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Boiler Short Cycling.

Gooch
Gooch Member Posts: 62
I believe my boiler is short cycling in my new home.

Here are some details.

1550sqft ranch house with walk out basement.
Radiant Tube in the basement slab.
Hot water coil mounted in duct work for main floor.
Indirect water heater.
HTP UFT-80W Boiler
No Outdoor reset.

The floor was wired to the CH terminals on the boiler and ran at a set temp of 100* F
The Hot Water coil and DHW were wired to a 2 Zone Taco Pump Control and ran at a set temp of 170*F

When the infloor would call for heat the boiler would run for a good amount of time 30+min
When the Hot water coil would call for heat the the boiler and fan would run for a short period of time, less than 10 minutes.

[URL=http://s61.photobucket.com/user/coryrood/media/Boiler_zps8sqksrva.jpg.html][IMG]http://i61.photobucket.com/albums/h72/coryrood/Boiler_zps8sqksrva.jpg[/IMG][/URL]

I started doing a bunch of research and thought i found the solution for my problem, I needed to lower the hot water coil water temp. Problem was that the boiler could only have two temps. DHW and CH. Since I don't want to change the DHW temp I needed to get the HWC on the CH circuit. But 100* wouldn't be hot enough for the HWC. I decided to install the outdoor sensor and enable reset. with the hottest water on the outdoor reset curve expected to be around ~140* I decided that I needed to install a TMV on the infloor. This is where I'm at today.

The slab and HWC are now on the Taco controler which is on the CH circuit of the boiler set with Outdoor reset.
The DHW is on the DHW circuit of the boiler and set to 170*F

boiler run times have not really increased with the HWC even with water temps as low as 110*F
boiler run times for the infloor have tanked though now. The boiler heats until the supply reaches a temp slightly above it's set point and then shuts down only to refire shortly there after because the infloor is still calling for heat and returning the 'cold' water and cooling the boiler below the set point. Why isn't the boiler modulating to maintain it's set point?

Thought I had my head wrapped around how this system should work but I seem to have made things worse.

Comments

  • Gordy
    Gordy Member Posts: 9,546
    Mass of connected load has huge effects on boiler cycling.

    A fan coil has little mass where concrete floors have a lot of mass. Once equilibrium is reached in a zone the cycling will begin. If set point (Tstat setting)for the zone has not been satisfied yet.



  • Gooch
    Gooch Member Posts: 62
    edited December 2016

    Gooch said:


    The Hot Water coil and DHW were wired to a 2 Zone Taco Pump Control and ran at a set temp of 170*F




    Describe the wiring that was done between the Taco Pump control and the boiler.

    Observe the boiler SWT during operation of the coil. Confirm whether or not the boiler is achieving the 170F setpoint prior to HL shutdown (unlikely).

    Originally the Isolated end switch came off the Taco Pump control to the DHW stat terminal on the boiler.
    The SWT would never get to the 170* setpoint unless I increased the tstat setting.

    The Indirect Tank pump was on zone 2 and the HWC was on Zone 1, Priority was turned on for Zone 2(DHW)
  • Gooch
    Gooch Member Posts: 62
    Gordy said:

    Mass of connected load has huge effects on boiler cycling.

    A fan coil has little mass where concrete floors have a lot of mass. Once equilibrium is reached in a zone the cycling will begin. If set point (Tstat setting)for the zone has not been satisfied yet.



    So how does one go about eliminating the cycling? add mass? at this point I'm really wondering if any of the system is installed correctly(Contractor)

    I was under the impression that the system would operate like this:

    An Infloor call would result in the boiler modulating to maintain a SWT until the call for heat ended.
  • Gordy
    Gordy Member Posts: 9,546
    Some pics of the boiler room would be helpful.

    Yes it should modulate to maintain ch set point.

  • Gooch
    Gooch Member Posts: 62
    edited December 2016
    Gordy said:

    Some pics of the boiler room would be helpful.

    Yes it should modulate to maintain ch set point.

    i61.photobucket.com/albums/h72/coryrood/Boiler_zps8sqksrva.jpg



    So, clearly, the boiler is not shutting down on limit. It's shuts down because the call from the coil has ended.

    What 'stat is controlling the coil and where is it located?

    a Rodgers White 5-2 thermostat located on the main floor, in a central location. No the boiler is not shutting down on limit, it is shutting down because the heat call has ended.
  • Gooch
    Gooch Member Posts: 62
    That will create a larger hot-cold-hot switch get though correct?
  • Gordy
    Gordy Member Posts: 9,546
    Depends on the Envelope. If its tight, and well insulated a tight differential is best. Large differential leaves large swings in temp, but does help short cycling.
  • Gooch
    Gooch Member Posts: 62
    edited December 2016
    New construction. Done pretty tight. I dropped the temp 3 hours ago and the house hasnt dropped any. It's 25*for outside today with no sun
  • Gooch
    Gooch Member Posts: 62
    edited December 2016
    Does the piping look right? This whole system just doesn't seem to work right.

    What would be causing the boiler to not maintain a SWT while running the inflows for an extended period of time?
  • Gooch
    Gooch Member Posts: 62
    edited December 2016
    My problem wasn't with not maintaining SWT with the HWC it's with the infloor. I do think I figured out what is causing it though. I installed a 3 way TMV on the infloor to protect it from temps over 110*F. when the outdoor reset calls for a water temp above this the TMV closes completely on the Hot side and effectively stops flow through the boiler. What did i do wrong here, how do i limit my infloor temp while still allowing a SWT above 110*F? I need to return the supply back to the boiler? Pressure Differential valve? Into the return? Wouldn't this cause the return temp to rise potentially above my infloor set point?

    I thought I had my head wrapped around this really well but obviously not, I feel like an idiot and wasted a bunch of money and got nowhere.


    Thanks for all your help.
  • Gooch
    Gooch Member Posts: 62
    I guess my thought was that the outdoor reset was a waste of time if it was only gonna control the infloor with a temp range of 90-110* so i wanted to get the HWC in my airhandler on the reset but was under the impression that 110* on the HWC wouldn't be warm enough. Am I way off base?
  • Gooch
    Gooch Member Posts: 62
    I guess i just thought the short cycling when the HWC runs was terrible for the boiler, let alone the boiler wasn't operating in the 'condensing' range? guess I know what I'll be doing tomorrow, putting it all back the way it was. Is there any way to lower the temp for the infloor?
  • Gordy
    Gordy Member Posts: 9,546
    It's not the uft's fault. The installer needed to select the right boiler with the necessary control package for the design. You can control up to three curves, and 3 different swt plus dhw with the right control package.

    Oversizing the fan coil would lower swt some.

    You could experiment with the odr on the hydro air zone to see how low you can go with swt. If possible you could then install the taco I series 3 way with odr on the rfh and use the uft odr on the hydro air.

    This really presents a problem when you install your rfh on the hydro air zone. Then you would need 3 swt one for slab, one for plates under hard floor, and one for plates under carpet. Plus hydro air if still used. It's all doable with third party controls.

    Just thinking out loud.
  • Gooch
    Gooch Member Posts: 62
    Gordy said:

    It's not the uft's fault. The installer needed to select the right boiler with the necessary control package for the design. You can control up to three curves, and 3 different swt plus dhw with the right control package.

    Oversizing the fan coil would lower swt some.

    You could experiment with the odr on the hydro air zone to see how low you can go with swt. If possible you could then install the taco I series 3 way with odr on the rfh and use the uft odr on the hydro air.

    This really presents a problem when you install your rfh on the hydro air zone. Then you would need 3 swt one for slab, one for plates under hard floor, and one for plates under carpet. Plus hydro air if still used. It's all doable with third party controls.

    Just thinking out loud.

    Thats my plan as of right now to see how low a temp i can run on the HRC. When i do the plates i was gonna route the tubing so the carpeted areas would be their own loops and could be slowed compared to the rest of the plates, depending on the design maybe even do 3/8 for the hard surface and 1/2" for the carpet.

    Appreciate the help.
  • Gooch
    Gooch Member Posts: 62
    What kind of 3rd party controller would i want to look for any recommendations?