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Thread: Adding Zones to Existing Single Run Boiler System

  1. #1
    DIY Junior Member imnomotor's Avatar
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    Default Adding Zones to Existing Single Run Boiler System

    I plan to add a Pex 1/2" floor heating into an under-development basement full bath. Floor has been poured, Pex is in place. I want to zone the floor heat system. Additionally, while I'm at it, I would like to zone the rest of the basement separately. I have baseboard water heat throughout the house, but the basement never warms adequately (thermostat on main floor). The basement system is in a loop of its own, so that could be isolated. Currently I have a 50-year-old boiler and a single pump - but all work well now. First, can I create the two zones on this system? Second, can I separately thermostat these zones without messing with the boiler or pump? Third, how do I do this? Can three separate thermostats control the one boiler and pump? I know this is a mouthful. If you can answer specific to this, can you point me in the right DIY direction?

  2. #2
    Retired Defense Industry Engineer jadnashua's Avatar
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    There are numerous ways this can be done. Taco makes an add-on relay box that can control multiple circulators and it OR's the call for heat to the boiler. You may need to add on more stuff, though, as the typical water temp for the baseboard is likely way too hot to run in in-floor radiant. So, some sort of mixing or tempering valve would need to be installed as well. With an older boiler, the run time on the radiant zones my short-cycle unless you add some buffer. Would need more info on what the actual loads are in the various zones to come up with the most ecconomical solution. Hopefully, you had someone run the numbers so you have the pex runs at the right spacing and density for the size of the heating area.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

  3. #3
    In the trades Dana's Avatar
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    With and old-school boiler you have to make sure the return water coming back from your radiation is over 130F (if gas), or over 140F (if oil) or the 50 year old boiler won't make it past age 52 or so. Your average water temp TO the slab radiant will likely never need to be over 100F, with 85-90F return. Slabs are inherently high mass and self-buffering (it can be set up to not short cycle), but you need a smart T-stat to keep it from over/undershooting the set point, even when you have the water temps dialed to something reasonable.

    The baseboard might still be able to deliver the design condition (coldest hours of the year) at 140F out too, but probably not at 120F (and running baseboard cooler than that isn't very reliable even when it DOES work.

    You have a real dual-temp hydronic design problem to deal with, and not something super amenable to design-by-web-forum. We might be able to come up with an architecture that works, but you'll have to design it, do the math, revise as needed, then test & tweak it in once it's up and kinda-working.

    If you're up to home-schooling yourself on the topic, you might start with John Seigenthaler's materials.

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    In the Trades Tom Sawyer's Avatar
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    You need to add either two more circulating pumps or three zone valves plus the manifold piping, purge and balance valves, a tempering valve for the slab heat zone, two more thermostats and either a Taco or Tekmar zone controller or a couple of relays. The 50 year old boiler needs to take a short trip to the scrap yard while you are at it.

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    In the trades Dana's Avatar
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    I'll second the motion to scrap the 50 year old boiler.

    On it's best day it was probably only getting 75% efficiency, and that day was in January 1962. With regular maintenance and a retrofit heat-purging controller like a Beckett Heat Manager or Intellicon it might still muster 70% for an as-used AFUE, but a right-sized low mass modulating-condensing boiler will average over 90% once it's tweaked in. Figure on at least a 25% fuel savings, even 40% savings isn't rare.

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