Hi Everyone, Newbie here,
I've read many posts regarding hydronic heating, but not quite where I'm headed.
So, here goes. All and any advice welcome!
I'm having a large shed built in my yard here in Blayney NSW. Yes it gets cold here, l know!
The slab will be the usual 100mm thick, and is 6M x 15M ie 95 square metres.
I am thinking of running the pex pipe in the slab.
Here's the plan.
Put in the pex pipe - 19mm pipe spaced at 9 inch centres
I will have a wood burning heater or fireplace as l have unlimited firewood on the farm.
I'm thinking of running copper pipe around the flue or in the back of the fire to heat the water.
I would like to use no pumps, just thermosyphon to move the water with a smaller header tank to allow for expansion.
Will this work?
I was worried I may boil the water however my steam buff father in law thinks my problem will be not hot enough with all that slab to heat.
The slab will not be insulated, (l know this isn't ideal) but the shed will be insulated.
I don't care if it's not toasty warm inside, even just to take the chill off the slabwould be good.
I also have two large slidind doors and two large windows facing north.
Thoughts ?
Thanks in advance
BC
Hydronic Slab heating
(21 posts) (6 voices)-
Brad Child
MemberPosted Thursday 28 Apr 2016 @ 12:22:52 am from IP # -
wlb
MemberSorry but your father-in-law is right, unless the heater is well below the level of the slab to allow thermosiphoning. And then you'll need well-insulated pipes from your heat exchanger to the slab or you'll lose what heat you have managed to put into the water.
You are also unlikely to get much useful heat transfer from a coil around the flue. I was faced with a similar challenge 35 years ago to heat the upper floor of a building by adding such a heater exchanger to a ground floor wood-fired heater. It didn't take long to work out that it wouldn't work and the project evolved down a more complex path. I have copy of a very old book on thermosiphoning central heating system design. I can post you a scanned copy if you like.
Posted Thursday 28 Apr 2016 @ 2:43:48 am from IP # -
ghostgum
MemberHave a look at the examples of solar air heating for sheds/barns at builditsolar.com
They should work well in a climate that is cold with winter sun.For slab hydronics, you can get thermo-siphon flow from the heater to a header tank above it, but you'll need a pump from the tank to the slab. I'd at least partly insulate the slab, e.g. waffle pod.
Posted Thursday 28 Apr 2016 @ 4:55:45 am from IP # -
Brad Child
MemberThanks for the replies.
I had this idea that the syphon would still work as the weight of the water is equalised, like a tube filled with water spirit level.
What am l missing?
It's sounding like its getting $$$. I was hoping for really simple. Fire is going- warm water is flowing through the slab.
Any ideas on how much the pipe would cost for 95m2?Posted Thursday 28 Apr 2016 @ 8:28:47 am from IP # -
Brad Child
MemberAlso, thanks wlb, I'd love to read that book if it's not too much trouble to send it to me.
Posted Thursday 28 Apr 2016 @ 8:32:27 am from IP # -
wlb
MemberBrad,
Send me a Private Message with your postal address and I'll print and post it to you.It's all about differences in density. When the water loses heat into the concrete slab and cools, it becomes more dense and must be able to fall down to the heater where it is reheated, expands and becomes less dense and rises back up to the slab and the cycle repeats. For thermosiphoning, the heat source must be lower than the point at which the heat is to be transported and used.
With your proposal, the water in the flue coil will boil and blow out through the vent and the cold water will remain stationary in the slab. If there is no open vent in the system, the pipe will rupture of explode, releasing high pressure steam.
Your system would require a circulating pump, which are small and don't consume much power, but you won't get much heat from your coil; not enough to make an impact on a large slab.Posted Thursday 28 Apr 2016 @ 9:52:12 am from IP # -
Brad Child
MemberAhhh, I see!! Yes, makes sense to me now.
Maybe a small pump and some coils or a zigzag in the back of the fire. I'm thinking of making my own fireplace so can design it in.Posted Thursday 28 Apr 2016 @ 11:03:08 am from IP # -
wlb
MemberIt must be open vented and have a header tank to allow for expansion and evaporation. If you do that, and have the right sort of pump, it will 'work'. But it will probably be ineffective, although you'll have the benefit of the radiant heat from the fire itself.
Posted Thursday 28 Apr 2016 @ 10:56:10 pm from IP # -
Phil A
MemberI'm not quite sure how I should address this issue without upsetting people I have respect for.
In respect to the above advice on thermo syphoning into a floor from a cylinder above, while sounding good in theory and at face value sounds correct, it is in fact misguided and inaccurate.
I'm not very good at explaining things sometimes but as I feel that this subject (thermo syphoning)is worth exploring Im going to try. It is one of my favourite subjects. I just love something for nothing!
As a matter of interest, I am sitting in a dwelling being heated by water being "thermo syphoned" or "gravity pumped" from a hot water cylinder above into a concrete slab below, yes.... it is possible.
The biggest problem is finding plumbers who understand how it works or are prepared to set it up. I will try and explain.
You must accept that the terms hot and cold are "relative" in engineering terms.
ie 60 degrees, while reasonable warm in domestic hot water terms, may in fact be the "cold" side of the system. You must get your head around this to understand these principles.
In this system, the heater, or in my case the furnace, becomes the "pumping device"In this circuit the "heater or furnace" is in the circuit between the cylinder and the floor on the return side. In other words the water is able to flow from the cylinder directly down into the floor, then out of the floor into the "inlet" side of the heating device.
From the outlet side of the heater it flows back up to the cylinder. If the water in the cylinder is at say 60 degrees and the water in the floor and the heater is 18 degrees then obviously there is no flow.
However, if you put fire in the heater and raise the water temperature sufficiently, the water from the cylinder becomes the "cold" side of the system and "pushes" the water down through the slab and then into the inlet side of the heater where the temperature is raised enough to allow it to be "pumped" back up to the cylinder.
In this case the "cold" water is 60 degrees, more than enough to put "heat" into the floor.
One obvious issue here is that once the heater cools down, the pumping effect stops. But I can assure you can thermo syphon "hot" water down into the floor.
The key is that the weight of the water in the "cold side of the system must "weigh"or have more mass, than the water on the hot side of the system.
The outlet from the cylinder is from the bottom of the cylinder to maintain the temperature differential.
There is other plumbing involved to enable operation of the heater without using the floor heating but for this exercise I have kept it as simple as possible.
Hope this helps.Posted Sunday 22 May 2016 @ 12:00:34 pm from IP # -
MrEGuy
MemberPhil, So what you are saying is that you can heat the water above the slab and it will then run upwards slightly into a storage cylinder. The outlet of the storage cylinder feeds through the slab then to the heater to replace the water that just went into the storage cylinder. The water on the outlet of the tank is still warm enough to heat the slab.
Brad, It sounds like a "Wet Back" heater that you are trying to make. If a coil around the flue does not work I would look at the various designs on the web that will have a better heat transfer.
I don't know much about them but if you are designing this yourself maybe you could have a rocket mass heater and run your coil in the "Mass" so that it has a better chance to exchange the heat.
Finally what is the shed used for? Is there any chance you could cover the floor with carper or something of that nature??
Posted Sunday 22 May 2016 @ 1:25:34 pm from IP # -
MrEGuy
MemberJust to clarify, are you trying to heat the shed with the hydronic system or just trying to stop the slab from being cold??
Will the fire be enough to heat the air in the shed, would ceiling fans help move the heated air around in the shed? Could more socks in the boots when working in the shed to stop the feet getting cold work?
It is an interesting project but I think if the problem was better understood a simpler solution could be found.
Posted Sunday 22 May 2016 @ 1:35:32 pm from IP # -
wlb
MemberYou've described the process beautifully Phil. But you've overlooked the absence of the necessary heat loss on the return (downhill) side of the circuit in Brad's plan. His idea was to simply put heat into the water from a fire above the slab and utilise a small expansion tank above that, and then have the hot water flow down into the slab to be cooled.
In order to get it to start and thermosiphon properly, some heat has to be removed on the downhill side of the header tank in order to obtain sufficient density and velocity to get the water to fall down to the slab (where it will be significantly cooled) and have enough momentum to get it back up to the heat source and continue the process.
As you say, hot water (or air) does not rise. Cold water (or air) falls and gets under the hot water (or air) and pushes it up.It all has to be tuned correctly too, so trimming valves are needed in the circuit. If you don't extract enough heat at the top of the return side, you don't get sufficient increase in density to cause the water to fall fast enough to get it through the large horizontal slab where a lot of heat will be extracted, and then back up the other side to the heater. However velocity through the heat exchangers is also critical. Too slow and too much heat is lost. Too fast and not enough is lost. In a properly tuned thermospihoning system, only a few degrees are lost through any one heat exchanger.
I designed a large system from scratch, with a lot of help, about 35 years ago. It's a tedious process with many inter-related factors; - pipe length, pipe diameter, pipe insulation, flow velocity, mass of water flowing per minute, temperature drop at each location.
To get it to work Brad will need to have a reasonable amount of heat extracted at a point above the slab. His problem is going to be that his heat source is small and inefficient and his heat exchanger is large and well below the heat source. There is nothing to get it all moving. What is most likely to happen is that the cold water in the slab will just lie there and the water in the coil will boil and blow steam out the vent of his expansion tank. This is also why it's essential that the vent is adequate.Posted Sunday 22 May 2016 @ 1:55:28 pm from IP # -
Phil A
MemberI don't believe I have overlooked anything. I am the one sitting here being kept warm by a system that everyone else said wouldn't work.
As a matter of interest, the momentum of the water is irrelevant. Momentum = weight X speed. As the temp in the system comes up, the water will flow as the smallest trickle, there is no momentum effort involved. As the water gets hotter the flow increases. As it cools it slows. It is the difference in the weight, or more correctly, the mass that creates the flow. The difference in mass is created by the heating on the hot side From memory, about 20gms mass differential at 40degrees temp differential for about 2 litres? (easily calculated if you need to).
If you heat the hot side enough (relatively speaking) then the heat loss on the cold side will take care of itself. When you allow water with heat units in it down hill (by decreasing the mass of the water on the hot side) it automatically drives off heat.
I have no trimming valves in the circuit.
Hope this helps.Posted Sunday 12 Jun 2016 @ 9:01:24 am from IP # -
Brad Child
MemberHi Everyone, thanks so much for the replies!
I'm sorry for the late reply as I only just logged on now. ( no email notification?)
Well firstly, my initial post makes me look a dill- 6 metres x 15 metres is 90sq not 95!
Anyway, the consensus from people who know because they have lived here all their lives, is that an unheated shed won't be very inviting in winter, so extra socks won't cut it.
I've priced the tubing and manifold etc and it's within budget.
970 sq ft is 3 rolls of 300 ft ( I'll leave out 70 sq ft near the roller door) on 12 inch spacing. Does this seem reasonable?
Now for heat source. I have unlimited fire wood although l still have to cut it etc. Maybe a large furnace type burner? Maybe rocket stove made out of large gas cylinder? I can make stuff.
I really want to make this work.
Phil, your ideas sound great! Can you give me some kinda specific designs for my installation please? Doesn't have to be pretty.
Many thanks to all for your help on this. I'm at ordering stage.
Kind Regards
Brad.Posted Thursday 16 Jun 2016 @ 10:03:53 am from IP # -
APR
MemberPhil A said:
Hope this helps.G'Day Phil,
What diameter pipe work is between the slab return manifold to the heater and on to the storage tank and down to the slab feed manifold?And, is the pipe up to the tank from the heater insulated and the pipe from the tank down to the slab not insulated?
Posted Thursday 16 Jun 2016 @ 11:08:53 am from IP # -
Phil A
MemberBrad, you may need to have good look at the heat input required. Reckon you will soon get tired of poking sticks into a rocket heater. A boiler or water jacket type heater would be more suitable. The heating effect required from the heater will minimal because the place will be warm any way. Without doing actual calculations and not knowing the thermal efficiency of your place, it sounds like 10 kw heating capacity would give you plenty of flexibility. I use a scandia stallion model HFB 717. it is 13kw boiler rating with about 10kw waste heat I'm guessing. It is only idling in our application and we have considered moving it into the shed as it gets too hot in here at times.(about 80 square metres. It would probably do triple this area if need be. We use about 6 metres of fire wood for the tassy winter. It is rated at 80% efficient.
The bigger the hw cylinder you can run the easier things get with this application. We have 400 litres. The plumber talked me out of 1000, but I will probably fit another one later. The bigger the cylinder the more flexible you are with the times you need to have to have the heater going. You don't necessarily want to be running the fire just because you want to run heat into the floor. You can run a small solar pump so you can pump some water into the floor when the fire is not going. If you use the system for domestic water as well then a heat exchanger type cylinder is better in this application. This is something you can make.
I am making a new one for another application that is 3500 litres. Mostly solar powered and do away with the heater to a large degree.
APR,
sounds like you are onto this already.
Pipework from the boiler to the cylinder (both directions)is 1".
The outlet from the floor manifold tees in about 1.5 metres from the heater, but this is not critical. This steps down from 3/4 to 1/2 inch about 400mm before it tees into the inlet to the heater. There is good reason for this. One thing important here is the use of a venturi arrangement where it tees into the heater inlet line, available from a plumber. It gives the return from the floor priority over the return from the cylinder above when it is running.
This allows correct heater operation when the floor heating is not up to temp. You will need a mixing valve to keep the floor supply less than 50 degrees. One side of this is from the outlet from the floor manifold. That side is 1" because that was the size that comes out of the return manifold and my solar pump (1") is fitted basically right beside it.
The other side is from the other end of the floor outlet manifold.
I ended up running a separate feed line from the lower cold thermosiphon line near the cylinder to the floor supply. This is 3/4 and goes via the mixing valve. There is good reason for this. I can explain further if required.
Yes the pipe from the heater is insulated and the pipe down from the tank to the floor inlet is not.
Oh and don't forget that the minimum slope on all thermo-syphon pipework is 50mm per metre, but the more you can run the better it will work.
Hope this gets you started.Posted Sunday 19 Jun 2016 @ 9:36:13 am from IP # -
APR
Memberwlb said:There is nothing to get it all moving. What is most likely to happen is that the cold water in the slab will just lie there and the water in the coil will boil and blow steam out the vent of his expansion tank. This is also why it's essential that the vent is adequate.
If water in the coil were to boil, that means you are entraining steam in the pipework up to the storage tank. This steam will be displacing water and will result in the heated side fluid having significantly less density than the pipework from the storage tank to the slab.
If the pipework from the heater to the storage tank were not insulated and lost a degree or two of heat, it is quite possible that any steam will condense out in the pipework and not be seen at the tank.
Posted Sunday 19 Jun 2016 @ 11:04:58 am from IP # -
Phil A
MemberIn my system the coil is vented at its own header supply tank. The coil only supplies domestic water.
The water in coil doesn't go into the heater or slab. The whole cylinder and slab and in my case the evacuated tubes also in the system are all closed circuit.
The cylinder runs its own header tank. The water level in the cylinder is deliberately run at approx. 150-200mm below the top of the cylinder giving an air space for steam to dissipate any energy.(form condensate) This then vents into the small header tank if necessary and is recycled. There is no waste steam or water.
One reason for running a bigger cylinder is to help eliminate the unnecessary boiling of the cylinder. Another reason is that above 60 degrees C you begin to get calcium and mineral separation, so near boiling temperatures should be avoided if possible anyway.PS I have just re read the post above a realise that you were talking about a coil around a flue and not the coil in the cylinder. We regularly have the water in the boiler gurgling to the point of boil. Of course the water in the cylinder will boil first due to the less pressure the water is under at the cylinder. The cylinder is the first place that you actually hear energetic boiling. The bubbles if created are simply transported back up to the cylinder and vented as described above. Of course they would also add to the pumping effect at the same time as the water in the up pipe would be even less dense.
The cure for inadvertent boiling is simply to dump some water into the slab via the pump.Posted Sunday 19 Jun 2016 @ 12:25:41 pm from IP # -
Brad Child
MemberHi to Everyone who helped me on this!
I didn't want to be one of those op's who get a whole lot of great advice and then disappear !
So, my slab is finally being poured today. We have had SOOOO much rain- most on record, so it's been very hard to get this job done.
After contemplating the fantastic advice and looking into complexity and cost, I'm not doing the hydronics. I can't justify it on the shed. I've already arranged plenty of north facing light and insulating the whole shed.
Father in law has a lovely old steam boiler l can have, so l will put that in and look for some radiators and do a simple above floor wall radiator system. The boiler alone may be enough once it warms up.
Thanks to everyone for the fantastic advice. I really appreciate it.
Kind Regards
BC.Posted Wednesday 7 Sep 2016 @ 10:44:58 pm from IP # -
Brad Child
MemberHi to Everyone who helped me on this!
I didn't want to be one of those op's who get a whole lot of great advice and then disappear !
So, my slab is finally being poured today. We have had SOOOO much rain- most on record, so it's been very hard to get this job done.
After contemplating the fantastic advice and looking into complexity and cost, I'm not doing the hydronics. I can't justify it on the shed. I've already arranged plenty of north facing light and insulating the whole shed.
Father in law has a lovely old steam boiler l can have, so l will put that in and look for some radiators and do a simple above floor wall radiator system. The boiler alone may be enough once it warms up.
Thanks to everyone for the fantastic advice. I really appreciate it.
Kind Regards
BC.Posted Wednesday 7 Sep 2016 @ 10:46:05 pm from IP #