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Dave Bush
Dave Bush Member Posts: 155
A contractor friend has a problem with several houses that have two or more hydro-air systems in them.

The deal is, apparently, that on very cold days in NEPA, the second floor units, which are in the attics, don't get the coils warm enough to make the aquastats (set at 140°F) to bring on the blowers. The pipes are hanging at about 130°F.

The problem appears to stem from the fact that these units were installed ith 1" copper from the boiler header, but knocked down to 3/4" for 30-40', which is, of course, now encased in sheetrock walls through 2 floors of living space.

Since, at 12 PSI, 3/4" will deliver 4 GPM, I believe that this low temp problem at the coils is coming from the 3/4" that is in the walls. If 1" had been installed, I don't believe we'd be seeing this problem, as THAT carries the requisite 8 GPM at 12 PSI.

Now, is it possible to get 8 GPM out of a 3/4" pipe, by increasing the pressure in the system, without causing awful noises and internal pipe damage? If it is possible, what would the calculation be to determine the required pressure?

These are standard CI boilers, conventional controls (No reset, et al.) so we're really severely limited in the maximum pressure.

I know that the real solution would be to tear out the 3/4" and put in 1", but we're trying to avoid doing that to these houses. (I don't know how many there are, but there are at least two, and the installs are not that old.)

The circs, BTW, are Taco 007's running up near the end of their curves as it is. He told me that the systems' design head is 19 GPM at 8' of head. Would going to a higher head circ help this situation out? I'm sort of at a loss as I'm not overly experienced with hydro-air systems.

I'd entertain any other suggestions on improving these things, also.

Thanks in advance,



  • dconnors
    dconnors Member Posts: 215
    i would try

    a higher head pump matched to design criteria
  • Carl PE
    Carl PE Member Posts: 203
    Hold on a minute.

    the second floor units, which are in the attics, don't get the coils warm enough to make the aquastats (set at 140°F) to bring on the blowers. The pipes are hanging at about 130°F.

    What temperature is your supply? The water should only lose a degree or two on it's way up there, unless you have insulation issues.

    Temperature and flow rate are two different problems.

  • Dave Bush
    Dave Bush Member Posts: 155
    Thnks for the replies.

    Carl, I see what you're saying and it makes sense. I'll talk to theguy later on and get back here with the supply temp. I was under the impression that it was around 160, but now that you mention it, I very well may have ****/u/me'd that...

    My original line of thinking was that with the reduced flow rate, that we might be possibly radiating some heat from the undersized piping that is buried in the walls, but that the reduced flow rate itself was causing the loss of temp at the aquastat. I suppose now, reading that, that that would be backwards thinking, as a reduced flow rate, at the same pressure, would actually cause a higher temp at the coil...
  • Dave Bush
    Dave Bush Member Posts: 155
    Well, that was my first reaction, too, Dave.

    Because that 007 is at teh top of the curve, but after reading Carl's reply, I think a light bulb might've come on.

    I'm just trying to figure out how several different houses could have the same problem. I'm going to ask him later if there is sufficient insulation in the chases....

    Film at 11.
  • tls_9
    tls_9 Member Posts: 89

    You have mentioned 12 psi several times and you said the units are in the seconed floor attic. Is 12 PSI adequate to get the water up there? Remember 12 psi in the basement is not 12 psi upstairs. Are you sure you have 12 psi, I've seen gages that are either off enough to fool you and gages that stuck where they sat for a wile. If 12 is enough and you do have 12, then I'd go with a higher head pump.
  • Dave Bush
    Dave Bush Member Posts: 155
    Like I said earlier,

    I'm not yet directly involved. I was asked to post this on the wall to see if there might be an easy answer to this.

    I wasn't able to speak to my friend today, but I will in the morning.

    12 psi was being used for a reference, only, as I haven't seen the gauges.

    The problem only manifests itself on at-or-below-design-temp days. The systems apparently work rather well until the ambient falls to freezing-the-balls-off-of-a-brass-monkey temps.

    I suspect that we'll be seeing that type of temp either tonight or tomorrow, also, and if he and I are both available, I intend to take a field trip with him to one of these problem houses to see if a second set of eyes can spot something out of the ordinary.

    But thanks all that replied. I'll make certain that he sees this thread real soon.
  • jerry scharf_2
    jerry scharf_2 Member Posts: 414
    more about the piping


    How are the flows to the two hydro-air zones controlled? Proportional or on/off, circulator or valve? What is exactly the lift distance from the expansion tank to the attic air handler?

    Here's my no information WAG. The upstairs Air Handler (AH) doesn't get flow when the downstairs unit is running. Without reset, the cycle times of the AHs will be proportional to the demand. As the demand increases, the downstairs zone continues to increase it's cycle time until the upstairs no longer has enough time to heat adequately. How I would test it would be to manually turn the upstairs AH on, set both thermostats to call for heat and watch the temp drop across the upper AH. Should tell you all you need, and you don't have to wait for cold weather.

    If this is right, then you need to figure out why. I would start with the system pressure. Here's why. If you are pumping away, the pump adds pressure to the distribution side. When the lower AH it not calling, the full pressure add of the pump goes to the attic unit. When the other AH calls, much of the dynamic pressure boost goes to the lower unit and thus no flow upstairs. The next thing for me would be to add a balancing valve to the lower AH, to make sure the flow to it isn't too much larger than the attic unit. It either of these don't solve it, then I would get a bigger pump.

    If my conjecture is correct, increasing the supply temps would make the problem go away, but not solve it. It's also why I wouldn't start with a bigger pump. It could again mask the problem without solving it.

    Let us know how it turns out.

  • joel_19
    joel_19 Member Posts: 931

    there is a flow problem but the tubing size isn't it. we've installed Ohh I don't know 150-200 in attics with 3/4 pipe some with 1/2" Never a problem and we will go to at least 10 bellow tonight.
    Where is the aquastat is it properly insulated? or is it picking up cold attic air? just because it didn't click on doesn't mean your water isn't warmer than that .Also with good ducts you can run hydro air with colder water than that we do it all the time no problem.

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