Questions about radiant heat system

[mx842]

Question

The tank is connected to the stove via thermo-syphon flow , correct? When you see 140degree water into the 20 foot line leading to the manifold, WHAT IS THE WATER TEMPERATURE at the bottom of the tank ? That would be the supply TO the wood stove. Is that also 140? It would be a good sign if it is.

I'm not sure, I can't remember if I ever checked the temp there I know it always feels cold when compared to the hot line going upward. I guess as that hot water leaves the tank out the hot supply line it is drawing cold water into the tank via the return from the manifolds keeping it cold at the bottom and hot at the top of the tank. Is that even possible? I often wondered what would happen if I left the pump off longer just to see how far the hot water would travel down the line to the manifold on it's own but never did try it. I think I did try once cutting both of the valves off at the supply and return at the top of the storage tank and let it run for awhile as I remember I did have hot water at the bottom supply line to the stove. That was awhile back and I think I was messing with different valves trying to figure out how to get the return temps up then it got really cold and I decided the sending 110 degree water down the line was better than nothing once I figured out how to do that I just left it alone.

 

[mx842]

mx,

One way to measure how many BTUs you are able to get from the fire would be to put a small circulator on the HX lines to the water heater. This would stir the tank enough to make it's entire temperature measurable over time and make it the same on the bottom as on the top.
Then you can see how long it takes to rise 40 gallons 100 degrees, for instance and calculate the BTUs you collected. The main floor area should see a minimum of about 10 BTU per sq ft to make it rise in a useful way. 25 degrees per foot would be better. 50 would be better yet.

Here's the suggestion: Install a circulator on the HX lines. Determine your BTU availability from the fire with temperature over time and gallons in the tank. Calc how many BTUs you need based on floor area.

Then you will know if you can make it work or not.

Then increase the flow rate through the floor so you can evenly deliver the energy.

That may be worth a try I don't have a small pump I could use. Would closing the supply and return valves going into the storage tank do the same thing by just letting it circulate by thermo-syphon flow and see how long it takes to get the temps at the top and bottom about the same?

I need to get a pump anyway but hate to buy something that's just to experiment with.

 

[Raspy]

mx,

You need a different pump for the floor, so you could use the one you have there now (B&G Series 100), on the HX. Shutting off the tank and letting it sit, if that is what you meant, will not equalize the temp. It will allow it to stratify and be at different temps top and bottom.

Do don't have to equalize the tank, but it was a suggestion to find out how many BTUs your system was producing so you could determine if it's capable of heating the slab.

 

[Raspy]

mx,

One more thought. I'm assuming your loop lengths are all about the same. So, if you get the manifold loop metering valves and shut off valves all wide open such that you are getting a gallon per minute flow, or even 3/4 gallon, per loop, that pump will be OK. Then you could find the smallest Grundfos or Taco pump for instant water recirc systems and put it on the HX circuit.

That would be cheaper, but you must prove first that the B&G pump can deliver to the loops.

The thermosyphon setup you have obviously works, but is not set up in the most affective way. You'll have to maximize everything in the system before you can realistically hope to heat your floor.

 

[mx842]

mx,

You need a different pump for the floor, so you could use the one you have there now (B&G Series 100), on the HX. Shutting off the tank and letting it sit, if that is what you meant, will not equalize the temp. It will allow it to stratify and be at different temps top and bottom.

Do don't have to equalize the tank, but it was a suggestion to find out how many BTUs your system was producing so you could determine if it's capable of heating the slab.

Ok, I see what you are saying, I was just thinking that with the two valves turned off at the top of the storage tank and with the hot water going in to the top and the cold water being drawn out the bottom that sooner or later it would come close to being the same temp......that is if it didn't blow up first I guess

I was checking around yesterday to see if I could find out how to figure out just how many gallons is in the whole system. I had all that stuff along with pictures of the layout of the pex throughout the building on another computer but it crashed and I lost all of that information. I could maybe recover it off the old hard drive but I just haven't had the time to mess with it. I know it is 8 loops and they all are from between 240 and 285 feet I do remember that. I was also trying to figure out what kind of head pressure that the supply and return loops plus the manifolds and floor loops came up to but I couldn't wrap my head around it.

 

[mx842]

mx,

One more thought. I'm assuming your loop lengths are all about the same. So, if you get the manifold loop metering valves and shut off valves all wide open such that you are getting a gallon per minute flow, or even 3/4 gallon, per loop, that pump will be OK. Then you could find the smallest Grundfos or Taco pump for instant water recirc systems and put it on the HX circuit.

That would be cheaper, but you must prove first that the B&G pump can deliver to the loops.

The thermosyphon setup you have obviously works, but is not set up in the most affective way. You'll have to maximize everything in the system before you can realistically hope to heat your floor.

One question how would I check to see if I'm getting the flow you suggested?

 

[Raspy]

mx,

The total gallons in the whole system doesn't matter. For calculation purposes, all you need are the gallons in the water heater tank to figure out your BTUs. Then the square footage of the slab to get the BTUs required (for rough numbers to determine if it will work).

The system pressure doesn't matter too much and doesn't affect flow rates or how much energy is delivered. Normal pressure for a hydronic system is 12 PSI. 10-20 is a fine working range. Always include a 30 PSI relief valve and an expansion tank to stabilize it. As log as it is full with no air, the pump will be happy. Normally you'd have a pressure regulator to fill the hydronic system from your street system and an air vent to get all the air out.

If you don't want to get that fancy, you could manually fill the system and leave a trapped air space above the pressure relief port on the side of the water heater. Don't use either of the top fittings and let the air accumulate there above the upper side fitting. This would become an air separator and expansion tank all in one. Then pump off the bottom of the tank toward your floor system and return through the upper side port. This upper port can also tee to your HX hot line. With this setup you'll have stabile pressure, good air management, the fewest parts, and the ability to separate the heat source from the delivery side to calculate BTUs. You can fill the whole system with gallon jugs of water and a funnel in the old "hot" port on top of the tank. Fill it to just above the upper side port and then cap the top two pipes (H and C). All air in the entire system will gather there which will get it away from where you don't want it, and it will stabilize the system pressure. Then you'll start out with atmospheric pressure in the whole system and it will work fine. It will probably climb to about 5 PSI when hot and the slab warm. Before capping the upper fittings, run the pumps for a while to get all the air to that area. Then add more water as needed to get the level above the upper side fittings. Then cap the "H" and "C" ports and you're done. Oh, one more thing. You can use the top "H" port to to install the 30 PSI relief valve, it will act as a cap and give you the safety you need.

Done. Fire it up! Get some number to see where you stand.

Review what I said about your PEX layout and direction of flow and make sure all loops are flowing as much as possible. You need another pump for the HX loop and, possibly a stronger one for the floor. Can you include a picture of the floor tubing layout that might show the design and tube spacing?

 

[Raspy]

One question how would I check to see if I'm getting the flow you suggested?

If you mean the flow through the floor loops, look at the flow meters on the manifold and adjust to fully open. The flow through the HX is less critical but should settle in to be nearly the same on the supply and return.

 

[quicksandfarmer]

Ok, I see what you are saying, I was just thinking that with the two valves turned off at the top of the storage tank and with the hot water going in to the top and the cold water being drawn out the bottom that sooner or later it would come close to being the same temp......that is if it didn't blow up first I guess

I was checking around yesterday to see if I could find out how to figure out just how many gallons is in the whole system. I had all that stuff along with pictures of the layout of the pex throughout the building on another computer but it crashed and I lost all of that information. I could maybe recover it off the old hard drive but I just haven't had the time to mess with it. I know it is 8 loops and they all are from between 240 and 285 feet I do remember that. I was also trying to figure out what kind of head pressure that the supply and return loops plus the manifolds and floor loops came up to but I couldn't wrap my head around it.

What is the diameter of the tubing in your loops? With that it should be easy to calculate the volume.

Half inch PEX is 0.00961 gallons per foot, so 2000 feet is 19 gallons. If you go up to 3/4 it's 0.01894 gallons per foot or 38 gallons for 2000 feet.

You can estimate flow rate using the flow curve of your pump.

 

[quicksandfarmer]

You can calculate the BTU/hour by multiplying the temperature change of the water times the gallons per minute times 500 (The 500 comes from the fact that BTU's are degrees times pounds, a gallon of water weighs 8.3 pounds so one GPM is 500 pounds per hour). This should be the same whether you measure it at the heat exchanger or at the floor tubing, heat is neither destroyed nor created when it transfers.

The question you're looking to answer is, what is the weak link? If the system isn't giving satisfactory performance, why not? My suspicion is it's going to come down to the stove. If it's not producing enough heat to warm the space, no amount of fiddling with the plumbing is going to change that. If it is, then you can make adjustments that will do a better job of distributing that heat in a more usable way and keeping your stove operating temperature up.