Click here to Find a Contractor in your area.
In fairness to all, we don't discuss pricing on the Wall. Thanks for your cooperation.
Need to contact us? Visit

Slab temp rise

I have a customer with a Bradford white direct vent water. It is serving a 35x35 garage with 5 lops of radiant 1/2 tube installed. It is unknown the legnth of the tubing. When we started the system the return water temp was around 50 degrees. It seems like it is taking forever to heat the slab. After 30 hrs the slab is now about 65 degrees. The input on the heater is 38,000 btu. The circulator return is pipd to the " hot" out on the water heater and the return piping is piped to the "cold" inlet. Not sure of the heat loss in the building.....


  • GordyGordy Member Posts: 7,111
    Temp rise.

    Well it all starts with a heat loss.

    Is the slab insulated?

    What's the outside temps since you commissioned the slab. It's not unusual for a concrete slab to take a couple of days to get up to temp.
  • DaddywarbuckssDaddywarbuckss Member Posts: 4
    edited November 2012
    Load calc

    Just did the MJ8 Load calc. This is one of those garages that is built into the side of a hill. (only the front of the slab is above grade, the rest is 4'+ below grade) I made the assumption that they actually insulated beneath the slab. Design temp is 0* ODT and 65* indoor. Total loss at design is 36,210 BTU. Installed is a 38K BTU bradford white water heater. I assume it is good for about 34K after efficiency loss.

    OH PS the outside temp was 29 overnight and 55 during the day. Boiler was turned on about 1pm. Indoor temp was roughly 45* and slab must have been about the same 50* maybe??? It ran for 30* hrs and we got the call from the customer. "hey it's not warm in here" This gentleman uses the garage periodically and wants a quick start up. I think the current set up is not the most practical way of accomplishing that.
  • GordyGordy Member Posts: 7,111

    So you did not do the install?

    30 btus a sf seems high for a garage.

    I don't know if I would assume slab insulation let alone what type was used if any.

    How about flow rates, and tubing circuit lengths? Spacing? Tightened up by the garage doors?

    And yes that type of set up,needs to be ramped up days before user time in building this is not oh I'm going to work in the garage today turn the heat on.

    I would say that water heater may give you 28000 after efficiency loss.

    Is it gas or electric? Assume gas.
  • DaddywarbuckssDaddywarbuckss Member Posts: 4
    Answers to above


    So you did not do the install? No. The HO did.

    30 btus a sf seems high for a

    I don't know if I would assume slab insulation let alone what type was used if any. Assumed was 1" foam board. HO couldn't remember

    How about flow rates, and tubing circuit lengths? Spacing? Tightened up by the garage doors?

    Flow rates unknown. 007 circ. 5 loops. HO does not know lengths. Probably 250? Each. Neither does he recall spacing 12"? Many Unknowns here

    And yes that type of set up,needs to be ramped up days before user time in building this is not oh I'm going to work in the garage today turn the heat on.

    I agree. Would it help to put a 90k btu mod con or standard boiler in? Is there any other way to quickly heat the floor? Max out GPM and btu I guess, but that would have disastrous results when slab was up to temp...

    I would say that water heater may give you 28000 after efficiency loss.

    I figured 88% efficiency. Propane direct vent..

    Is it gas or electric? Assume gas.
  • GordyGordy Member Posts: 7,111
    edited November 2012
    Steady state slab

    I think I would up the heat source.

    250' loops plus leaders to the manifold is about right for that size of slab.

    Convince the HO that he needs to pick a set point, and leave it alone.

    Sorry, but sounds suspicious if he can't remember if the slab was insulated or not. I would bet not. Insulation would help the response time in the right r value. Along with perimeter insulation. Even being in the earth there needs to be a thermal break in the slab.

    Check the delta t of each loop while in operation after a couple days should be between 10 and 20 degrees. Is it still running? Is the slab getting warmer?

    I believe a 007 will give enough flow provided there is not a rouge loop of excessive length.

    I don't think that water heater is going to give you that high of an efficiency.
  • Paul48Paul48 Member Posts: 4,229

    the inlet and outlet of the circulator to determine the head and length of the loops.
  • GordyGordy Member Posts: 7,111

    Should be able to get .75 gpm per loop with a 007. Don't know if there is radical pressure drop in the S/R piping.
  • DaddywarbuckssDaddywarbuckss Member Posts: 4
    Spoke with home owner today

    I didn't have the chance to head out to visit this job today as I am off for the holiday until Monday. I did have a chance to speak with the HO. The slab is up to temp after about 40 hrs run time. ( about a degree per hour ) He stated that there is "bubble wrap" buried below the tube. He had no answer for whether or not there was insulation buried along the slab edges.

    I did find out that the previous boiler was a cast iron WM of 70k input and probably 80% or so efficiency (just a guess).

    The HO also said that the old boiler would heat the space up over night.... Or at least in 12 or so hrs. He also said he feels that the new water heater is really "sucking up" the propane. ( is it me, or does it not take the same amount of energy to heat that slab from 45* to 85*).

    I think the answer in this case is to avoid trying to reason with the HO and just go with the larger power plant. Man this business of heating and cooling structures is such a subjective practice. This system clearly works correctly, just not to his liking.... You can lead a horse to water.... People who disregard mathematics and science very much frustrate me!
  • GordyGordy Member Posts: 7,111
    Uh huh

    Bubble wrap? Packing peanuts would have been better.

    Clear sign the water heater is running at a much lower efficiency than the old boiler.

    What temps was he running through the slab with the boiler? I'll bet 150 or more. More than a water heater can keep up with. Why did he ditch the boiler?

    My rad pad says you can get 30 btus a sf with .75 GPM with 12" oc and 250 foot loops. That's 5' of head not counting the s/r piping.

    To hold 65 degrees he needs an avg water temp of 95 degrees

    Sell him a little 50 k mod/con. You'll both be happy.
  • Harvey RamerHarvey Ramer Member Posts: 1,911
    It is possible to calculate

    the time it takes to heat such a slab, within a slight margin of error. But no matter, I would take a close look at that water heater. I have a feeling that the heat exchanger never quits condensing. If that is the case you are getting very little heat out of your flame. That leaves you with 2 options; install a 4-way I-series mixing valve with boiler protection or put in a real boiler.

    If you put in a real boiler and you want optimal performance from the slab try this setup; Put the boiler on an outdoor reset control. Place the outdoor reset sensor in a place where the morning sun hits it but not the midday or afternoon sun. Use a 009 DT circ or something similiar as long as it is a Delta-T and can over pump the slab. Instead of doing primary secondary piping, pump directly through the boiler. Put in a flow switch to prevent the boiler from firing until there is enough water being circulated to meet it's minimum firing rate. Over size the boiler a bit. that will help with recovery. It will not short cycle. The slab is a huge buffer. If you choose this setup, you have to use every component I just mentioned. Other wise it will be a flop.

    I use this method on slabs and it works superbly. No flywheel effect and quick recovery.

    Ramer Mechanical
    To learn more about this professional, click here to visit their ad in Find A Contractor.
  • GordanGordan Member Posts: 891
    Real boilers need protection, too

    Unless you mean a condensing boiler, which might be implied by the phrase "minimum firing rate", you'd probably want to have a boiler bypass to prevent too cold return water from hitting the boiler's heat exchanger.
  • NRT_RobNRT_Rob Member Posts: 1,009

    no need for a flow switch. single zone system, on is on. if it's zone valved, you need to be more careful.

    this water heater is definitely below 30kBTU/hr output... not many 88% efficiency water heaters for space heat out there. but let's be super nice and say it gets 30kBTU/hr. If you have a 4" slab, it would heat a 35x35 slab 1 degree F every 20 minutes if we can dump the full output of the water heater, which we should be able to do with this loop field. WITH NO HEAT LOAD AND WITH A DRY SLAB. if this is a fresh slab, initial startups can take extra long.

    BUT, you have to deduct the heat load. you say design load is 37kBTU, basically, and you are basically at an average of half load. so now you only have about 12kBTU/hr left on your generous 30k output water heater. Now you're more like 1 deg F per HOUR of runtime. from 40 to 70 would take about 30 hours. and that's just slab temp... if you are starting with a room temp slab, that means you are picking up 1 BTUs/sq ft of output to the room per two hours of runtime, and until your output hits your heat loss, you're short.

    not exactly accurate: load is lower when the room is colder, so actual performance could be a bit faster. also, this doesn't account for the heat absorption of the dirt in contact with the slab if the edges aren't insulated. It presumes your load is correct. and it presumes that the slab is dry, again.

    but it should illustrate that there is no fast response here.

    If you want to juice up recovery with output, don't bother putting in a heat source sized for more than 30 to 40 BTUs/linear foot of pipe in this case. that's probably about the max emission you'll get. maybe slightly more. I am making that number up though as a ballpark aid.

    really though if you want fast recovery you don't want slab radiant....
    Rob Brown
    Designer for Rockport Mechanical
    in beautiful Rockport Maine.
  • zacmobilezacmobile Member Posts: 211
    thermal constipation

    I did a quick calculation in my heatloss spreadsheet and with no perimeter or underslab insulation I come up with 51btu/sqft (62,475 total) required but the floor would only be able to provide 25btu/sqft (30,625 total) if tubing is at 12" centres.

    This still says it all:

    "So you did not do the install? No. The HO did."

    To be fair I've seen some pretty good homeowner installations they are the exception not the rule.
This discussion has been closed.


It looks like you're new here. If you want to get involved, click one of these buttons!