from a different coal breaker who has worn out screens or worn crusher parts. I can burn pea coal into the 60's at low thermostatic control settings so I save it for the early and late parts of the heating season or when we get a warm spell.

The Saey Hanover 1 is a hopper-fed 60,000 BTUh stove with a thermostatic air-shutter to regulate the fire. It has a top-loading center hopper which is why I made the collector easily removable. Between the coal on the grate and the full hopper it holds about 50 pounds at a time so I usually add 15-20 pounds twice a day and empty the ash pan once a day. Lifting the collector off the top was always a design feature I knew I'd need. In fact, I built a rectangular frame which you can see in the pictures behind the stove. It was meant to hold a 50 foot coil of copper tubing cemented into the frame with plaster of Paris, but I found this air-conditioner before I ever got around to building it. I may still have to go that route if this one doesn't produce more heat. It will connect to the existing flexible hoses and be equipped with a hinge at the rear so it flips up vertically for coal loading.

Update 12/6/07:

The heat exchanger has been working well and I had a tank of 150 degree water this morning. Guess I've got to start building the heat-dump radiant floor heating system for my bathroom. To get rid of the excess hot water, I started doing laundry with hot water and warm rinse. Obviously this is wasteful if you're paying the oil-man or electric company for hot water, but I'm using waste heat off the stove and burning $200/ton coal instead of $3.00/gallon oil (200 gal. is equivalent to 1 ton of coal) or 15 cents/kilowatt electricity so who cares.

One little problem cropped up today which may become a real issue if not solved. The swaged ends on the flexible braided hoses I use to allow moving the collector began leaking right after starting the washing machine. Apparently when the washing machine took it's 20+ gallons of hot water the tank was refilled with 56 degree stuff which began circulating to the collector. Apparently the wide temperature swings caused the polymer lined flex-hoses to creep a little, loosening their seal on the barbed fittings. I guess I'll have to find a source for Teflon hoses instead of these cheap home-center supply lines. For now, I re-swaged the ends with a tool I've had for years that's used to swage aluminum ends onto steel cable. It's not as pretty as the original 8-point swage, but it stopped the dripping. Teflon flex-hoses appear to be swaged continuously with a much longer barbed end to tolerate more abuse. Of course I'm sure they're mch more expensive...

As for dumping surplus heat, I want to install another pump to take water from the top of the tank before the tempering valve and circulate it through an under-floor radiant "dry-system" loop between the floor joists of the bathroom. That should warm up the freezing tile floor and help use up that excess hot water in the tank. I don't want the tank to get over 150 degrees for safety's sake but I also never want to hit the 160 degree temperature cutoff on the pump which could cause steam to be created. I'm debating whether to use PEX, PEX-Aluminum-PEX, or just plain old type L or M copper tubing. I'm good at soldering and hardly ever have a bad solder-joint so I may just buy copper tubing and not risk the "newfangled" PEX for this purpose. Any input on the relative merits of each would be appreciated.

I've been doing some additional pipe insulation too. I wrapped all the pipe going to and from the collector with radiant-barrier aluminized bubble-wrap type insulation. Same stuff as the water-heater wrap but coated with aluminum on both sides. The same material will go under the radiant flooring between the joists to reflect 98% of the I/R energy upward toward the floor above. From what I read, it's good stuff though kind-of expensive. A 48 inch wide by 25 foot long roll costs about $45 at the local home-center.

After almost a month of operation, I'm very satisfied with my external heat exchanger on top of my hopper-fed coal stove. Over the last 4 weeks, I've had more than enough hot water for my 1-person house and even had enough excess to be able to use hot or warm water for the laundry and to take longer hot showers without worrying about running short of hot water or spinning the electric meter off the wall.

On average, I've been maintaining about 120 - 150 degree water in my 80 gallon tank. I've been fiddling with the differential controller's settings in the last couple of days trying to maximize the temperature in the tank. I activated a collector minimum temperature of 135 degrees before the pump will turn on, making sure the collector won't be dumping cooler water into the tank than the minimum desired top-of-tank temperature. This may reduce the total heat returned to the tank but generally results in higher peak tank temps and reduced tank cooling.

When you use hot water from the tank, it is replaced by cold (50 degree) municipal water. This cold water partially mixes with the water near the bottom of the tank, sending 60-70 degree water to the heat exchanger. With my 5 GPM pump, the exchanger can only add about 10 degrees to this heat if running continuously. This would cause the return water to be cooler than the water at the top of the tank, mixing with it and dropping the tank temperature.

I readjusted the Delta-T settings so the pump comes on at 15 degrees hotter at the collector and turns off at 10 degrees. This wasn't enough though so I added the minimum turn-on temperature setting at 135 degrees. Now the pump cycles on and off to keep the collector temperature between 125 and 135 degrees minimum. The water returning to the tank is always warmer than the minimum I want to have in the tank (125 degrees) so the tank never cools off to the point where the mixing valve is bottomed out.

This may slow the recovery rate after hot water use, but results in continuous 120 degree water available at the taps unless I've used ALL the 80 gallons of water in the tank. Now, even on warmer days when the stove is regulated down to a lower temperature, the stagnant water in the collector reaches 135 degrees before the pump sends it to the tank and replaces it with cold water. This cycling goes on until the water at the bottom of the tank is above 125 degrees at which point it usually runs continuously again. In fact, as the temps come up, the off-time is reduced as the batch of water needs less time to raise to the turn-on temperature.

So far, so good. I haven't gotten a full month's electric bill yet to compare costs, but I anticipate saving about $20 TO $30 a month compared to using the 4500 watt electric heating elements in the tank. We pay about 15 cents/kilowatt-hour around here. People in more expensive areas would, of course, save more.

Nice Job Bob! Hey I have been thinking of using my Solar Hot Water system with the coal stove. Basically, my solar runs from Spring thru fall, so from Nov til march/april, it's just shut down and idle. But all I need to do is take the existing loop and set it up to send the water to a heat exchanger like you have to, pre-heat my well water starting at 55 degrees. I have a sepersate 30 gallon electric tank as a backup that I'm using now. All I need is a splitter value to redirect the water! So you just used an auto AC raditor to pickup heat? Jim