How to best heat the SLAB ?

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Re: How to best heat the SLAB ?

Postby Wirregar » Fri Oct 02, 2009 12:09 am

FYI; we are in Gippsland, Victoria. Have ample supplies of wood, and it is free as it comes from my land. So Wood is attractive cost wise. A new more efficient Wood heater would probably be the best option at this stage.

To Answer some other questions, The house is approx 45-50Sq. which the slab would heat. So a large volume of water is needed to be heated. Of course with all slab heaters, it takes 24-48 Hours to get slightly warm.. So it is the sort of heating you want to keep running once you start it up.

Being in Gippsland, I doubt Solar would provide enough heat during the months we need it. Short days and cold temps (around 4-10c during Winter) And as we are on bottled gas, supplementing the solar with LPG is not an option I want to entertain.

Diesel, or Bio Diesel could be an option, are they more efficient or cleaner to burn than wood fired furnaces?
Maybe I will look into that?

So at this stage, Wood fired or Diesel Fired Furnace is where I am heading..
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Re: How to best heat the SLAB ?

Postby greg c » Wed Oct 07, 2009 8:13 pm

I would thoroughly investigate solar heating. You don't need any storage as such, use the slab as storage. All you will need to buy are the collectors, a more suitable pump, a differential controller and a one way valve to stop reverse thermosiphoning at night. You obviously need an expansion tank but I'll bet you have one of those on the system already. With the money it would take to do anything else, you can buy a lot of solar collectors and heat the house with zero emissions. I would work out how much water firstly, and knowing that for shower temp hot water we normally use 2 square meters per 150l, I think you could let that run out a bit to say 250l. The beauty of solar is it runs every day for nothing, and if it is sized properly and the collectors angled to maximise winter gain It should be able to keep the house at a fairly even temperature.

I would forget diesel, filthy fuel best used in transport. If you need to burn something I'd say wood is the only option on a cost basis, and it is renewable.
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Re: How to best heat the SLAB ?

Postby Gordon-Loomberah » Wed Oct 07, 2009 8:47 pm

A big Yes from me to what greg c says, go for evacuated tubes, supplemented by wood burning. A big NO to diesel.
Burning wood is totally renewable, you just have to plant enough trees to cover your use of wood.
I have 78 evacuated tubes supplying my hydronics radiators, which only cost a $few thousand.The 250l storage tank is not quite enough, but I will be plumbing in another 1000l tank soon, and we are in the process of building the house and have no curtains over most of the windows yet, and havent finished insulating and lining all the walls. I'll be adding the heating of the wetback stove to the system sometime soon, and then it will be more than enough heat for cold frosty winter nights.

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Re: How to best heat the SLAB ?

Postby munter » Thu Oct 08, 2009 11:52 am

Given the relatively low temperature that I understand slab heating works at you could potentially use a low pressure solar colleciton system to keep the costs down. Low pressure evacuated tube collectors are cheaper because they don't have the heat pipes and manifolds and just run the water to be heated directly in the evac tubes. The downside is that a cracked tube results in a system leak which requires a trip to the roof to fix.
I would think that the large thermal storage you would have in the slab would alleviate the need to have large storage tanks but also that water is probably the appropriate medium due to its higher specific heat compared to air. The exchange of heat will be much more compact using water.
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Re: How to best heat the SLAB ?

Postby andarm » Thu Oct 08, 2009 9:10 pm

With an evac. tube system a broken tube doesn't equate to a water leak, just a loss of insulation around that specific collector.
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Re: How to best heat the SLAB ?

Postby Gordon-Loomberah » Thu Oct 08, 2009 9:22 pm

andarm wrote:With an evac. tube system a broken tube doesn't equate to a water leak, just a loss of insulation around that specific collector.


That all depends on whether the tube has water in it, and how broken it is. With heat pipes (no water in the tube) you will just see a slight reduction in heating. With water filled tubes you can lose all your water if the inner and outer glass break.

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Re: How to best heat the SLAB ?

Postby andarm » Thu Oct 08, 2009 9:28 pm

U tube - the water is held within the copper pipe
Heat pipe - the phase change medium is held within the copper pipe.

Neither have the water in contact with the glass. The actual tube is only for insulative purposes
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Re: How to best heat the SLAB ?

Postby munter » Fri Oct 09, 2009 11:24 am

Both the systems you describe are high pressure systems capable of handling mains pressure. There is another approach to the use of evacuated tubes which does away with heat pipes and U-tubes and runs the water directly in the evacuated tube. These are referred to as low pressure systems as they are not capable of working at mains pressure.

These low pressure systems are cheap because they dispense with the heat pipes and copper tubes but have the draw-back of potential leaks when a tube cracks and the issue of sealing around the top of the tube where it enters the tank/manifold. I'm not sure if they are equally as effective as heat pipe or U-tube systems but I suspect not. Given the lower cost per unit they might still be the most effective solution for hydronic heating.
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Re: How to best heat the SLAB ?

Postby Gordon-Loomberah » Fri Oct 09, 2009 12:36 pm

munter wrote:These low pressure systems are cheap because they dispense with the heat pipes and copper tubes but have the draw-back of potential leaks when a tube cracks and the issue of sealing around the top of the tube where it enters the tank/manifold. I'm not sure if they are equally as effective as heat pipe or U-tube systems but I suspect not. Given the lower cost per unit they might still be the most effective solution for hydronic heating.


My hydronics system has a 30 water filled glass tubes + 2 X 24 tube manifolds with heat pipes. I've only had it running 4 months, but no leaks so far. Its a bit hard to know if the heat pipes are more effective than the water filled tubes, but I know all of them together can get the 250l tank boiling by late morning on sunny days, even in winter.

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Re: How to best heat the SLAB ?

Postby beachbum » Wed Nov 18, 2009 9:15 am

How about something really simple and cheap?
Use the KISS principle.
(Keep it simple, Stupid! )
A couple of rolls of 50mm agricultural poly pipe connected together and laid out in a fanwise fashion with a pump which is turned on and off by a light sensitive switch.
The poly pipe and pump is connected in series with the tubing in the slab and is filled with water to create a closed loop system.
When the sun shines the water in the poly pipe is heated and the pump is turned on, thus circulating warmed water through the slab which will absorb the heat and returning cooled water to the poly pipe for reheating.
If the slab ever gets too hot for comfort, you just turn off the power to the pump.
The system could be refined a bit by adding a temperature differential switch to the circuitry so that the pump can't start until the water in the poly pipe is hotter than the slab.
You will be surprised at how much heat energy is captured by black poly pipe when laid out in the sun.
I had an irrigation system in Tasmania that had a 80m section which could not be buried due to sheet rock in the vicinity - The water became too hot for comfort, even in the middle of a Tasmanian winter.
Two rolls of 100m each would do a pretty good job on a house slab.If not enough, just add more rolls. The pressure loss due to friction is negligible, so extra lengths will not increase the friction head substantially and the existing pump will probably suffice.
As a bonus, you could also use the system to cool your slab (and house) in summer, simply by reversing the light and temperature differential switches. ie the pump turns on at night when the outside air temperature is less than that of the slab in the house, thus transferring the heat to the great outdoors.
However, there is one serious downside to a system like this, and that is its low WAF score.
(Wife acceptability factor)
She who must be obeyed, will probably think that a bit of poly pipe on the ground in front of the house looks unsightly and will veto it.
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