Radiant floor slab experiences?

[thunderskunk]

Hi!

For those with radiant floor slabs using hot water through pex:

• How long does it take to heat your space from cold? Say from 60 to 70 F?
• Does the room take just as long to heat up from a cold slab than a hot slab if you open the garage door?

The long story: I built my 33x40 garage with 12’ ceilings and radiant floor heat. I’m running a machine shop with no anticipated need to open the doors much, at least that was the plan. Now we’ve got a guy doing some mechanic work, so the doors will get opened several times a day. The plumber mentioned I may want a second source of heat which… threw me in a tizzy. I wasn’t expecting that curveball, but I feel like he’s right…

 

[Earp69]

Temperature rise from 60-70 depends on how many ft of pex you have in the floor,obviosuly the more hot water pumping through the floor the merrier. My shop is 36x40, I ran 1750 ft of pex even though rule of thumb is 1 linear ft per square foot of floor so I should have ran 1440 ft of pex.My floor will raise temp quickly I think, 5 degrees may take a 1/2 hour or so.

You do not need a second source of heat, radiant floor heat is the absolute best at recovering from big doors opening. You have a huge slab that's acting as a heat sink, you'd be amazed at how fast temp recovers/if it even drops. The biggest thing you want is your thermostat to go off of slab temp instead of air temp to prevent short cycling. If you go off of air temp, anytime you open the garage door your unit will cycle. If you go off of floor temp this is not the case.

 

[jblnut]

Question 1 - What do you mean heat from cold ? Do you mean from whatever temp the slab is the first time you turn heat on in the fall up to 70f or from a 60f slab to a 70f slab ? I've never kept track of how long it takes the first time in the fall ....

I keep my 54x72x18' farm shop at a balmy 58f and heat it up to 68-70f a few times over winter for birthday parties and other such events and I'll set it higher in the evening and it'll be to temp in the morning.

Question 2 - Not quite sure what you're asking but ..... I have a 16x12 and a 26x16 overhead and open the 16x12 multiple times daily and the big door a few times a week. Depending on the outside temp of course ..... The temp drops inside but comes back after less than 10 minutes usually. The doors are open long enough to drive something in or out and never stand open.

I have a gas line installed for a hanging heater and have never thought about installing one. It doesn't bother me to have a little temp difference once in a while. I have a close neighbor that has a topper heater and it's super annoying. It seems like it runs and runs and runs and then the floor heat doesn't run as his floor stat is on air temp only. He ends up setting the topper 5f below the floor heat and then it never runs. Likely solvable issues but he doesn't have a floor stat. To make a topper heater work well you need a floor stat I think.

 

[TurnipTruck]

I put radiant in my last shop, a 24x62 Quonset. The very first time I ever ran the boiler was on a Halloween, and it was 20*F outside. The 100kbtu boiler ran at 100% for 24 hours to raise the interior temp from 40* to 57*. Once the building and stuff got heat soaked, the boiler ran pretty much once a day for an hour or so every winter thereafter.

 

[like2wheel]

I have a close neighbor that has a topper heater and it's super annoying. It seems like it runs and runs and runs and then the floor heat doesn't run as his floor stat is on air temp only. He ends up setting the topper 5f below the floor heat and then it never runs. Likely solvable issues but he doesn't have a floor stat. To make a topper heater work well you need a floor stat I think.

What is a "topper heater"?

 

[jblnut]

What is a "topper heater"?

A hanging LP forced air heater heater that runs when the temp drops quickly to heat the air back up. It "tops off" the temp quicker than the radiant does. His shop was built in the early 90's when in floor radiant wasn't as common in farm shops so it was the thing to do at the time. It works but seems loud and annoying to someone that doesn't have one. I love my radiant heat and would happily do it all again.

 

[PoorUB]

Hi!

For those with radiant floor slabs using hot water through pex:

• How long does it take to heat your space from cold? Say from 60 to 70 F?
• Does the room take just as long to heat up from a cold slab than a hot slab if you open the garage door?

The long story: I built my 33x40 garage with 12’ ceilings and radiant floor heat. I’m running a machine shop with no anticipated need to open the doors much, at least that was the plan. Now we’ve got a guy doing some mechanic work, so the doors will get opened several times a day. The plumber mentioned I may want a second source of heat which… threw me in a tizzy. I wasn’t expecting that curveball, but I feel like he’s right…

Your first point, Do you mean from a set point of 60 then bumped up to 70 degrees? If yes, it can take a couple days. With floor heat set it to the temp you desire and leave it there. You can not do a night set back. The floor needs to be 5-10 degrees warmer than the space temp. If you want it 60 degrees the floor will settle out 65-70 degrees. If you turn up the temp to 70 degrees you need to heat up all that slab to 75-80 degrees.

Second point, The space temp can recover in just a few minutes. I worked at a HVAC shop with floor heat. Even in -20F winters when they were moving rolls of sheet metal, or equipment in and out, it was not unusual for the overhead door to be open for an hour. 20-30 minutes after closing the door it felt warm again.

I would not worry about a second source of heat. My bet is you will not need one.

 

[Earp69]

I have 12 foot ceilings also, I put in two large ceiling fans assuming I would have to push the heat down. I was extremely surprised when the ceiling temp was actually 2 or 3 degrees below slab temp without any fans running. Your machines will really appreciate the floor heat also, no more rust.

I put radiant in my last shop, a 24x62 Quonset. The very first time I ever ran the boiler was on a Halloween, and it was 20*F outside. The 100kbtu boiler ran at 100% for 24 hours to raise the interior temp from 40* to 57*. Once the building and stuff got heat soaked, the boiler ran pretty much once a day for an hour or so every winter thereafter.

There are many factors obviously. How well is your slab insulated, temp of the ground under the slab, water temp going into the floor etc. I send 110 degree water into the floor, and can't remember exactly but I think the return is 70 degrees or so.