Welcome! Here are the website rules, as well as some tips for using this forum.
Need to contact us? Visit https://heatinghelp.com/contact-us/.
Click here to Find a Contractor in your area.

Is this normal? Garage taking hours to get back to temp

HelpInAK Member Posts: 41
I received some great help here last winter when I was learning the ropes with the heating system as a new homeowner.

Radiant heat, propane Lochinvar Noble 199k btu.

We rarely use the garage to park our cars, but with temps dipping into the -30F range this week we decided to give our cars a break. I’ve noticed that the garage can take hours to get back up to set point temp. It’s been nearly 4 hours right now and the temp has rebounded only from 44 to 47 (set point at 49). Outside temp is about -20F and the garage was open for maybe 25 seconds.

Is it normal to have to wait this long after using the garage to get back up to set point or should I have someone take a look at my system? I know we are introducing two large chunks of cold metal that need to be heated but this seems excessively long. We haven’t used any hot water since arriving home because I wanted to see how long it would take to reheat uninterrupted.

Additional details if it matters: output water temp is 135F, passes a mixing valve and looking at the temp gauge appears to be about 80F entering the slab. Return water temp reads 127F.

Any help is greatly appreciated, thank you!


  • kcopp
    kcopp Member Posts: 4,452
    If it a radiant slab, you betcha. That's A LOT of mass to heat up. it can take a day easily. By all rights you should be leaving it at one temperature walk away.
  • pecmsg
    pecmsg Member Posts: 4,968
    Less then 1° per hour, that's fine.

    Agree with @kcopp set it and forget it!
  • ethicalpaul
    ethicalpaul Member Posts: 5,802
    edited December 2019
    > I know we are introducing two large chunks of cold metal that need to be heated but this seems excessively long

    Just so I'm clear...you aren't just heating up the cars, but you are also heating the slab and the entire garage and everything inside it from very cold, is that right?

    Said another way: You aren't just opening the warm garage and driving the car in and seeing it take hours to rebound from that, right?

    Another question for the pros: @HelpInAK says the water from the mixing valve going to the slab is 80 degrees. Is that a good temperature to send out to the slab? It seems low to admittedly ignorant me.

    NJ Steam Homeowner.
    Free NJ and remote steam advice: https://heatinghelp.com/find-a-contractor/detail/new-jersey-steam-help/
    See my sight glass boiler videos: https://bit.ly/3sZW1el

  • hot_rod
    hot_rod Member Posts: 22,505
    Garages are often piped with wide tube spacing as they are not considered comfort zones. So it may have limited heat input which will also slow down recovery.

    SWT would be dictated by the initial design.

    Classic example of appropriate SWT and input to avoid shocking the slab :)
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • mattmia2
    mattmia2 Member Posts: 9,981
    Radiant slab probably isn't the right heat emitter for intermittent short term use.
  • Icarus
    Icarus Member Posts: 143
    As has been pointed out, it is simply a matter of the thermal mass. Consider the mass of the slab, the mass of all the stuff in the garage (and the heat loss from the garage, with a ~70F Delta, it is going to take a LONG time to heat.

    As an anecdote, we live part of the year in Northern Ontario Canada, where winter temps of -30-40F is quite common. When we leave home for a few weeks the small cabin (wood heated) begins to revert to ambient, usually ~0F. Everything in it gets to that temp. When we come home, light the fire, the air gets warm quite quickly, but the rest of the cabin takes days to thaw out, even with a 35k btu woodstove cranking out, a 400 sq ft cabin that is well insulated. Usually, a couple of days after getting home, we will pull a can of something off the shelf, and it will still be frozen solid. The biggest challenge was warming the bed up the first night. A combination of heated mattress pads and flax seed bags heated on the stove makes the first night tolerable.

    So, heating your mass with such a large delta is going to take a while.

  • HelpInAK
    HelpInAK Member Posts: 41
    Makes sense, thanks everyone.
  • HelpInAK
    HelpInAK Member Posts: 41
    @ethicalpaul no, the garage is maintained at 49F. After opening and moving the cars inside, it will drop to around 43 and take hours to rebound back to 49. I didn’t think that big slab would lose so much heat after 30 seconds of exposure to the cold.
  • Fred
    Fred Member Posts: 8,542
    It isn't the slab that lost the heat. It's that 4 or 5 tons of -20 degree steel that you pulled in the garage that cooled the temp of the garage.