In-Floor Heating for a Shop & Greenhouse - Considerations ?

[rScotty]

SNIP....
But definitely do the math before laying any tubing.

I agree with that. Math saves a lot of work. But floor heat can be different from traditional calcs.
When you start figuring how many BTU you need, give some thought toward the RATE at which heat moves in and out of different surfaces and through different materials. You can use math for rate of change calculations as well as for simpler sums. For floor heat, heat input can be very slow compared to outflow. That makes a difference, but the good news is that it will work even if you make a mistake. And is easy to change as long as there are enough loops of pipe in the floor to begin with. So err on the side of too many loops if you have any doubts.

Floor heat involves a lot of different surfaces and heat exchange steps. If doing it on the back of an envelope, I'd try to treat each transfer across a surface as a separate transaction, then estimate or calculate exchange rates as well as BTUs.

Do the math both ways, and think about the way that the "rate of heat transfer" will cause a floor heat system to differ from forced air heat calculations.

Another design difference to think about is that in floor heat systems, heat can be stored in a tank to be used later. That is something that is hard to do with warm air. HWH tanks are great for storing warm fluid.

And remember - no matter what you do, math is just a tool. Don't get hung up. Floor heat is great to have and will work in spite of mistakes in calculations. Floor heat is basically continuous parallel loops of warn fluid in and cold out - AND ANY AMOUNT OF BTU IS AN ADVANTAGE. All it takes is plumbing in two additonal valves to be able to accept heated fluid from any source. Doubling the input BTUs is simple.

A fist-sized silent Taco pump will drive fluid through several thousand feet of PEX tubing. In a low temperature type floor heat (the best kind) all the manifolding and valves can be standard plumbing fittings.

Most all the various ways to heat the fluid are safe, basic, and can be easiliy changed - so don't be afraid to experiment.
enjoy,
rScotty

 

[AnotherBueller]

As others have said with more words, concrete floors have a very slow heat transfer rate to objects in the room and anything on the floor will insulate that warm floor from the stuff in the room.
Concrete is VERY unresponsive to rapid temperature change.

 

[dstig1]

Quicksandfarmer - I am puzzled by your comments as both my house and shop are fully and only heated by hydronic radiant tubing. The shop is slab on grade and the house is basement slab, main level, and smaller upper level. The house is tubing in the slab and the main floor was engineered to hold 1.5" on concrete on top of the subfloor in which the tubing was placed. Another layer of subfloor was put on top of this on sleepers. The small upper level was done with staple-up. Both buildings are SIPs and tightly sealed and very well insulated. Outside Minneapolis. The shop has no issue heating from the floor and it is 2600 sf with 13' ceilings (R48 ceiling).

The house actually runs into problems in very cold subzero weather with not being able to keep up 100% so during long cold snaps I sometimes have to turn on the furnace to help out a bit. The furnace is mainly there for the AC in summer and is used in the fall and a bit in the spring to avoid turning on the floors during times of changing conditions. My issue appears to be the pressure drop in the on-demand heater creating too much back pressure to allow for high enough flow to get enough heat output. from the floors. I think I might have better luck with a tank HW supply, but I am fairly stuck at this point... I tried many things to increase the flow without great success.

 

[quicksandfarmer]

but the good news is that it will work even if you make a mistake. And is easy to change as long as there are enough loops of pipe in the floor to begin with.

There's basically three ways it doesn't work:
* In order to get enough output to heat the place the floor has to be so hot you can't walk on it; or
* The floor puts out so much heat that to keep the room at a reasonable temperature the floor is so cool you don't get that "toasty toe" feel you were going for; or
* The heat responds so slowly that you vacillate between freezing and boiling.

When you get one of those situations, it's usually baked into the tubing that's in the floor, and there's no easy way to fix it. And almost always, what you find is that if someone had jotted a few numbers on the back of an envelope before starting it would have been entirely predicted.

 

[rScotty]

Quicksandfarmer - I am puzzled by your comments as both my house and shop are fully and only heated by hydronic radiant tubing. The shop is slab on grade and the house is basement slab, main level, and smaller upper level. The house is tubing in the slab and the main floor was engineered to hold 1.5" on concrete on top of the subfloor in which the tubing was placed. Another layer of subfloor was put on top of this on sleepers. The small upper level was done with staple-up. Both buildings are SIPs and tightly sealed and very well insulated. Outside Minneapolis. The shop has no issue heating from the floor and it is 2600 sf with 13' ceilings (R48 ceiling).

The house actually runs into problems in very cold subzero weather with not being able to keep up 100% so during long cold snaps I sometimes have to turn on the furnace to help out a bit. The furnace is mainly there for the AC in summer and is used in the fall and a bit in the spring to avoid turning on the floors during times of changing conditions. My issue appears to be the pressure drop in the on-demand heater creating too much back pressure to allow for high enough flow to get enough heat output. from the floors. I think I might have better luck with a tank HW supply, but I am fairly stuck at this point... I tried many things to increase the flow without great success.

Why do you think that increasing the flow will make much difference to the heat in the house?
Have you done some tests? There isn't much of a temperature differential between the concrete next to a heat tube and the fluid inside, so I wouldn't expect the heat to move into or thru the concrete very quickly.

My thinking is that if you are able to ncrease the flow rate through the piping, you will see the return flow will be warmer than before - and the concrete surface temperature will not change much.

But that is just my thinking - not experimental evidence, so it could be wrong. Good thing that being wrong is a key part of being innovative....
rScotty

 

[quicksandfarmer]

Quicksandfarmer - I am puzzled by your comments as both my house and shop are fully and only heated by hydronic radiant tubing. The shop is slab on grade and the house is basement slab, main level, and smaller upper level. The house is tubing in the slab and the main floor was engineered to hold 1.5" on concrete on top of the subfloor in which the tubing was placed. Another layer of subfloor was put on top of this on sleepers. The small upper level was done with staple-up. Both buildings are SIPs and tightly sealed and very well insulated. Outside Minneapolis. The shop has no issue heating from the floor and it is 2600 sf with 13' ceilings (R48 ceiling).

The house actually runs into problems in very cold subzero weather with not being able to keep up 100% so during long cold snaps I sometimes have to turn on the furnace to help out a bit. The furnace is mainly there for the AC in summer and is used in the fall and a bit in the spring to avoid turning on the floors during times of changing conditions. My issue appears to be the pressure drop in the on-demand heater creating too much back pressure to allow for high enough flow to get enough heat output. from the floors. I think I might have better luck with a tank HW supply, but I am fairly stuck at this point... I tried many things to increase the flow without great success.

It sounds like you have a pretty good system.

The amount of heat the floor puts into the house is determined entirely by the temperature difference between the floor and the inside air. So if you find the floor isn't putting out enough heat to keep you comfortable on the coldest days you need to increase the floor temperature.

There's two ways to do that. One is to increase the flow rate of the water. As the water flows through the floor it cools. The faster it flows the less it cools and the warmer the average temperature of the water in the floor. That's what you've already tried, and there's a limit to how effective it can be, the water never gets hotter than its source temperature and at a certain point even ridiculous flow rates don't change anything.

The other way is to increase the water temperature. Have you tried that? A common solution with floor heat is to have what's called "outdoor reset" where there's a sensor outdoors and the water temperature is adjusted according to the outdoor temperature, the colder it is outside the hotter the water is.

 

[rScotty]

There's basically three ways it doesn't work:
* In order to get enough output to heat the place the floor has to be so hot you can't walk on it; or
* The floor puts out so much heat that to keep the room at a reasonable temperature the floor is so cool you don't get that "toasty toe" feel you were going for; or
* The heat responds so slowly that you vacillate between freezing and boiling.

When you get one of those situations, it's usually baked into the tubing that's in the floor, and there's no easy way to fix it. And almost always, what you find is that if someone had jotted a few numbers on the back of an envelope before starting it would have been entirely predicted.

Sounds to me like you are describing a situation where a boiler is running very hot fluid to heat a floor that doesn't have much mass. Most of my comments are directed differently - I prefer heavy hydronic floors/slabs from 3 to 6" thick and running HWH temperatures of up to 120Fmax rather than at boiler temperatures.

But I've actually seen the problems you describe in a couple of systems - one of which i designed myself. That one was built using a commercial system of PEX heat piping inserted into thin extruded aluminum "radiating members" that were mounted between the sleepers under a hardwood floor.
Beautiful installation, expensive idea, poor performance.

Another was an overhead basement ceiling system designed by a homeowner to heat the floor above. It used a solar/propane boiler and high temperatures. In that one, the pipes were fastened between the joists for the floor above in a way that put them in contact with the bottom of the low mass concrete & wood composite floor above. In that installation, the idea was to have the heat pipes accessible from below instead of being buried within the concrete. It ended up requiring hotter fluid than I felt comfortable with, and heated the air below more than the concrete above.

Both systems worked, but could have worked better.
rScotty

 

[quicksandfarmer]

Sounds to me like you are describing a situation where a boiler is running very hot fluid to heat a floor that doesn't have much mass. Most of my comments are directed differently - I prefer heavy hydronic floors/slabs from 3 to 6" thick and running HWH temperatures of up to 120Fmax rather than at boiler temperatures.

The heat output of a floor is determined entirely by the temperature difference between the floor and the inside air. What the floor is made of, how the heat gets into it, etc. doesn't matter. Of course those things ultimately matter, but down the road. If the heating load and floor area of the building are such that you can't meet the heating load with a reasonable floor temperature, that's it. No amount of plumbing is going to change that.

This is what I meant earlier when I said, "if someone had jotted a few numbers on the back of an envelope before starting it would have been entirely predicted."

 

[rScotty]

The heat output of a floor is determined entirely by the temperature difference between the floor and the inside air. What the floor is made of, how the heat gets into it, etc. doesn't matter. Of course those things ultimately matter, but down the road. If the heating load and floor area of the building are such that you can't meet the heating load with a reasonable floor temperature, that's it. No amount of plumbing is going to change that.

This is what I meant earlier when I said, "if someone had jotted a few numbers on the back of an envelope before starting it would have been entirely predicted."

I hear what you are saying. My reply is that it isn't that simple. Some types of calculation may be OK, but the simplest sort of generic total BTU back of envelope calculations can point to the wrong solution by ignoring how a floor will work for the people living on it. Total energy calculations work a lot better for forced air or wood stoves in a room than for heated floors.

An example of things not always is the thermal mass and heat transfer of the floor. Simple calcs say how hot the surface is, but equally important is how it can maintain that temperature. Also, that people will tolerate a greater variability in air temperature than in floor temperature. And I think that plumbing will change both of those things.

I prefer to sum more detailed individual calcuations rather than express them as one single number. That's the only real difference in our methods. Put another way, my feet prefer a different temperature than my hands do.

It's OK by me if you feel differently. And if you get value from doing things that way, then that's OK with me too. Difference is the spice of life.
Besides, I'm old and I'm easy.
rScotty

 

[quicksandfarmer]

It sounds like you have a pretty good system.

The amount of heat the floor puts into the house is determined entirely by the temperature difference between the floor and the inside air. So if you find the floor isn't putting out enough heat to keep you comfortable on the coldest days you need to increase the floor temperature.

There's two ways to do that. One is to increase the flow rate of the water. As the water flows through the floor it cools. The faster it flows the less it cools and the warmer the average temperature of the water in the floor. That's what you've already tried, and there's a limit to how effective it can be, the water never gets hotter than its source temperature and at a certain point even ridiculous flow rates don't change anything.

The other way is to increase the water temperature. Have you tried that? A common solution with floor heat is to have what's called "outdoor reset" where there's a sensor outdoors and the water temperature is adjusted according to the outdoor temperature, the colder it is outside the hotter the water is.

Just to throw around some numbers: imagine you have a house where the heated floor maintains your set temperature when it's 25F outside but not when it's 10F. Additionally, imagine your thermostat is set to 70F, your boiler is producing water at 120F. When it's 25F outside the water coming back from the floor is at 110F and the floor itself is at 85F. What would it have to look like when it's 10F outside?

There are four different heat transfers in this system: boiler to water, water to floor, floor to air, and house to outside. In order for the temperature to hold steady they all have to match. And they're all driven by temperature difference. So let's look at them one by one.

At 25F outside and 70F inside, there's 45F difference between inside and outside. At 10F there's 60F difference, exactly one third more (I picked the numbers to make the math easy). So whatever amount of heat is needed at 25F, a third more is needed at 10F.

With floor at 85F, there's 15F between the floor and the interior. To get a third more heat, that difference needs to be a third higher, 20F. So the floor needs to be at 90F.

With the water leaving the boiler at 120F and returning at 110F, the average water temperature is 115F. With the floor at 85F that's 30F of difference. To get a third more heat that needs to be 40F -- with the floor at 90F. So the average water temperature needs to be 130F.

The amount of heat transmitted by the water is equal to the flow rate times the difference between the out temperature and the in temperature. With the flow constant, to increase the heat transfer by a third the temperature change has to increase by a third, so 13.3F instead of 10F. So if your average water temperature is 130F, and the change is 13.3F, it needs to be leaving the boiler at 136.6F and returning at 123.3F.

In summary, if a boiler setting of 120F is perfect when it is 25F outside, you need a setting of 136.6F when it's 10F outside.