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Forum Index : Solar : solar hot water
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| angus Newbie  Joined: 08/12/2009 Location: AustraliaPosts: 7 |
Hi. Not so much solar hot water, but more as wood fired hot water. My household burns wood for home heating- lots of wood- and about 24/7 for about 9 months of the year. A lot of heat energy goes up the flue.I have checked the flue temp at around 136`C So what I am hoping to do is to install a wetback in the flue and then pipe the hot water to an insulated hot water tank .The tank is copper and about 500 litres. The tank will not be pressurized and the water will be moved by thermosyphon.Any over heating will vent to the atmosphere above the roof line. This is very much like the old briquette hot water systems of the 1960's. Question is - How can I stop my wood heater from boiling the copper storage tank dry? My parents used to just shut down the briquette heater (back in the 60's), but I cannot do this because my heater is used to heat the house. Can I just shut down the lower inlet to the wetback and allow it to boil dry? The wetback will be made out of stainless steel so it will be okay being boiled dry. Mac your plumbing knowledge could come in handy here. 
Angus.. (The older I get, the better I was). |
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| VK4AYQ Guru  Joined: 02/12/2009 Location: AustraliaPosts: 2539 |
Hi Angus Just a suggestion, I had this problem and used a thermostat from a car to bypass the water through a car radiator mounted out in the cold to dissipate the excess heat. I also installed a small hot water pump and pumped the hot water to the cold end of the house to spread heating around better. It is risky to run a wetback dry as it could result in a flash steam explosion in the system. All the best Bob Foolin Around |
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Downwind Guru Joined: 09/09/2009 Location: AustraliaPosts: 2333 |
To start with you should do a search on Heat Robbers i think they are called which rob hear from the flue, there is losts of warnings on extracting heat from the flues as it causes problems with the flue clogging up and the up draft of the smoke. Basically cooling the smoke in the flue is a bad system and not the way to go here. A friend has a wood stove that has a water jacket at the back of the fire/stove. It is a closed loop system that runs glyocnol (radiator coolant) in the piping. The hot water service tank has a coil in it for a heat exchanger, it works on thermo syphon. The closed loop system with glyconol stops the water boiling in the pipes and gives better heat transfer. The size of the coil or heat exchanger on the back of the stove will greatly control how much heat is transfered to the hot water tank also the placement of the lower coils will also control the heat transfer. If it was me i would use the neck off an old car radiator with a radiator cap for pressure relief and an overflow reservoir so the system could suck the coolant back in when it cooled down same as a car would.(mounted at the highest point) Running a pressurized system the fluid boiling point is rasied greatly. These systems are avaliable commercially and work very well. I think best to copy a proven system and leave the flue alone. Pete. Sometimes it just works |
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| dwyer Guru Joined: 19/09/2005 Location: AustraliaPosts: 574 |
Hi Angus (The wetback will be made out of stainless steel) For safety reason It not good ideas using stainless in the stove as it will crack up 
Dwyer |
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| DTMC Newbie Joined: 12/02/2010 Location: Posts: 11 |
Angus, I have done what you propose with a SS coil (dozen turns of 1/2" on 4" dia) in the middle to upper end of the flue on a low pressure system. I was shooting for 60C tank temperature. A small pump was used to circulate water through the coil with a check valve on the feed side and solenoid and relief valve on the hot side. Shut down was a flick of a switch or thermostat on tank and boil dry on a pressure relief valve. Temperatures in the upper end of the flue were moderate enough that the coil was not at risk. On shut down it would vent ~400ml of steam safely into the vacuum distillation unit. Fuel was waste oil and to deal with soot and ash a vibrator (electric motor with unbalanced mass) was permanently mounted (thermally isolated) to the flue and used each day when the burner was shut down. Once or twice a year I'd run a broom down the flue. The only problem that I have ever had was low water quality (bore water) causing salt build up in the coil. A jacket would have been better in the short term, but still a problem. Long term I built a vacuum distillation unit above the heater coil with the flue as a direct heat source. This not only saved the heater coil, but also the rest of the plumbing and the garden. I'd do this even on mains water. |
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SSW_squall Senior Member  Joined: 20/03/2010 Location: AustraliaPosts: 111 |
Hi peoples, I've been meaning to put up some info on my hot water wood fired heating system for a while and this looks like a good juncture to do so: Our hot water service tank is a large one: approx 300L and is located under the house, below the level of the floor that the slow combustion wood fire is situated on. So a pump was the only option to move the water up to the fire and back down to the tank, in other words a thermosyphon would not work. I've coiled up a single 6m length of annealed 1/2 inch copper pipe into a helix about 600mm high that fits inside the flue. I've cut a small notch at the base of flue pipe where the pipes enter and exit These pipes go through the floor and are connected directly to the hot water service by a small pump that takes water from the bottom of the tank and deposits it back into the top of the tank. 
The pump is a 12v ultra reliable automotive grade unit:Davis Craig pump It has a magnetic drive impeller (no shart seals to fail) and is rated for 120 deg water at a max preasure of 36PSI, in practice this rating is quite genereous as the pressure is less than 30PSI and the temperature from the bottom of the tank rarely gets above about 50deg. 
The pump is operated by a simple thermostatic switch on the "in" pipe right next to the flue. thermostatic switch The thermostat switch is the normally open type and closes at a fixed temperature of 50 degrees, so in normal use the pump runs for about 10 seconds every minute. As more cold water enters the pipe going into the flue the switch opens, after this cold water heats and triggers the switch the water exiting the flue has heated up nicely to about 75 degrees. In this way the most of the water in the coil is at least partially warm so this stop the coil from sooting up the flue too much. In anycase i havn't had any real issue with the flue sooting up although I have stuck a peice of wire up the flue once to dislodge the soot when the fire seemed to be getting abit smoky. (OF course anyone with a flued fire should be doing this anyway) The pump has a 7.2Ah 12v backup battery to ensure that it doesn't ever lose power, and there is always water under 10PSI gravity pressure from the 22000L tank that feeds the house. What would happen if the pump failed?? (like someone switched it off and forgot to turn it back on) This has actually happened a couple of times over the last year, i've lit the fire and realized half an hour later that the pump wasn't running. Well not much really... The water in the coil turns to steam but there is less than 0.5L of water in the coil so this just displaces the water back into the hotwater service. Because there is no non-return valve on the hot water service that is connected to the DAVEY pressure pump this extra pressure goes into the pressure bladder on top of the pump and is comfortably absorbed. When i did switch on the circulating pump there were a few bubbling sound coming from the the hot water service as the steam was forced out of the coil, but this stopped after about 30 seconds or so. I've set the off peak time-switch to heat only in the very early morning (4-6AM) so this ensures that we always have hot water for showers in the morning. But allows the fire to heat the water all day and evening when the fire is burning. Without fire going the hot water service uses anywhere between 6 and 12kWh a night to heat the tank up, but with the fire going this is reduced to about 3 or even 1kWh depending on how long the fire has been burning for Works like dream, but only in winter (like my hydro rig) when we use the fire!! I would like to get a evacuated tube solar hot water service and run the flue heating loop in parallel with the solar collector heating loop... I hope this adds some fuel to the fire 
AB Einstein: Everything should be made as simple as possible, but not one bit simpler |
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Gizmo Admin Group  Joined: 05/06/2004 Location: AustraliaPosts: 5078 |
Great post AB, good info for anyone thinking of doing the same. I have a big old how water system I've been saving for such a project ( only solar heated instead of fire ). I'm thinking it might be worth while adding a fire box for those long wet season days where we wont see the sun for weeks. Glenn The best time to plant a tree was twenty years ago, the second best time is right now. JAQ |
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oztules Guru Joined: 26/07/2007 Location: AustraliaPosts: 1686 |
Thanks AB Good description. ..........oztules Village idiot...or... just another hack out of his depth |
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| SSW_squall Senior Member Joined: 20/03/2010 Location: AustraliaPosts: 111 |
Thanks guys's The only small issues i have had was the the rubber pipe that connects the pump to the copper pipe, which was really stupidity on my part Firstly i grabed an old car heater hose that i *THOUGHT* would be OK And i had this connected such that is was curved through 90 degrees to the bottom of the tank. As the DAVEY pressure pump switches on and off this hose would move slightly as it expanded and contracted, i noted this with some interest and though nothing of it... That is until i came home from work one day and it sprung a pin hole leak which was threatening to turn into a flood!! Now the 90 degree turn is made of copper pipe and silver soldered, so the rubber pipe section is short and straight and is most importantly NEW. Man i felt like such a dick for using that old $#!t-house hose. Some times you've just gotta learn the hard way?! That happened almost a year ago now, it's been hassel free since then. AB Einstein: Everything should be made as simple as possible, but not one bit simpler |
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| VK4AYQ Guru Joined: 02/12/2009 Location: AustraliaPosts: 2539 |
Hi Guys Just a little experiment I did on my new solar hot water system, I have connected the batteries to the booster element in the tank and found it draws 3 amps at 28 volts and that helps keep the water warm during prolonged overcast times. The element is a 3600 watt 240 volt one, so it would be a real power sucker on 240v at our new tariff of 19.41 cents per kw. They are fitting a new off peak power meter, when they get around to it, so the opportunity was there to try it now. It makes a good little dump load when the batteries get full. I also tested the element on the old system, a 2400 watt element and it gets very hot in free air, it burnt all the old scale off the element and draws a little over 2 amps, not a lot but it all helps, when the diversion load controller cuts in the voltage goes higher so that would mean a little extra energy being diverted. This is I think a starting point for the energy storage unit I am building as I found that there are some low voltage elements available on ebay. My experiment with converting old elements as suggested in the forum didn't go to good fumble fingers and magic smoke release. All the best Bob Foolin Around |
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Phil73 Newbie Joined: 22/07/2010 Location: AustraliaPosts: 2 |
Hi Guys, I was thinking of doing a similar thing as Angus to boost my close coupled solar system over the winter when our wood heater is on almost 24/7. I was thinking of making a heating coil form 3/4 inch soft copper pipe the would wrap around the flue pipe in an upwards spiral with about 20 wraps. The hot water tank on the roof is located 8m horizortally form the flue and their would be a 2m rise from the top of the ceiling to the hot water tank on the roof. I would locate the heating coil between the heater and ceiling, inside the flue guard which would basically hide it. Can anyone tell me whether a thermosyphon system should work or is the distance too far? Also could there be corrosion problems with the steel flue being in contact with the copper pipe? My research found that it should be ok as there will be no moisture between the flue and copper pipe to act as an electrolyte? Thanks Phil |
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| VK4AYQ Guru Joined: 02/12/2009 Location: AustraliaPosts: 2539 |
Hi Phill I used the same system on my HWS and it worked well, except when I first did it I had a air lock in the pipe due to a high spot in the circuit, it went 4 mtr across the ceiling cavity and 3 mtr vertical with standard lagging sleeve on the pipe. The flue was 4" stainless steel and no corrosion problem in the ten years I used it. I did have to wrap a heat shield around the copper coils on the flue to get it to work properly, I used a couple of rolls of heat shield that is used to wrap exhaust pipes with, It is a fiber glass type material woven into a 2" wide tape. All the best Bob Foolin Around |
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| greyim Newbie Joined: 01/05/2014 Location: AustraliaPosts: 1 |
Hows it going now? I have a similar situation, putting the hwc up high is problematic.... so am thinking of 12v pump etc same as you. My cylinder is a Saxon Copperflow water jackey coil exchange type so the wetback circuit would be low pressure anyway. |
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