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Boiler return temperature protection

HeloDog Member Posts: 34
I’m getting estimates to replace my existing cast iron boiler and would like some insight into how best to provide return water temperature protection given my existing system. Also any improvements to my existing piping layout would be appreciated.

I have radiant heating in the floor (single story) with six zones. I’ve included a diagram below. I greyed out the three zone valves in the upper left because the pipe disappears behind a wall so I’m just guessing they must exist because I have six thermostats in the house and only three zone valves in the mechanical room.

None of the proposals or HVAC people have suggested any protection is needed. They plan to use the existing piping and make as little change as possible. So if I want anything different, I’m pretty sure I’m going to have to ask for it.

Slant/Fin and Weil-McLain cast iron boilers have been proposed so I've read over their installation manuals. For radiant floors, both the Weil-McLain and Slant/Fin manuals recommend using primary/secondary piping or a method they call Boiler-Bypass to provide return water temperature protection. Slant Fin also says a three-way valve arrangement will work. Meanwhile, Weil-McLain says do NOT use a method they call a System-bypass method for radiant floors because it could result in excessive water temperature in the radiant tubing.

Unfortunately, my existing piping looks to me like that System-bypass method Weil-McLain says not to use for radiant floors. That doesn't sound good. Although it seems to me that in my setup, the boiler aquastat and temperature gauge are providing over temperature protection to the radiant tubing so I don't get why that would be a concern.

So is it as simple as adding a temperature gauge on the return line next the boiler, replacing the mixing ball valve with an actual mixing valve and checking the temperature occasionally?

Or maybe adding three-way thermostatic mixing valve(s)?

Or can the piping in the mechanical room be easily rearranged into a primary / secondary loop - despite not having all the zone valves in the mechanical room?

I'm confused.


  • SuperTech
    SuperTech Member Posts: 1,792
    I would recommend an variable speed injection pumping primary secondary setup for boiler protection, or look into a Taco I series valve with outdoor reset capabilities. Gotta keep return temperature high enough before the burner shuts down, also want to avoid low stack temperature as much as possible
  • HomerJSmith
    HomerJSmith Member Posts: 1,562
    edited December 2019
    HeloDog, you're not reading the manual correctly.

    "Figure 24 is an alternative piping method that provides return water temperature control for boilers installed on converted gravity systems(large water content or steam systems).Do not apply the piping of Figure 24 on radiant heat-ing systems. It provides no method regulating the water temperature provided to the system and could result in excessive water temperature in the radiant tubing." Converted gravity systems. It's not you.

    I use an Taco i-Series set point, two port mixing valve with mixing temperature controlled by a sensor. I locate it between the output of the pump and close to the cold return to the boiler with the sensor at the boiler return.


    Using a ball valve for controlling the flow on a bypass is not a good idea. As your drawing shows. A better choice would be a globe valve properly oriented. (it's static) The best choice is related above with the Taco. (it's dynamic)

  • HeloDog
    HeloDog Member Posts: 34
    According to the design notes in the Taco iSeries mixing valve brochure, you need primary/secondary piping in order for them to work properly. I don't have that. So how difficult would it be to convert my existing piping to a primary/secondary piping layout? Is it even possible?

    HomerJSmith: I absolutely agree with you, they are saying the System-Bypass piping solution isn’t meant for my situation - a radiant heating system. My point was that it seems to be what I’m currently stuck with, right? Part of the flow from my circulator bypasses the system loop and is diverted to the boiler loop. That seems to be the defining element of a System-Bypass piping arrangement.

    You’re also right that the ball valve for a mixing valve isn’t great. I assume the house builder took a shortcut to save time/money. I’ll take a look at the valves you suggested.
  • old_washer
    old_washer Member Posts: 4
    I found this information about the difference between "Boiler and System Bypasses" useful. Maybe you'll find it useful, too.
  • hot_rod
    hot_rod Member Posts: 16,278
    If you go with a conventional non condensing boiler I would pipe in a 100% assured boiler protection. Bypass piping or bypass pumps are not a guaranteed method. Proper protection requires a "device" that can respond to temperature at the boiler return. Numerous ways to accomplish that function.

    Copper tube boilers enjoy primary secondary or hydraulic separators if you go that route again.

    Additionally you want temperature mixing for the low temperature zones.

    Here is one simple, effective way to do that. A reset control could also be used on the boiler or on the mixed zones with a motorized valve instead of thermostatic.

    The separator cleans up piping and provides 4 critical features, air, dirt, magnetic and hydraulic separation at 97% efficiencies.

    Of course a mod con boiler would eliminate the need for boiler protection. Most include reset control also.
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • HeloDog
    HeloDog Member Posts: 34
    Thank you. Really enjoyed that article. Makes complete sense. It points out that the advice given will conflict with most manufacturer manuals - and it does with the Weil-McLain manual. But it makes sense.

    Based on that article, my existing piping arrangement is fine for a system that needs to raise the temperature of the water returning to the boiler. It didn't address the concern the Weil-McLain manual had with this set-up (system water too hot for the radiant tubing) but it seems to me that the aqustat provides that protection. I don't understand their concern.

    So the new installer could leave the piping arrangement alone and just put in a decent mixing valve plus a temperature gauge near the return. Done. I could check it periodically and manually make sure the return is hot enough.

    An automated way to monitor the temperature and control the mixing would be nice but probably not necessary - unless an outdoor reset control is also added. Then it probably would be necessary, right?