Welcome! Here are the website rules, as well as some tips for using this forum.
Need to contact us? Visit https://heatinghelp.com/contact-us/.
Click here to Find a Contractor in your area.

System bypass affecting flow to radiators?

TomTA
TomTA Member Posts: 20
edited January 2021 in THE MAIN WALL
I have a question about a system bypass, and how it could affect flow to the emitters. In my system, the output side of a Taco 007 (1.25" output) goes into a T; from there a 1" line goes straight back to the return (maybe 6' of pipe with 2 elbows and a ball valve), while a 1.25" line goes to feed 8 big cast iron Victorian radiators in a three branch direct return system (with the 1.25" line quickly being throttled down to 1" or 3/4" as branches go off to the side).

Intuitively, it seems that the water coming out of the circulator would take the easiest way back, and try to push water through the 1" bypass line, with circulation going to the radiators only after the resistance in the bypass line got too high due to high flow.

The reason I ask is that (a) it just seems counterintuitive, and (b) when starting from a cold start, the exposed piping to the 2nd floor radiators heats up really slowly; you can feel the heat slowly rising through the pipe. Sort of reminds me of my first house with a gravity system.

Comments

  • Jamie Hall
    Jamie Hall Member Posts: 23,161
    Well, your intuition seems basically correct. The gimmick is the ball valve on the bypass. There are competing objectives here. One, you need to ensure that the return temperature to the boiler is tolerable; if it is a non-condensing boiler (you don't say what kind it is), the return water needs to heat up -- usually above 140 -- to ensure that you don't get condensation in the flue. Another, you need to be sure that there is enough flow through the boiler to absorb the heat the burner is producing. Then you need to ensure that there is circulation through the emitters so you actually get warm.

    Are these three compatible with a fixed valve position and a single pump? No, they aren't, which is why primary/secondary pumping and independent boiler and emitter circuits, with thermally regulated mixing valves makes such good sense.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • EBEBRATT-Ed
    EBEBRATT-Ed Member Posts: 15,452
    @TomTA

    get rid of the ball valve in the bypass pipe and the tee. Put in a 3 way diverting valve controlled by boiler return temperature in place of the tee and set the temperature to 130 degrees. The sensor goes between the diverting valve and the boiler.

    On a cold start the temp to the rads will come up slow until the mixed boiler return comes up to 130
  • hot_rod
    hot_rod Member Posts: 22,019
    Thermodynamics come in to play in gravity systems. I've always read the upper stories in gravity systems heat quicker and need flow restriction due to the hot water rising to the high points driven by the buoyancy temperature delta introduced, It seemed odd to me to put flow restriction on the most distant radiators to get the closest to boiler radiators to heat properly. Dan Holohan talks about that in his seminars in regards to gravity systems in high rise buildings.

    The bypass I think you are talking about is intended to be more of a mixing device than a flow balancing, or limiting valve device, per say.
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • TomTA
    TomTA Member Posts: 20
    edited January 2021
    I appreciate the answers. If I could lay this out in a little more detail, and get some more feedback, I'd appreciate it.

    The 1.25" boiler supply comes out of the boiler, and immediately comes to a T with a 1" outlet which supplies the indirect (with its own circulator a little downstream). The remaining boiler output continues to the circulator for the radiators. It exits the circulator and comes to the "T" I mentioned; 1" back to the return as system bypass, 1.25" to the radiators.

    The 1.25" going to the radiators comes to another "T"; 1.25" output goes to the south side of the house to feed 2 radiators (one up, one down). The other 1.25" output heads north. Northbound pipe interrupted by "T" which sends water to the east side of the house.

    Northbound part continues to serve 2 radiators, one up, one down.

    The problem appears on the east side of the house. It's got the longest physical run, and serves 4 radiators (2 up, 2 down). The downstairs radiators heat up well. It's the two upstairs one which almost seem like a gravity system with slow heating. I'm wondering if the circulator is running out of "oomph" to push the water through those last radiators, and the supply water on the east circuit basically just goes through downstairs radiators and then home.

    It seems as though the uninterrupted 1" system bypass piping could easily divert so much of the Taco 007 output that flow to the furthest radiators is compromised; basically taking a constant 6-8 GPM of flow away before it even gets to the distribution system. The radiators closer to the system heat up well and quickly; it's the two that are furthest away (and where 4 radiators share the same supply/return piping) that are questionable.

    Thanks. This is really a fascinating topic (to me, at least), but I just can't quite wrap my head around it.

  • hot_rod
    hot_rod Member Posts: 22,019
    A drawing or a pic would help see what you have. At some point a system with a lot of various loops or connections could really benefit from some balance valves. It often becomes the only way to ensure the required or desired flow is going where it needs to under any and all conditions.

    A simple piped boiler bypass should always have a globe or balance valve IMO. If not, it is like you are experience, just a path of least resistance and a "wild" maybe inadequate balance attempt.

    Here is my take on hydronic system balance.

    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • mattmia2
    mattmia2 Member Posts: 9,572
    Though the bypass could be better, it is unlikely your problem. Make sure you don't have air in the radiators that aren't heating adequately. Close off the valves partly on the radiators that are overheating to force more flow to the radiators that aren't heating enough and to force longer cycles. Is the thermostat by any chance near one of the radiators that heats first?