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digital tstat guns and balancing

keyote Member Posts: 659
Was about to give up on tsat gun, erratic readings and reading much lower than boiler sensors. I was guessing you cant get good readings through copper pipe, and i suspected some issue with type L and M might be mixed. But it worked on toilet water accurately so i then tried it on the pex in and out of the manifold much better.but still a bit puzzling.
So the boiler reads 128 swt 117 rwt =DT 11. and the readings with the gun on the loops as they enter exit the manifold were 119 swt 107rwt [ the manifold gauges are a degree hotter. so far so good as my manifolds are at the top of a 1" copper x 27' tall pair of risers, I thought OK Im losing about 8 degrees before the manifold because I havnt yet insulated the risers.Which made me feel better the water needed to be so hot on a 33 degree day. But then I thought if Im losing seven degrees on my way up shouldnt I also be losing some more on the way down. I understand the loss should be less, but the pipes are open to the interior of the house which is maybe 70 at most, I realize the risers are only a 2-3 inches apart so that might help still seems impossible to have almost no loss on the return riser. Any ideas?

Another question is how accurate is the reading on a 1/2 pex pipe?
The design day water was supposed to be 128 degrees, and produce an 84 degree surface temp on floors, which is the max you really want.
If i really have 120 degree water on a 33 degree day seems like my plateless design is going to need much hotter water on 17 degree design day. Now this may not be a problem because maybe, probably the floors right now are not actually 84 degrees [I've disturbed the tenants too much to check and suspect the digital thermometer wont really be very accurate on the wood floors without plates.]
The heat loss said that floor needed about 16,000 btu on design day divided by 1000-1100 sqft is about 15.5 - 16 per sq ft.
If Im right that the floor cant actually be too hot until its properly heating on design day then higher swt is the easiest solution? does that sq ft number sound right?


  • Jamie Hall
    Jamie Hall Member Posts: 18,977
    Not a comment on the temps -- but to get reasonable readings on various types of pipe, or at least consistent readings, try wrapping black electrical tape around the pipe and reading off that, rather than the metal of the pipe itself...
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
    Mark EathertonBoon
  • keyote
    keyote Member Posts: 659
    Thanks jamie, Hat ?
  • SWEI
    SWEI Member Posts: 7,356
    edited December 2016
    Also, there is a common misconception that IR temp guns measure the temperature on the exact spot where the laser hits. The spot actually marks the center of a circle, and the temperature reported is the average of all surfaces inside that circle.

    Many low cost IR guns have spot ratios around 10:1, meaning the diameter of the spot (90% of the measurement) is 1/10th of the distance from the sensor to the surface. So if you hold the gun two feet away, you are measuring a 2-1/2" diameter circle at the pipe.

    By comparison, the Fluke 568 has a 50:1 spot ratio.