Alternative Technology Assn Forums

1. 

   Bullwitter
   Member

   A few years ago I tried redirecting the cold water feed line to my (tank-on-roof-flat-plate) Solahart HWS via a heat transfer coil in a Beasley, open-vented hot water tank. The Beasley tank is heated by a thermo-syphon connection to a wood-burning heater.

   It did not work!

   Drawing warmer feed water into the Solahart tank makes no perceptable difference to its efficiency. I'm not really clear about why this is so, but think it has to do with the large size of the Solahart storage tank and the heat layering configeration therein.

   I am told there is a way around this but don't know what it is. Grateful if anyone can throw any light (or heat) on this subject?

   Posted Monday 25 Apr 2011 @ 1:32:01 am from IP #
2. 

   Skysolarman
   Member

   Hi Bullwitter , i wonder if you maybe getting reverse circulation from your thermosyphon system at night time once the wood burner has finished and by the morning the tank is cold...reverse circulation is worsened if the open vented tank is "boiled" by the wood fire , the higher the tank temperature the worse the thermosyphon if there is a thermosyphon problem , check the tank temp in the morning to isolate if this is the problem . If thats ok maybe its because your thermostat controlling the element is not controlled...for example if the preheat water temp from the open vented tank is 40 degrees and the thermostat is set to 60 it will always click the element on regardless

   Posted Monday 25 Apr 2011 @ 6:43:00 am from IP #
3. 

   Bullwitter
   Member

   Thanks Skysolarman, I will check for reverse thermosyphon in the wee hours. If there IS reverse thermosyphon, how is it prevented? The Basley tank never exceeds about 40 degrees.

   I'm not sure I understand the comment about the Solarhart thermostat. As far as I can tell, it was working properly before I redirected the feed water through the Beasley tank, nothing appeared to change during the redirection and likewise how that I have disconnected the redirection.

   One reason I disconnected the feed-water redirection is that Solahart are about to move my solar heater along the roof to make way for more PVs. Interestingly, the new position for the Solar water heater is almost directly over the wood burning heater (as close as the brick chmney will allow). If wood-preheating ever works it should do so with the new relative positions of the solar and wood heaters. The other reason is that it didn't work.

   My gut feeling is that my whole set up is wrong. Obviously I can't connect the pipes from the stove to the Solarhart tank as, being a closed system, it would blow up!! If I could replace the Solarhart tank with the open-vented Beasley tank the problem would be solved. I don't want to do that as it would be ugly, etc etc. Perhaps I need a smallish heat exchange tank in the ceiling directly under the Solahart which:
   1. is open-vented and heated from the fireplace below. and
   2. contains a heat exchanger which is part of the glycol circuit of the Solathart.

   The difference between that and what I have now (until recently) is that heat would be carried into the Solarhart tank by thermosyphon whenever the fire is going rather that just whenever water was drawn into the tank.

   What do you think?

   Posted Monday 25 Apr 2011 @ 7:50:44 am from IP #
4. 

   Gavin_S
   Member

   Hey Bullwitter,
   This thread is a bit old and may be dead, but it is an issue I have been fiddling with for a few years.

   We have owner built a house 5 years ago and at the time installed a Solarhart with no electrical backup. As a backup in the winter we attached a wood buring heater, with wet back and because it is a high pressure closed system were told you must use an open vented heat exchanger on the Solarhart. This turned out to be quite inefficient. With the heater burning for 5 - 6 hours a night there wasn't really enough hot water for a family of 4. Lots of luke warm showers on winter mornings.

   I bypassed the heat exchanger and circulated the hot water directly through the Solarhart tank. This improved efficiency and we could all have quick hot showers but no long hot baths and I'm still not happy with it.

   I am now adding a flue hot water jacket to try and increase the amount of hot water. I'm also thinking of adding a pump to circulate the water through the system.
   Would this increase efficiency over just relying on convection?

   A few questions/comments re the previous posts.

   I was under the impression that reverse thermosyphon/convection isn't possible. If the hot water tank is at the high point, then once the fire is out, cold water settle at the bottom and without heat to circulate it, hot water rises and nothing happens.

   Regarding connecting a wood heater to a closed system - the solar collectors are connected to a closed system. With hot WA summers the sun gets the water much much hotter than I've ever managed with the wood heater. And is it a closed system if it has a pressure blow off valve?

   The other problem is that the cold water entering the hot water system when you turn on the tap on those chilly winters mornings is only 3-4 degrees.

   Any feedback greatly appreciated.

   G.

   Posted Sunday 11 Mar 2012 @ 8:13:05 am from IP #
5. 

   Bullwitter
   Member

   Hi Gavin,
   Thanks for a very interesting reply. What you have actually done has mirrored my ‘thought journey’ and saved me spending $1,000 on an open-vented heat exchanger. I baulked as I suspected it wouldn’t be very effectivebecause it would have meant THREE heat exchangers in a row! Each one less than perfect - adding up to big losses.

   You have eliminated one heat exchanger - connecting your fireplace wetback directly to the Solahart tank – without blowing it up. I follow, and accept, your logic that the fire is no more dangerous than solar panels - as an “uncontrolled” heat source. I am keen to hear how the addition of flue hot-water jacket changes things. This is another heat exchanger, but I presume it is plumbed in with the wetback so they operate as one – right?

   However, I have gone off in a different direction. My wetback (thermosyphon) connected to the Beasley tank located on the adjacent bed room works sooooo well for heating that bedroom that I don’t want to change it. Instead, I am using my saved $1,000 to try “solar-boosted solar”. By that I mean an array of vertical (or near vertical) evacuated tubes on a nearby northern wall. They should contribute almost nothing in Summer (when not needed) but hopefully quite a bit in winter when the low sun beats directly onto that wall. They would be directly linked to the Solahart tank (as are the existing flat plate collectors). My problem now is how long can thermosyphon pipes be before the thermosyphon slows down too much.

   I will report on progress, and welcome comments.

   PS. You are right about the thermosyphon not reversing if the tank is a decent height. Cold water entering the tank at 3-4 degrees on a winter morning is – well – the way the world works and your Solahart tank set up should keep it away from the hot outlet. About 20 years ago it was fashionable to tinker with solar pre-heaters, but they were found to be ineffective - and I reinvented this square wheel again myself!!

   Posted Sunday 11 Mar 2012 @ 11:42:53 am from IP #
6. 

   Joe
   Member

   For Thermo Siphon to work the inlet and outlet pipes need a fall:- have to have a minimum gradient of >150mm/1000mm run and the pipes >20mm dia fully lagged.
   as long as the pre heater is below the Solar Hart tank and has sufficient fall in the pipes then it will work.
   The Solar Hart should have a high temp/Pressure relief valve that will dump water if the tank gets to hot, a friend of mine fitted a pressure relief on the cold feed to minimise the hot water loss on pressure rise it dumps the excess pressure into his house water storage tank so the Solar Hrart now only relieves on temperature
   Joe

   Posted Sunday 11 Mar 2012 @ 6:33:06 pm from IP #

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