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Wrong from the get go

ChunkstyleChunkstyle Member Posts: 4
edited December 2014 in Radiant Heating
Hi people,

I was hoping to get some advise on what could be causing my boiler to be short cycling in my shops radiant heating system.
The shop is a 24' x 40' with a full upstairs. Bottom of shop has been divided into a 24 x 28 main shop and a 12x 24 finish room. All three areas are on separate zones.
Ground floor zones are 1/2" pex tubing run in a 6" slab. Main shop has a 3 loop manifold that is fed from zone valve with 3/4" kitec. Finish room space has one loop of 1/2" pex 90' long.
Upstairs has a 8 loop manifold fed from control board with 3/4" kitec. Pex tubing is 3/8" heating the 2" thick gypcrete slab.
Boiler is a Munchkin 80.
Primary piping has the circulating pump on the inbound side of boiler. Primary piping is connected to the secondary with two closely spaced T's, 3.5" on center. Pipe is 1" diameter copper.
Secondary loop piping has the circulating pump located beyond the expansion tank and feeds any of the three zones.
The problem is when two of the three zones call for heat on their own, which is almost always with the upstairs being 90% of the time. Sequence is the zones valve opens, circulating pumps start whirring away then the boiler fires up. Problem is the boiler will exceed it's set point of 105F and short cycle. Placing my hand different location of the primary and secondary piping I have determined that some of the primary's outbound heated water is being diverted back into the in bound T at the primary/ secondary T connections. This "prewarms" the inbound boiler water and causes the boiler to exceed the set point on initial firing. I can open a second zone at the same time as the first and there is enough flow of cold return water to keep the set point from being exceeded. I can then shut the second zone off and the boiler can modulate itself to maintain the 105f.
As I see it, the heated outbound water is in the primary loop is getting 'stolen on the inbound side at the T intersection due to the different flow rates between the primary and secondary loops. My dilemma has been how to best cure this problem. The Company that built the control board is no longer in business to be of any assistance. When it was it was of little help. They wanted to charge me $150/hr. just to come out and look at it. Since I did the install (built the barn myself) they used that for cover and an excuse to charge. Control was designed and built by them and all installation was followed to the letter but it made no difference. It was suggested by them to install a ball valve between the T's of the primary and secondary loops to shut off the ability of the hot outbound water to be diverted back to the cold inbound. I'm dubious of any advice and wanted to get a second opinion. Would anyone be able to offer any insights for an optimum solution to this situation. I have attached pics of the boiler with the primary and secondary piping. Any advice would be appreciated.


  • ChunkstyleChunkstyle Member Posts: 4
    Quick pic of zone valves.
    If there are any other shots I could take that could be useful let me know and I'll post them.
    Thanks again for any help.
  • IronmanIronman Member Posts: 6,155
    edited December 2014
    Two things come to mind:
    1. You're experiencing greater flow through the boiler loop than through the zone loop. This is especially so when only one zone is open. You need to make sure that your circ's are set to the correct speeds. That requires proper calculations for the zone loop. Also, with only one zone calling, the boiler is over-sized in comparison to the load.
    You need to make sure that all air is properly purged from the zone loops. Also, consider reducing the number of zones. Mod/cons and zoning is generally undesirable and often unnecessary, particularly with a high mass like a slab.

    2. If the boiler has the ability to use a "system" sensor, then, use that instead of it working from the supply sensor in the boiler. Connect it on the supply side of the zone loop, downstream of the closely spaced Tees.
    Bob Boan
    You can choose to do what you want, but you cannot choose the consequences.
  • IronmanIronman Member Posts: 6,155
    What is the on center spacing and length of your loops?
    Bob Boan
    You can choose to do what you want, but you cannot choose the consequences.
  • icesailoricesailor Member Posts: 7,265
    I personally feel that it is bad piping practice to connect two closely spaced tees on a vertical run where the tees are connected to turns from the horizontal to the vertical (bottom) and the vertical to the horizontal (top). There can be way too much turbulence in the vertical pipe for the primary (short horizontal) pipes connection to sort out.

    Horizontal = Laminar.
    Vertical = Circular.

    Turns are turbulent until sorted out.

    A ECM circulator as the secondary pump might help. With zone valves, some manufacturers recommend adjustable bypass valves.
  • ChunkstyleChunkstyle Member Posts: 4

    The second story 3/8" pex loops were spaced 6" on center. Have to dig out my old hand drawn schematic of the loops layout to get the lengths. They did not exceed the maximum lengths allowed for 3/8" pex by the supplier.
    Downstairs main floor was 1/2" pex 12" o.c. Again, loops did not exceed the e maximum run by supplier. Lengths were noted and will have to dig out notes.
    My initial goal was to have my finish room on seperate zone so that I might be able to leave the shop for the night and let sprayed components finish curd up for redcoats next day. I can see that a 90' pex loop is probably to small for the munchkin to modulate down to and short cycle. Don't see why the upper should cept that the supply/ return lines are 3/4 Kitec whereas the primary loop is 1".
    Obviously I am trying to sort this out myself now as I have been frustrated by paying others to and still have the problems in what I thought was a fairly simple control board. What I've learned in search of a solution is that there is a good deflation of craftsmanship in designing and installing these systems!
    Would you have a link to a good resource for calculating pump speeds? More reading ahead.
    Thank you for the critique. I will do as you recommend and get those T intersections oriented in a horizontal position. I assume the max 4x pipe diameter spacing of T's is still applied. My understanding is that a minumum of 4" after and 8" before the T's are required. Would you agree?

    Thank you both for the help. It is greatly appreciated.
  • icesailoricesailor Member Posts: 7,265
    "" Icesailor,
    Thank you for the critique. I will do as you recommend and get those T intersections oriented in a horizontal position. I assume the max 4x pipe diameter spacing of T's is still applied. My understanding is that a minumum of 4" after and 8" before the T's are required. Would you agree? ""

    Don't change it on account of me. That's just my professional and experienced opinion. I just try not to ever do it that way. I'd be trying a ECM circulator with those zone valves first. That might solve your problems. It ain't broke that bad. Don't get too radical in your repair. Keep it simple. Simple problems seldom have difficult answers.
  • ToddAKToddAK Member Posts: 4
    Add a differential pressure by-pass on the end of your manifold to maintain a somewhat constant flow rate on your system loop, this will help maintain a little more of a constant. What is your differential "CH" set to on the boiler programing? If the differential is to great the unit will go to a higher fire rate then needed trying to make up the differential and over heat. A lower differential like 7-10 degrees is actually helpful in a lot of cases.
  • ChunkstyleChunkstyle Member Posts: 4
    Hi ToddAK,
    Differential was set at 20 when I checked. I since set it back to 8 and the short cycling for the second floor zone has stopped. Thanks! I called a few Heating and plumbing supply stores in my area and they acted surprised that I would want a differential pressure bypass valve. Both supply houses said they don't sell nor stock them anymore as they aren't used much these days..... I did notice that the Munchkin layouts called for one in the configuration I have but the control fabricator omitted it in the build.
    I'll be shopping for the Groundfos alpha to replace the secondary pump as you recommended. I'll also get those T's oriented as you suggested. I don't mind the effort and want it as correct as possible. Thanks again.
  • icesailoricesailor Member Posts: 7,265
    I'll tell you where you can get one if you want one and want to call someone.

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