Questions about radiant heat system

[mx842]

I've been playing around with different ways to heat my shop for several years now and have almost got it to where it is doing ok but would like to do a few things that would make it better.

What I have is a wood stove with a ss tank attached to the top of the stove with the exhaust going out through the center of the tank. The water goes in one end and travels around the sections and out the other end. As the water heats up and leaves the SS tank it rises up a section of 1" copper pipe to a 35 gal water heater. This heated water dumps into the top side port where the pop off valve was and then back to the stove through the old bottom drain port of the hot water heater.

Normally when I start the stove up cold I cut off my circulation pump that I have stationed at my manifold which is connected to this storage tank by roughly 80' of 1" copper pipe. I let the natural rise and fall of the heated and colder water circulate from the stove to the water tank until it heats up the tank to around 120 to 130 degrees. By this time there is hot water already going out of the top hot water port and into the supply line headed to the manifold.

I have a small pressure tank that is plumbed in the supply line about 15' from this hot water outlet port and once I feel hot water leaving the pressure tank I then turn on my pump. Normally when I turn on the pump the water temp leaving the stove has gotten to between 140 and 150 degrees. Once the pump is turned on the water stabilizes in the storage tank to around 120 degrees and stays there as long as I keep the fire going. I keep the fire going all through the day by adding a few pieces of wood every hour or so and at night when I go in I bank it full and cut the draft down low so it don't burn out to quickly. I will normally have a few coals in the bottom in the morning and I can still feel heat on the sides of the stove but the water has cooled down to where I started the day before.

I got through last season like this and even on the coldest days and nights the shop was fairly comfy. I have a couple more tanks I have come across this summer and I want to add one, maybe both to the system for more storage.My question is what would be better.....to run this extra tank in series with the pipping or run it parallel to the tank like it is at the stove? The supply run to the point where I will tie to the pipping to the second tank is about 9 feet off the floor and the bottom of the second tank will be about a foot higher that that.

My thinking is if I 'T' off the hot line to the hot side port on the tank and then 'T' off the cold pipe returning to the stove it would act more as a mixing valve so to speak than a storage tank. I was thinking of trying it this way to see what would happen but thought about asking about it before I ran all that pipe.

The biggest problem I have is that even dumping 110 degree water in the slab the return is always to cold when it gets back to the stove and this causes problems with creosote build up. I have to rod out the stove pipe every couple days to keep the keep it drawing.


As it is now I'm dumping roughly 100 degree water into the manifold, that is connected to roughly 2000 ft of 1/2" pex and by the time it leaves the return manifold it is cold or around 50 degrees. After a couple days of keeping a good fire all day The center sections of the shop floor will warm up 65 degrees in places and cool off as it gets to the outside walls where they stay on average around 50 to 55 degrees.

 

[kc5dlo]

Mix some of the heated water with the return water. Sometimes a simple ball valve between the hot water and return line will work.

 

[quicksandfarmer]

The biggest problem I have is that even dumping 110 degree water in the slab the return is always to cold when it gets back to the stove and this causes problems with creosote build up. I have to rod out the stove pipe every couple days to keep the keep it drawing.

The long and the short of it is you are taking more heat out of the fire than it has to give, so it's burning cold. You're going to continue to see creosote problems unless you figure out a way to take less heat or have your fire produce more heat. I'm no expert but a quick google tells me that the recommendation is to maintain your water at 140F or higher to avoid creosote buildup.

But you don't want to be sending water that hot into the floor. I think what you want is a thermostat on the stove tank that turns on at 140F. That controls a circulating pump. You also want a mixing valve that feeds the floor, that is set to 110F. The hot input to the mixing valve is the stove tank, the cold input is the return from the floor. The stove tank will stay at 140F, when it starts getting hotter the circulator pump kicks in and the mixing valve opens and warm water goes out into the floor and cold water comes back into the tank.

Think of the cooling system of a car, where the task is keeping the engine at the proper temperature. Your design goal is keeping the stove at the proper temperature, then shedding excess heat if you have it.

That's the "take less heat" solution, heat is never taken if the stove tank is below 140F. The question is whether there is going to be enough heat to warm your shop. If not, then you have to look into "make more heat" solutions -- more wood, or a bigger stove.

 

[mx842]

The long and the short of it is you are taking more heat out of the fire than it has to give, so it's burning cold. You're going to continue to see creosote problems unless you figure out a way to take less heat or have your fire produce more heat. I'm no expert but a quick google tells me that the recommendation is to maintain your water at 140F or higher to avoid creosote buildup.

But you don't want to be sending water that hot into the floor. I think what you want is a thermostat on the stove tank that turns on at 140F. That controls a circulating pump. You also want a mixing valve that feeds the floor, that is set to 110F. The hot input to the mixing valve is the stove tank, the cold input is the return from the floor. The stove tank will stay at 140F, when it starts getting hotter the circulator pump kicks in and the mixing valve opens and warm water goes out into the floor and cold water comes back into the tank.

Think of the cooling system of a car, where the task is keeping the engine at the proper temperature. Your design goal is keeping the stove at the proper temperature, then shedding excess heat if you have it.

That's the "take less heat" solution, heat is never taken if the stove tank is below 140F. The question is whether there is going to be enough heat to warm your shop. If not, then you have to look into "make more heat" solutions -- more wood, or a bigger stove.

I think you are right on with the stove not running hot enough. I figured this out by the end of the season last year and knew I would have to make changes before this heating season, that's what I'm doing now. When I first put this thing together I didn't know what to expect but now that I know it has no problem making 140 degree water I can make some changes to the system to make it do what's needed. I'm thinking that if I can add these two other water tanks which will give me another 80 gallons to play with I can run the stove hotter and then the run my loops off of the last tank in the line with a mixing valve in-between to temper the return water back to the stove I can keep the water temps up without worrying about it overheating. I have more than enough load to run safely the way it is but would like to have enough storage to make it through the night.

 

[quicksandfarmer]

So it sounds like you want to have three operating modes -- warming up, heating, and storing. In all three modes you want to keep the stainless tank hot.

In warming up mode, you aren't letting the water circulate at all, you're just trying to get the stainless tank to temperature.

In heating mode, you're taking hot water out of the stainless tank and mixing it with cold water to get about 110F, and then circulating that into your floor loop. The water you're taking out of the stove tank you're replacing with return water from the floor loop.

In storage mode, you want to store the water as hot as possible, because the hotter it is the more heat you can store in the same volume. Your storage tanks sound small for what you're describing though. Let's say you burn a cord of wood a month, which would be typical around here. A cord is roughly 30 million BTU, depending on the type of wood, so let's say you're burning a million BTU a day. Let's also say you're burning half the time, so you need to store half a million BTU. Let's say you can store at 200F and your inside temperature is 60F, a temperature difference of 140F. A BTU is the amount of heat to raise one pound of water one degree, so to store half a million BTU with a 140F temperature difference you need 500,000/140= 3500 pounds of water, or just over 400 gallons.

When you heat that water to 200F, you want to do it gradually so that your stove tank doesn't get cold. You need some sort of thermostat that controls how much heat leaves the stove. This could be the same thermostat and circulator that you use for heating, the control logic is the same: don't let the stove get too cool.

Then you have to figure out how to get that heat out. It might very well be that with 400 gallons of 200F water you don't have to do anything other than let it cool and the heat escapes into the room evenly enough. If not then you have to incorporate some mechanism for getting the heat out.

In any case the big thing is keeping your stove running hot.

 

[mx842]

So it sounds like you want to have three operating modes -- warming up, heating, and storing. In all three modes you want to keep the stainless tank hot.

In warming up mode, you aren't letting the water circulate at all, you're just trying to get the stainless tank to temperature.

In heating mode, you're taking hot water out of the stainless tank and mixing it with cold water to get about 110F, and then circulating that into your floor loop. The water you're taking out of the stove tank you're replacing with return water from the floor loop.

In storage mode, you want to store the water as hot as possible, because the hotter it is the more heat you can store in the same volume. Your storage tanks sound small for what you're describing though. Let's say you burn a cord of wood a month, which would be typical around here. A cord is roughly 30 million BTU, depending on the type of wood, so let's say you're burning a million BTU a day. Let's also say you're burning half the time, so you need to store half a million BTU. Let's say you can store at 200F and your inside temperature is 60F, a temperature difference of 140F. A BTU is the amount of heat to raise one pound of water one degree, so to store half a million BTU with a 140F temperature difference you need 500,000/140= 3500 pounds of water, or just over 400 gallons.

When you heat that water to 200F, you want to do it gradually so that your stove tank doesn't get cold. You need some sort of thermostat that controls how much heat leaves the stove. This could be the same thermostat and circulator that you use for heating, the control logic is the same: don't let the stove get too cool.

Then you have to figure out how to get that heat out. It might very well be that with 400 gallons of 200F water you don't have to do anything other than let it cool and the heat escapes into the room evenly enough. If not then you have to incorporate some mechanism for getting the heat out.

In any case the big thing is keeping your stove running hot.

You are almost there,:drink: accept I would call it a basically a 2 mode system the way it is. Let's start out with a cold start and go from there. I fire the stove and normally I have cut off the pump at the manifold. I've tried it both ways and it will still get up to temp it just takes longer if I leave pump running. The pump only circulates the water out in and out of the normal hot and cold ports of the water heater storage tank. This water heater tank acts as a mixing valve that mixes the circulated water from the loops with the heated water that is made by the stove.


With the pump off and the fire started it takes roughly 15 minutes for the fire to get hot enough to start the natural flow of the heated water wanting to rise. The only way it can rise is out the 1" copper outlet pipe that is roughly 10' long that dumps into the side port of the water heater tank where the pressure relief valve was. I still have the safety valve there I just added a Tee there for the valve to sit on top and the water to run straight in. I have a Tee at the bottom of the tank where the tank drain valve was the drain valve is teed off with the cold water running straight out of this tee back to the in port of the ss stove top heater.

This is a constant thing as long as there is heat in the stove it will transfer heated and cold to and from the storage tank weather the pump is running or not. I have been controlling the loop temp by how hot I keep the stove. I did manage to be able to get the storage tank up to 130 by taking the insulation jacket off the 80' run over to the manifold and by doing that the water had cooled down to 100/110 degrees going into the manifold. What I'm hoping is that if I add this other 50 gal water heater to the system right over the manifolds where I can send 180 degree water from the storage tank at the stove then add the third 30 gal tank along side the 50 with a mixing valve in-between and temper that tank down to 100 degrees or so for the loops to run off of I can keep the fire hot enough to where it doesn't make as much creosote.

I know the ideal thing would be to have more storage but I haven't been able to find one that I can afford at this time so I'll have to make do with what I have i guess. I do want to add these two other tanks but was wondering what the best way to pipe them would be.

 

[mx842]

I found a few pics that I took while I was putting this together and maybe they will make things a little clearer.

 

[quicksandfarmer]

I'm trying to be helpful here, and I read through all of your posts again, but I just don't see how having more water in the system helps. It looks like you've got a very good system and a simple change would make it work really well.

For the tank on the stove, you want it to quickly heat up and then hold its temperature. You want as little water as possible without causing big swings in temperature (boiling when the circulator turns off, rapid drops when the circulator comes on).

For the floor loop, you want just enough water to fill the tubing, any more and it just means waiting for the heat to take effect.

If you want to store heat, my recommendation is to heat the slab as much as possible and store the heat there. If you heat the slab to the point that it's uncomfortable to walk on, and the shop is still cold the next morning, then we can talk about other ways of storing heat.

You already have a circulator pump. I would add a thermostatic mixing valve Google and aGoogle. This will cost about $200 for the parts but will give you a really good system.

The aquastat connects to the stove tank, and the electric leads go to the circulator pump. Put the mixing valve so the valve output goes on the intake side of the circulator. The hot input to the valve goes to the output of the stove tank. The cold input to the valve connects to a tee, which connects the return to the stove tank and the return from the floor loops. I would start off with the aquastat at 160F and the mixing valve at 110F and then adjust.

When you start off, everything is cold, the aquastat is off so the circulator is off. The stove tank water won't circulate. As your fire warms, the stove tank water starts to warm. When it hits the set point of the aquastat, the circulator turns on, and the circulator starts water through the mixing valve. The thermostat in the mixing valve adjusts the portion coming from the stove and the portion coming from the return loop to keep the water at 110F. Whatever water is pulled out of the stove tank is replaced by water coming out of the loop return.

Once it starts running, look at how it runs and make adjustments. If the circulator stops and starts a lot, it means you're pulling more heat than the stove can provide. You want to get to the point where the circulator is cycling occasionally, that's the point where you're pulling the maximum heat your fire can support. Adjust the temperature on the mixing valve until either the circulator starts kicking off, or the floor gets uncomfortably hot.

If you get to the point where the floor is uncomfortably hot in spots but you still want more heat, a bigger circulator will pull more water and more heat.

If you get to the point where the water coming back from the slab isn't cold, then your limiting factor is the ability of the slab to absorb and radiate heat. This is where you can start talking about storing heat. Although I think your best way to store heat is to warm the building, so what I would look into is additional radiator capacity.

You also want to adjust the aquastat to the point where you don't get creosote buildup. That's probably trial and error.

 

[Sawyer Rob]

The biggest problem I have is that even dumping 110 degree water in the slab the return is always to cold when it gets back to the stove and this causes problems with creosote build up. I have to rod out the stove pipe every couple days to keep the keep it drawing.

If you would "properly" dry your firewood before burning it, you wouldn't have all that creosote build up...

SR

 

[CalG]

Quicksand farmer speaks accurately

As described in the original post, there is already too much "heat sink capacity", and not enough heat.
If the floor water was returning with about a 10 degree F drop from supply temp., You would have a well matched system.

But that is not the case. It's coming back cold. That is not good. It wouldn't even be good for an oil fired system.
Minimize the water volume according to the suggestions given, and optimize the heat storage IN THE FLOOR.
Those thermostatic mixing valves are the ticket.

One heat source
One primary circuit and storage tank
Two circulators (one if a thermo-siphon system works for the primary)
one control box
One thermostatic mixer
one temp sensor for the primary circuit
one thermostat to trigger the floor circulator.
two thermometers to keep track of primary and "mixed" temperatures
Two wood sheds, each full of one season's fuel needs . (keep 'em full a year ahead)

That would be a useful system