Questions about radiant heat system

[pmsmechanic]

Storing the heat in the floor is a valid suggestion. The water should not be coming back to the boiler cold. That points to the fact that the floor isn't warm enough.

I have friends that have worked on in floor heated shops. If a chinook came along after a long cold spell the shop would be way too hot. The extra heat was heat stored in the floor. Often the doors would have to be opened just so the mechanics could work in the shop.

For those who don't know what a chinook is. It's a sudden warm air system that give a large warm change in air temperature. Mostly accompanied by wind I've seen two feet of snow disappear over night.

 

[quicksandfarmer]

How can you store enough heat in the floor to last all night? The floor has a fixed mass and you can't heat it to 180 deg. I think the OP has a good idea with the water tank if he needs to store heat.

I built a house with hot water heat in the concrete floor. I used a heat pump, so I didn't need to store heat, but if I did I would have probably used a water tank.

On page one of this thread I did a back of the envelope calculation that OP needs to store a half-million BTU's overnight, and it would take 400 gallons of water heated to 200F and cooled to 60F to provide that. Let's say instead you're going to use concrete heated to 80F and cooled to 60F.

Concrete has a specific heat of 0.18, which means a pound of concrete holds 0.18 as much heat as a pound of water, or you need 5.5 lbs on concrete to hold the same heat as one pound of water. To store a half-million BTU's while dropping from 80F to 60F you'd need 25,000 lbs of water, to hold it in concrete you'd need 139,000 lbs of concrete. Concrete is about 4,000 lbs per yard, so that's 35 yards. At 4 inches thick that's 2800 square feet, or about 40 by 70 feet. I don't know how big OP's shop is, or how thick the floor is, but that's not an inconceivable size. And I don't know how well insulated the floor is, which would be a big factor.

It's a heck of a lot simpler to heat the floor up to 80 during the day and let it cool overnight than to mess around with storage tanks and multiple stages. As I said in an earlier post, if you heat the floor as hot as you can stand it during the day, and it's still too cold the next morning, come back and we'll talk about storage solutions.

The more important point is that if the stove can't put out enough heat to heat the slab to 80, then it can't put out enough heat to store the water either, heat's the same either way.

 

[CalG]

Storing the heat in the floor is a valid suggestion. The water should not be coming back to the boiler cold. That points to the fact that the floor isn't warm enough.

I have friends that have worked on in floor heated shops. If a chinook came along after a long cold spell the shop would be way too hot. The extra heat was heat stored in the floor. Often the doors would have to be opened just so the mechanics could work in the shop.

For those who don't know what a chinook is. It's a sudden warm air system that give a large warm change in air temperature. Mostly accompanied by wind I've seen two feet of snow disappear over night.

The nice thing about a radiant slab heating system is that when the air temperature get high, you can just open the doors and windows without a severe energy loss penalty. The Slab is only going to give it's heat in proportion to the RADIANT losses with some consideration for conductance . But you have stated "Warm Chinook winds" ')

Add to that, the floor will still be there for you when the winds die and the sun sets on a clear cold sky. BTDT ;-)

 

[mx842]

thanks guys.........But maybe I should ask this question. What is the hottest water that you can safely dump in a slab?

 

[CalG]

MX

Since the invention of thermostatic mixing valves, No one has tried or cared to find out.

 

[quicksandfarmer]

thanks guys.........But maybe I should ask this question. What is the hottest water that you can safely dump in a slab?

Hot water won't damage concrete, they use it for cooling towers in nuclear power plants.

The reason that the water is typically cooler in a floor system than in radiators is that your feet will get uncomfortably hot when the temperature starts getting above skin temperature. You want the water to be as hot as you can stand. The hotter the water, the better job it does at it's primary task, which is transferring heat into the living space. Hotter is always better.

It's difficult to predict how a heated slab is going to perform, so you're going to rely on trial and error. With an adjustable thermostatic valve, adjust the temperature up until it feels too hot.

 

[dadster4]

In the early days of radiant heating, large factories in Sweden had a sudden increase in their workers suffering from shin splints found to be caused by the temperature of the radiant floor being too high. In all the installations I have done, the water temp entering the slab has never been higher than 110 degrees. Over time a higher temp can damage the slab from too much thermal stress. The idea of using the slab itself as heat storage has no benefit, the slab temp stays more stable due to it's mass, but is still the "radiator" of the system and will not "store" heat. I have only designed systems using a hot water heater or condensing boiler as a heat source which can both cycle on and off on demand, so using a woodstove that needs to stay hot, creates a new twist. A mixing valve and small circulating pump on your primary loop (the heat exchanger and water storage tank) will help to maintain a higher return temp to help reduce creosote in the stove. Another mixing valve on the secondary loop (the water storage tank and loops) will maintain a constant temp supplied to the slab.

 

[mx842]

In the early days of radiant heating, large factories in Sweden had a sudden increase in their workers suffering from shin splints found to be caused by the temperature of the radiant floor being too high. In all the installations I have done, the water temp entering the slab has never been higher than 110 degrees. Over time a higher temp can damage the slab from too much thermal stress. The idea of using the slab itself as heat storage has no benefit, the slab temp stays more stable due to it's mass, but is still the "radiator" of the system and will not "store" heat. I have only designed systems using a hot water heater or condensing boiler as a heat source which can both cycle on and off on demand, so using a woodstove that needs to stay hot, creates a new twist. A mixing valve and small circulating pump on your primary loop (the heat exchanger and water storage tank) will help to maintain a higher return temp to help reduce creosote in the stove. Another mixing valve on the secondary loop (the water storage tank and loops) will maintain a constant temp supplied to the slab.

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.

Like I said in another post originally I had planned to put an outside boiler in-between the shop and the house to heat both but just haven't had the time and money to build or buy one. I had an old wood boiler that I played around with for two years but I finally scrapped the boiler part of that thing and built this heat exchanger and added it to the old stove. When I laid everything out I put my manifold setup in the front of the building but because of space in that area I couldn't put the stove in there so I had to build an additional room on the back of the building as a boiler room. That's why I have the long run of pipe between the two parts. It would be a whole lot simpler if the two parts were in the same general area but it's a little late for that.

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.

 

[mx842]

Hot water won't damage concrete, they use it for cooling towers in nuclear power plants.

The reason that the water is typically cooler in a floor system than in radiators is that your feet will get uncomfortably hot when the temperature starts getting above skin temperature. You want the water to be as hot as you can stand. The hotter the water, the better job it does at it's primary task, which is transferring heat into the living space. Hotter is always better.

It's difficult to predict how a heated slab is going to perform, so you're going to rely on trial and error. With an adjustable thermostatic valve, adjust the temperature up until it feels too hot.

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.

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.

 

[dadster4]

"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.