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Circulator speed and Radiant

I define cold as lower than 10*
Steve from Denver, CO


  • Steve Garson_2
    Steve Garson_2 Member Posts: 708
    Circulator speed and Radiant

    My radiant heat loop has a circulator with three speeds. It is set at the middle speed. When the weather is 0* like it has been, the room temp goes no higher than 66*, so the backup electric heat is needed to raise the temp a bit more. The room has a north wall with cathedral ceilings and probably should have had a HW baseboard and 2-stage T-stat, but it's too late for that now.

    Would it may a difference to move the circulator to the higher speed on real cold days?
    Steve from Denver, CO
  • Brad White
    Brad White Member Posts: 2,398
    It cannot hurt, Steve

    Only way to know is to try it. Your best bet for effect though is to increase the supply water temperature incrementally within reason.

    I will also assume that your tubing lengths are not excessive and that you have reasonable flow to start with.

    Here is a rough explanation of the flow rate effect alone:

    Flow rate typically has minimal effect on most emitters. (Dropping the flow in half if you are running a 180-160-170 supply-return-average temperature system will drop your average temperature only ten degrees and you will still get about 90% of output.)

    However, with low-temperature, low-delta-T systems this flow to output difference is magnified.

    Most floor radiant systems use a lower delta-T than other emitters. Rather than 20 degrees, 10, 8 or even 6 degrees is common. This is principally to minimize perceived cold spots, to keep the floor temperature more even and homogenous.

    Say a given circuit now uses 1.0 GPM and has a 12 degree Delta-T (about 6,000 BTUH) because your slab temperature is stressed, not keeping up. Say your average water temperature in this scenario is 109 degrees (115 supply and 103 return).

    If you go to high speed you will not double the flow but you might add, depending on the current operating point and pump curve versus system curve, maybe 25% for discussion's sake.

    Now you would be running 1.25 GPM at 115 supply. Your delta-T initially would drop to about 9.6 degrees from 12 degrees. Your return water would rise to 105.4 degrees and your average water temperature (on which your floor output is rated) would rise to 110.2 from 109 in rough numbers.

    This increased temperature will eventually settle to a balance point between increased floor output and water temperatures, but you get the idea. The effect is marginal compared to increasing your water setpoint.

    I would recommend you step up the pump first and see, but that will take time if a high mass floor. Next I would raise your water temperature. You may find that raising the water temperature will allow you to keep the pump speed where it is now though.

    Does that make sense?

    "If you do not know the answer, say, "I do not know the answer", and you will be correct!"

    -Ernie White, my Dad
  • ALH_4
    ALH_4 Member Posts: 1,790

    Do you have outdoor reset on this system? How warm do your floors get when it is cold out? Is there a thermometer on the supply pipe to the radiant floor?

    An infrared thermometer is the most fun you will ever have for $59. With one you can take a lot of the guess work out of optimizing your system.
  • Define cold...

    I was called in to look at a "problem child" job done by one of Denvers older respected boiler companies. The occupants of this mansion had thier office in the basement, where the hydronic contractor had installed a radiant ceiling. They installed a 3 way off a parallel loop with a dedicated pump for the RCH and a seperate 3 way and pump for the upper floor RFH system serving the main floor. When I walekd into the house, I was hit with a big blast of radiant energy. I walked over to the stat and found it turned to 77 degrees F. When asked why they had it so high, the owners said they spent the majority of their time in the basement working, and when they came upstairs, it felt cool, so they were trying to balance out the discomfort. The floors were running at 85 degrees F on the main floor, and the MRT was close to 75 degrees F. When I went down stairs, I found out what MEAN radiant temperature really meant. The basement was 85 degrees F, and the thermostat was hanging by one wire on the wall.

    When queried about this condition, the GC said he's had the boyla guys out numerous times and they had exhausted their internal resources, threw up their hands and walked away. The ceiling was at 95 degrees F.

    A cursory review of the mechanical room found that they had tied the RCH distribution system in as a parallel off the heat source as opposed to a P/S pipng arrangement, so every time the boyla fired to satisfy ANY call for heat, the RCH system got heat and flow wether it wanted it or not!

    I tried my best to explain to the HO that his main floor was NOT cold, but that his body felt that is was because everytime he came from hell into heaven, the MRT was lower and his body was telling him so. He said, "I really don't give a crap about MR T or Mrs T, just make me comfortable!"

    So, as an experiment, I shut off the supply and return to the basement RCH system, showed him how he could turn it back on, and left him with my business card. I also lowered his main floor stat to 70 degrees F and told him that if his next door neighbor came into his house, he'd break out in a sweat. He said, "Oh yeah, he DOES!"

    There are many other problems with the system, that are inherent to a "design as you solder" system. These guys were GOOD pipe fitters, and had everything been high temp, they'd probably have gotten by, but with three different temperature requirments, they missed thier mark BIG time on this job.

    THe moral of the story, radiant and hydronics may not be rocket science, but you MUST pay attention to details, and when the customer says they are uncomfortable, LISTEN to them...

    I'm certain I will gain another customer for life. TOo bad the consumer has had to live with the discomfort they have for the last three years...

  • Steve Garson_2
    Steve Garson_2 Member Posts: 708

    Good information from everyone. Many thanks.

    My radiant is just for an addition: master BR and Bath. The temp control is manual. The ideal water temp during winter is 140*. I jack it up to 150* when it's real cold.

    The infrared thermometer shows 80* on the hardwood floors. My understanding is that you shouldn't have more than 80* on oak floors. Assuming that is correct, the problem is very likely that the heating guy who the GC hired had limited radiant experience. I doubt he factored in what would cover the floor: dresser, chest, oriental rug, love seat, king size bed which all blocks heat to some level. Under the rug is toasty and when it's real cold, ie under 5* I roll up the rug:)

    I only wish it was done right when it was built. But fortunately I had the foresight to install and electric baseboard "just-in-case" they didn't engineer things correctly. The moral of the story: if you get radiant installed, hire someone who specializes in it. Just any heating/plumbing firm won't necessarily do the best job. And when you have cathedral ceilings, extra heat will be needed when it's at design temp.
    Steve from Denver, CO
  • Weezbo
    Weezbo Member Posts: 6,232
    Excellent observation on MEAN radiant temp *~/:)

    i call it Severe :) Bake the brain syndrome...that just me :)

    nice fix to the glitch :) something that i have found is you can definitely determine how cool 80 degrees IS when 104 is pounding away at the brain, melting you to your knees when you walk into a room :)
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