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slab in basement how long to heatup

matty Member Posts: 1
how long to heat up 4" slab  in basement and once its heats up  how much over the temp the thermostat is set at can i expect. this is an older system

 with  160 temp water running  through the basement  zone


  • Jean-David Beyer
    Jean-David Beyer Member Posts: 2,666
    A long time...

    I have a system with hot water in a slab. My current boiler and controls and zoning are all different and have not been through a winter yet.

    The old system ran about 160 water through the slab when the thermostat felt cold. It would take about 4 hours to warm up the slab. The overshoot was around 5 degrees. It cycled unacceptably fast; like 30 seconds on, 45 seconds off.

    After I set the boiler control to keep it between 130F and 140F, and diddled one of the valves, the overshoot went down a bit. I got the cycling to slow down a bit; like 90 seconds on, 120 seconds off. Since this was not meant to be a condensing boiler, I was probably damaging the boiler, but it never got around to leaking.

    The new boiler is a mod|con with outdoor reset, so I expect the overshoot to be greatly reduced. Likewise the rapid cycling.
  • GMcD
    GMcD Member Posts: 477
    Slab warm-up time

    It depends.  If the slab has insulation underneath, as well as at the edges, a 4" slab can warm up to setpoint in as little as 8-10 hours.  If the slab is missing the edge insulation and has no insulation under it, it can take days, or even forever as the ground will suck a lot of heat out of the slab until the local ground temperature and slab temperature come into equilibrium.

    A normal room thermostat is a device that senses "air" temperature.  The slab will be radiating heat, and not necessarily heating up the air at the same rate, hence many people with improper slab control get overshooting/undershooting because the slab's response time to a change in heating water temperature is a heck of a lot longer than what the room temperature change time can be with a poor building envelope. 

    A temperature sensor imbedded in the slab to monitor the slab temperature can help reduce the overshooting (Check Tekmar's line of Controls), but a slab heating system coupled to a poorly insulated building envelope that allows quick changes in the room loads is a bad combination, and all the band-aids, baling wire, and chewing gum in the world won't help.
  • Randy Baerg
    Randy Baerg Member Posts: 26
    floor heating temperature & control.

    First off your supply temp of 160°f is way too high for radiant floor heating. You will experience excessive temperature over / undershooting without providing a lower temperature. typical design is around 110-120°F. You should add a mixing / reset control that would allow you to operate at a lower temperature, do outdoor reset and still provide boiler protection (ie keep it from condensing).

    Secondly you should also consider a proper radiant thermostat. These work on PWM (pulse width modulation) which provides nice long cycle lengths, e.g. 20 minutes, to reduce the short cycling you would get otherwise. tekmar control systems provides several products that should work with your system

    By running at a lower temperature you will will have much less room temperature swing, reduced cycling and probably better efficiency as well. Let's see... improved comfort, lower energy use, reduced wear & tear....not bad.

    Your concern about how fast the slab should warm up is moot when you control radiant properly because you are actually trying to maintain the room temperature with a fairly constant slab temperature with constant circulation.
  • CMadatMe
    CMadatMe Member Posts: 3,086
    How Long?

    An impossible question to answer. For starters the max water temp for Pex in concrete is 140 degree water. Once you start driving over that temp the concrete begins to crystalize around the tubing thus slowly reducing btu output until it cannot produce any at all. No different than scale build up in a coil.

    Back to your question. You should never have overshoot of the therm if the system water temp is properly controlled. If you have a basic thermostatic mixing valve and running the same temp on a 20 or 30 degree day that you need at zero for the radiant it is a water temp control issue. In a properly designed radiant system we do not use 70 degrees as our indoor design temperature to calculate heat loss and water temp. We use 65. Some may use 68 but a properly designed radiant system never uses 70 degrees.

    If you are overshooting you need a much lower water temp. Have a radiant heat loss done (notice I did not say a heat loss I said a radiant heat loss). Set the water temp needed for your design day. If a thermostatic mixing vlv, throw it in the garbage and consider using some type of control that will modulate the radiant water temp based on outdoor temp. The Taco RMB is what I call radiant for dummies, all the piping and pumps are done for you and it works great and is an easy installation. Check it out at [url=http://www.taco-hvac.com]www.taco-hvac.com
    "The bitter taste of a poor installation remains much longer than the sweet taste of the lowest price."
  • NRT_Rob
    NRT_Rob Member Posts: 1,013

    Properly designed radiant systems use 70 degrees all the time.

    because people in radiant homes do NOT typically turn their thermostats down to 65. A study was done on that to prove it. people who spend the money on radiant typically soak up the extra comfort rather than turning down their thermostats. in other words, 65 might be as comfortable as 68-70 for a non-radiant system... but that wasn't comfortable to begin with for most people, and now they can be really comfortable and tend to choose to be.

    there are exceptions to every rule and you can design any way you like, but 70 is a completely acceptable and, in my opinion, a required design temp. Explaining to someone who has a radiant system why they CAN'T get to 70 would be a fairly awkward conversation, I would think.

    Also, a basement slab is likely to max out with a 100 deg max temp. 3-way tempering can do that just fine.
    Rob Brown
    Designer for Rockport Mechanical
    in beautiful Rockport Maine.
  • Gordy
    Gordy Member Posts: 9,537
    Agree with Rob

     We set our thermostat at 70*.   The study he is refering to, and theory are completely true.  I'm bundled up in the winter all day in my line of work. The last thing I want to do is stroll around my castle in sweat pants, and sweat shirt. 65 for us is a bit chilly. Maybe I'm getting old.

  • CMadatMe
    CMadatMe Member Posts: 3,086
    You can design

    for what ever temp you want but a thermostatic mixing valve is the most primitive way to control radiant water temp. As stated you are looking for comfort. Why would you run the same water temp that you need on your design temp day on a 20 or 30 degree day? Isn't that the opposite of comfort. Why turn a system that wants outdoor  reset into nothing more than a on/off system?

    I'd like to read this study that was referenced, does anyone have a link for it? Back to your I want to see 70 degrees on my thermostat. Didn't the gentleman who originally posted give the reason. The floor overshoots....With a lower water and design temp we can control the overshooting. When the zone is satisfied and that mass keeps giving off heat we can keep the room comfortable. This is more pertinent when there is no floor sensor.
    "The bitter taste of a poor installation remains much longer than the sweet taste of the lowest price."
  • NRT_Rob
    NRT_Rob Member Posts: 1,013
    this is a basement

    if 100 degrees were the max temp requirement as it likely is in a basement, dropping that 10 or 20 degrees with resetisn't a really big deal in a low-swing environment like a basement slab. with a decent thermostat you would never in a million years be able to tell the difference between 100 degree bang-bang in a basement slab vs reset mixing. With higher temps, maybe... if it were main living space with variable loads, maybe... but not in a basement slab.

    it may be "primitive" but it works quite well, thanks and knocks off a several hundred bucks immediately plus any replacement costs down the road that aren't doing much otherwise.. adding reset mixing for a basement is, frankly, wildly overkill if you don't need it for anything else. he currently has swings because he's running 160 degree water, which is ridiculous... and running 100 degrees would fix that in a jiffy, especially with a thermostat replacement.

    as for the radiant heating study, the report is from the CMHC, and requires money to get it. however, here is an article referencing the study: http://www.pmmag.com/Articles/Industry_News/1f4783d4ebfc7010VgnVCM100000f932a8c0____

    here is the page on radiant heat from the people who did the study:


    and from that page, they reference the study if you want to buy it from them. from the page;

    "Many manufacturers claim that radiant floor heating is more economical to operate because the temperature setting may be set to 20ºC (68ºF) rather than the usual 21-22ºC (70-72ºF) as required by other types of systems. A study by CMHC (Thermostat Settings in Houses with In-Floor Heating, #01-106) has shown that people tend to keep their thermostats set the same as if they had a forced air system. Even so, the warmest air is at the floor where it is desired (and not at the ceiling) and there is reduced heat loss through the ceiling and walls.

    Zoning a variety of rooms with the options for different temperatures has the potential to reduce energy consumption."

    of course they are working with Celsius because they are canadian.
    Rob Brown
    Designer for Rockport Mechanical
    in beautiful Rockport Maine.
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