Questions about radiant heat system

[CalG]

If it were me........;-)

Based on what I perceive as too little flow and not enough total heat input. (Based on HOT water in and COLD water return)

I might consider closing all but two of the loops and seeing how "hot", you can get the return. If two loops can maintain a 10 degree drop, add a third loop. If not, close off one and run the other.
Two important aspects of a wood fired hydronic radiant system are. Don't extract more heat than necessary to support efficient fire box and flue temperatures, and keep the return temperature close to the supply temperature.

 

[bigdeano]

The more water flow you have through the slab the closer the inlet and outlet temps will be and the more even the slab temp will be. Maybe you just need a more powerful pump.

 

[quicksandfarmer]

That was kind of what I was thinking but would I need to add the second mixing valve? I had about decided that for this season I would add one into the supply and return lines at the storage tank and dump the mixed water into the cold intake line between the tank and the heat exchanger. I'm not sure weather it would be better to dump the mixed water into the storage tank or directly into the the cold line as I mentioned. And as I mentioned before I would really like to keep the gravity/thermal exchange setup I have now if possible. It works really well the way it is but I don't know what will happen once I add the mixing valve into the picture.

Think of your system as two parts -- the "hot loop" which is the stove tank, which you want to keep around 140F, and the "warm loop" which is the floor coils and you want to keep around 110F. (Those numbers are just guesses). To keep the system operating properly you want to keep both loops at the proper temperature, and you want to have some sort of thermostatic control.

Here's the thing: you want the system to be able to be separated into components for the purpose of diagnosing what is happening. If you have some sort of thermostatic control, you can see whether each loop is doing what it is supposed to, and the behavior of each loop will be consistent so it will be easier to isolate problems. Right now, you're indicating that you're not completely satisfied with the system's performance, and you can't tell if it's because the manifolds aren't working, the circulator is too small, the stove is just too small, the water temperature is wrong, or whatever. Put in thermostatic control -- a mixing valve for the warm loop, an aquastat for the hot loop -- and you'll be able to tell right away where the problem is by observing the behavior.

Here's another thing: those numbers of 140F and 110F are just guesses. The best way to get the right numbers is to try different ones and see what works best. With thermostatic control, it's as simple as turning a dial. Maybe 140 is too cool, you'll still get creosote building up. Maybe 160 or 180 is better. Just turn a knob and find out.

 

[Raspy]

The first problem is too low of a flow rate through the floor tubing. Your tube lengths look about right (2000 divided by 7 loops), but you are only getting heat in the middle of the floor. Sounds like you have a spiral tube layout. Make sure your loops are all open at the flow meters and the shutoffs on top of each loop. Then balance the flows at the flow meters. Your pump is a low head design so you might want to put two in series or change it to a Grundfos 2664 or 2699 three speed. Since you are getting heat in the center of the room first I'd suggest you have the flow reversed and are heating from the center out instead of the perimeter in. Is that the case?

Second problem is pulling too much temperature from the fire. Slowing the water flow to raise the fire temp is not going to help you. The fire temp needs to be much higher. A radiant reflector under the heat exchanger might do the trick, or a smaller heat exchanger. This type of system is always problematic with carbon in the exhaust or carbon on the heat exchanger. And your Heat exchanger output BTUs are probably much less than you think.

Using the water heater as a buffer tank is a good thing because it will help stabilize the system when the fire is going but the circulator is not, such as during a power failure. It gives you time to reduce the fire without overheating the loop. The mass of the slab is so big compared to the mass of the water in your water heater, that it won't hurt your recovery time. You won't be "storing" significant heat there and it's buffering affect will allow you to skip any tempering valves. I can't see that a tempering valve has any value here.

Overall, I'd guess the BTU delivery is too low to be practical and the circulation rate is too low to give even heat. Too much energy is being pulled out of the fire for it to run hot and clean.

There is no constant return temp, or differential temp that means everything is fine, the dynamics of the system are aways changing as it heats up. But you should be able to see a rising temp on the return within 10 minutes or so and it would be nice if it settled in at around 20 degrees or less compared to the supply. The lowest practical temperature to deliver is about 90 degrees, but that will be very slow to heat. 120 degrees will give better response. But remember it's BTUs you are looking for not so much delivery temp. The floor will absorb more energy with a faster flow rate, so get the flow rate up and the supply/return temps close without worrying about the delivery temp, then let the floor rise and the supply temp rise together until you are comfortable. Comfort will typically be fine with an average floor temp of about 75 degrees, but if you have poor insulation you may have to go higher.

 

[quicksandfarmer]

I'm not saying you are right or wrong I'll just say that I for sure I don't know. I'll say this though I did a massive amount of research on radiant heat before I poured the floor and everything I looked at said not to dump really hot water into the floor and 100 on the low end and 120 at the high end. I know I've dumped 140 degree water into it before I realized the stove had heated up that much in that short period of time.

The reason for keeping the water relatively cool in a floor system is not to avoid damaging the concrete but for comfort. A temperature of 110F on bare skin is where it begins to get uncomfortable, as was noted above exposing feet to excessive warmth can lead to injury over time. Most people shoot to keep the floor temperature between 90 and 100.

Floor systems are notoriously hard to predict, so it's hard to know what input temperature is going to give the floor temperature you're shooting for. A good starting point is to set the water at 110 and see what the floor temperature ends up at and then adjust accordingly.

You got me thinking when you were talking about getting the proper differential between the supply and return. I think 20 degrees is about a normal target. I think I'm going to pull apart those manifolds one more time and look at the valves to see if I can see anything wrong and if that don't work I guess I'll have to add valves to both sides and adjust them that way.

A low return temperature means the slab could radiate more heat, if it were getting more heat. A warm return temperature means the slab can't radiate all the heat it's getting and is returning some. If the slab isn't getting enough heat, there are two possible causes: 1. The stove isn't producing enough heat. 2. There's not enough water circulating.

If you were set up with an aquastat and a mixing valve you could tell which is the case very quickly. If the aquastat is cycling on and off, the stove is at its limit for producing heat, the aquastat is turning the pump off to let the stove catch up. If the aquastat is continuously on, the stove is producing more heat than the circulator pump can distribute into the slab.

If it's a circulation problem, it could be that the pump is undersized, or there could be a problem with the piping that is limiting the flow of water. In either case you want more gallons per minute going through.

 

[CalG]

On the subject of water inlet temperatures.

It really does not matter what the initial temperature of the water is IF the flow rate is so small as to pull the water temperature down below an acceptable level in the first ten feet in the floor.

The original post suggests that the return water temperature is in the mid fifties F. That is GROUND temperature. The "hot water" may have given up all it's available heat long before it arrives at the return thermometer.

Heat extraction and distribution.

And while I'm being critical, There is NO WAY you are going to get enough BTUs to warm up 2000 linear feet out of that wood stove with the "stove top and flue collar" arrangement as your photos show. It's just not going to happen.
Sure , the holding tank gets hot, but is because the heat isn't getting out to the floor. (More flow is needed)

Just to be fair, I've got a thin slab hydronic radiant system in two additions of my own home. Both run off the Peerless 170K BTU oil fired boiler, as does the DHW. The water goes out via thermostatic mixing valves at 112 F, and the floor starts feeling good on bare feet when the return is above 85F. The system is NOT called to deliver heat constantly, just evenings and week ends. (the wonders of programmable thermostats ;-)
I have also cobbled up a thermo-syphon system that takes heat directly from the down stairs wood stove (Fischer) via a pir of Cast iron radiators placed on top of the stove, and pipes it up to a single cast iron radiator in the dining room. It takes a LOT of wood stove heat to bring even a small amount of heat to the living space. At tops, this small "heat exchanger" runs at 130F. But it feels great when it's there. ;-)

I would love to integrate the wood heat (Our main source) with the hydronic system. The fragile electrical grid suggests that such a plan will either be costly and a maintanence headache, or just plain silly. Good only to show all my "preparer" friends, of which I have none.


That is my position, and I'm stick'n to it. ;-)

 

[mx842]

"As I said before and as quicksandfarmer I think it was, also mentioned I need to find out why there is such a temp difference between the supply and return temps at the manifolds. I'm thinking the way I have to fire the boiler now to keep the water at a safe temp for the floor it is never at a constant enough temp to effectively get the floor warm. I know I also need to figure out the manifold flow control gismo's on these manifolds. They seem to dump the same amount of water no matter how I adjust them and I'm not sure if it is me or that they are just not working."
A heated slab works much differently than "regular" radiators, convection tube heat, and hot water coils. Radiant slabs most often use a condensing boiler as a heat source which operates at peak efficiency when the return water comes back cold, not the standard 10 degree drop in temp as other heat exchangers. The system needs to operate for several days in order to heat the slab to a stable state. So far I don't think this has been the case, considering all the variables you are working with.
It is difficult to clearly see the manifold setup you have. Does your return manifold have floating flow indicators in the clear top? To balance the loops; 1- with the circulating pump operating, close all the valves on the supply manifold (you will not damage the pump in any way doing this). 2- open each valve fully open, one at a time, and verify that the return flow indicator for that loop shows full flow (this ensures that the loop is completely filled with water). 3- Slightly open all supply manifold valves, until flow is indicated in all the loops, a little at a time and give it a few minutes to stabilize. 4- open any valves a little more that show the lowest flow, until doing so has no further effect(you may be surprised how little some valves need to be opened). 5- When the system is operating(being positive here) if there are any areas of the floor that you want warmer, do not open it's supply valve any more- close the other loops down a bit.
The mixing valve will not operate on the gravity loop without a pump in the circuit, but you only need a very small pump. And , yes, the mixed water should tee into the cold line between the tank and heater, the goal in this "primary loop" is to get the water HOT and keep the return to the heater a little less than that.

Thanks............The manifold system is like the ones you see at Pex Universe 5-Port St. Steel Complete Radiant Heat Manifold Package - PexUniverse

The setup has the hot supply on the bottom and the return is on the top of the rack. They have the flow meters on the hot side with valves on each loop that controls the output of the water in each loop. It's a clean setup and looks easy enough but those flow meters are hard to figure out plus they have cheezy temp gauges that don't work worth a crap. I've replaced those several times and I've never gotten two out of the box that will read the same one always seems to read 10 to 15 degrees higher than the other when the system is cold. I've checked them in hot and cold water and it makes no difference.

I just piped in two new temp gauges on the supply and return lines just where they dump into and out of the manifold and that should be easier to read the temp differentials of the two sides. Now if I just could get the proper differential between the two sets of manifolds I'll be all set.

We had a hard freeze last year and before I got this setup up and running the floor surface was down in the high 20's. I was scared that the pex was toast but in about 3 days after firing the heater the average floor temps were in the mid 50's with low to mid 40's around the outer edges and lower to mid 60's in the middle of the room. I dodged that bullet somehow and sure don't want to see that happen again. That was a kind of a freak thing that happens around here every now and again, mostly the winters here are pretty mild compared to other parts of the country.

Also, I may as well throw this into the mix too since I will have to run piping for this part of the system. Over the summer I built a ventilation system for the building. Right now it's just mainly for cleaning the air inside the shop. When I built my air handler I did it in a way where I could add a plate heat exchanger at a later time if I wanted. I was thinking I could use it as a heat dump in case something went wrong. I know this would require some kind of control system; which I know I am going to need anyway but this could be added quite easily and while I have the system empty I may go ahead and stub in that pipe now if I can figure out how it needs to be piped into the system I already have. Anyway that's something I can worry about another day.

 

[mx842]

On the subject of water inlet temperatures.

It really does not matter what the initial temperature of the water is IF the flow rate is so small as to pull the water temperature down below an acceptable level in the first ten feet in the floor.

The original post suggests that the return water temperature is in the mid fifties F. That is GROUND temperature. The "hot water" may have given up all it's available heat long before it arrives at the return thermometer.

Heat extraction and distribution.

And while I'm being critical, There is NO WAY you are going to get enough BTUs to warm up 2000 linear feet out of that wood stove with the "stove top and flue collar" arrangement as your photos show. It's just not going to happen.
Sure , the holding tank gets hot, but is because the heat isn't getting out to the floor. (More flow is needed)

Just to be fair, I've got a thin slab hydronic radiant system in two additions of my own home. Both run off the Peerless 170K BTU oil fired boiler, as does the DHW. The water goes out via thermostatic mixing valves at 112 F, and the floor starts feeling good on bare feet when the return is above 85C. The system is NOT called to deliver heat constantly, just evenings and week ends. (the wonders of programmable thermostats ;-)
I have also cobbled up a thermo-syphon system that takes heat directly from the down stairs wood stove (Fischer) via a pir of Cast iron radiators placed on top of the stove, and pipes it up to a single cast iron radiator in the dining room. It takes a LOT of wood stove heat to bring even a small amount of heat to the living space. At tops, this small "heat exchanger" runs at 130F. But it feels great when it's there. ;-)

I would love to integrate the wood heat (Our main source) with the hydronic system. The fragile electrical grid suggests that such a plan will either be costly and a maintanence headache, or just plain silly. Good only to show all my "preparer" friends, of which I have none.


That is my position, and I'm stick'n to it. ;-)

I don't know.......you may be right but I do believe this heat exchanger can and will put out enough heat to warm my floor. It's just a matter of what kind of control system I can come up with. Here is what I know........When I fire the stove with the pump off within 20 minutes I have 140 degree water going into the hot water heater storage tank that sits along side the stove. If I leave the pump off for another 10 to 15 minutes I then have 140 or better degree water going out the supply line on the storage tank and through 20' of pipe to the pressure tank and out the wall headed to the manifolds. At this point this is all happening by the natural flow of the hot water rising out of the heat exchanger circulating the water in the storage tank to a point where it starts flowing out the hot port on the supply tank. I have never really tried to see what would happen if I just let her burn and keep throwing wood in the firebox.

If I fire the stove with small wood, kindling, or something like the pile of OSB scraps I have stacked behind the building it heats up pretty fast and the numbers I posted above are what happens if I fill the fire box about 1/3 full of OSB scraps. By the time this burns down these are the numbers I am getting out of this setup. As I said before I normally cut the pump on once the water gets to 140 at the pressure tank and throw in some regular wood and let it go. Depending on what kind of wood I add and how much I put in plus how much air I put into the stove everything equalizes out to between 100 to 110 degrees at the manifold. The only limiting factor to how much heat this stove top heater will make is the amount of wood and air I put in the firebox and basically that has been how I have been controlling the water temps up to this point and is what I want to change.

This year I am a little better prepared and I will have good dry wood to burn that I have had stored from last season which will make it better on me and the stove. I have around 20+ cords of wood laying on the ground that I have been trying to process and right now it's a race to see who will come out on top/////me or the termites. I've been splitting this wood and when I come across wood that is still pretty good but has termites in it I split this wood smaller then spray it with ant and termite killer then let it lay there for a couple days to make sure they are all dead then throw it on the pile. some of the wood that has been laying there the longest looks good on the outside but when you pick it up you can tell it is pretty much eaten up on the inside and that goes straight to the fire pile termites and ants an all, that I always have built next to the splitter. I'm about a third of the way done right now and I hope within the next several weeks I'll have it all cut, split and covered up. Next year I hope to have a wood shed to store it in but there is so little time.

 

[CalG]

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.

 

[Raspy]

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.