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Thread: What size pex for heating loop?

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  1. #1
    DIY Junior Member madhavok's Avatar
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    Default What size pex for heating loop?

    Hello all,

    I have a two story, two family house that is on 1 heating loop, 1 zone, for hot water radiators up & down. The current configuration is a 1.25" threaded pipe loop and is operated by a taco 007. Small sections of the loop which have failed were repaired by the previous owner with 1" copper (not sure if this matters). The radiators tee off and back to the main loop with 1/2" pipe.

    The first floor is basically 2x4 sleepers on slab with a subfloor and hardwood. The heating loop runs around the perimeter of the house (160') and lays on the slab inbetween the 2x4 sleepers.

    Ok, now that I got the lengthy (sorry) background information out here is my question. Because of hurricane sandy the floors are ripped out and I'd like to add a second heating loop / zone for the first floor. My plan was to leave the upstairs radiators on the existing loop and plug the T fittings that were hooked up to the first floor radiators. Run a new pex loop for the first floor radiators and replace the old weil mclain gas boiler with two high efficient somethings.

    How do I determine what size the new pex loop should be for the first floor? I was guessing 1" should be more than adequate? Also anyone see any problems with this other than obviously disonnecting the first floor radiators from the existing loop without breaking 60 year old piping.

    I appreciate any advice here.

  2. #2
    In the trades Dana's Avatar
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    On the new floor it's worth putting down 1.5" XPS insulating foam (R7.5) on the floor and securing the new subfloor to the slab with TapCons 24" o.c. rather than repeating the 2x4 sleeper mistake. This makes the floor warmer and less susceptible to mold/rot issues, as well as saving on the heating bills. If there is no vapor barrier in the slab (or if you don't know), it's cheap insurance to put down 6-10mil poly sheeting under the XPS as both a slip-surface and vapor barrier against ground moisture. At current natural gas pricing this is not strictly a financial return on heating costs issue (though it does have a long term financial rationale, see Table 2, p10- you're in climate zone 4) but it's huge on comfort and indoor air quality (by keeping the mold spore counts down.)

    It sounds like the original system is a "mono-flow Tee" configuration, in which case plugging lopping the Tees and going with a direct flow approach may lead to room-by-room temperature imbalances.

    Replacing sections of corroded 1.25" iron with 1" copper has negligible effect on flow, but if you're re-configuring to a lower-flow rate and series-plumbed radiators rather than a mono-flow Teed setup it's unlikely you would need anything bigger than 3/4" PEX. Even if you leave one zone as monoflow it's likely that you'd still be able to achieve sufficient flow with 3/4" PEX if it's just running between the boiler and each end of the pre-existing (now shorter by half) section of mono-flow, but leaving it at 1" wouldn't hurt.

    Going with two boilers instead of one is likely to leave you with TWO boilers oversized for their loads, whereas if you actually did some design work up front (there's a concept for ya! ;-) I wish more pros actually did that in the northeast. :-( ) it would be cheaper and more efficient running it as two roughly-equal zones on one boiler (probably set up with a separate pump on each zone rather than one monster pump and high-flow zone valves.)

    All good heating system designs start with a room-by-room heat load calculation (ACCA Manual-J, or even I=B=R is good enough for a house this size in your climate), using realistic indoor and the 99% outdoor design temperature for your location. (For most of the Sandy-affected areas of NY & L.I. using +15F as a design temp would work.) Without that you have no idea how much radiation or flow you would need in each room and zone, or what size boiler would operate most efficiently.

    If experience any guide it's likely that the existing boiler is probably more than 2x oversized for the actual heat loads, so it's not really a great guide. If you oversize the boiler it will suffer efficiency losses from cycling, and incur higher maintenance costs to boot. Now that you're breaking it up into two zones, oversizing issues loom much larger, since you now have less than half the radiation available on the smaller zone that needs to be able to deliver 100% of the boiler's heat to avoid cycling. But if you use a high-efficiency modulating-condensing boiler right-sized for the whole-house load, most of those have at least a 4:1 turn-down ratio, and will simply modulate it's firing rate to match the load. But a mod-con boiler at 3x oversizing may still have real problems managing single-zone calls efficiently.

    You also have too look at the total radiator sizing on each zone/room relative to it's load, since that determines the maximum temperature required to deliver the heat, and how low the water temp can get without causing excessive on/off cycling on the boiler. Mod-con boilers all come with "outdoor reset" controls, that use the outside temperature to determine the output temperature of the boiler. This is done to increase the efficiency, which is maximized at the lowest water temp that doesn't over-cycle the boiler. You can't get any better than ~87% efficiency out of a condensing boiler if the water coming back from the radiators is over 125F or so, which isn't going to happen with 180F outbound water. But with 110F return water it'll be in the mid-90s, and with sub-100F return water it can hit the high-90s. High-mass old-school radiators are GREAT at delivering predictable heat at lower temps, unlike fin-tube baseboard, so this is probably going to work out pretty well if you design it right.

    The real answer to your pipe sizing question will fall out of the flow requirement math based on how you configure the radiators and zones, and the lengths/pumping head the loops present. In most series-plumbed situations 3/4" is going to be fine after all design-ducks are in a row, but as I stated earlier, you'll probably need/want to leave it as a mono-flow Tee, otherwise the heat balance will be all over the place, since the temperature of the water entering the last radiator on the loop will be 10s of degrees colder than the first one in line. But with only half the radiators on a given loop it won't need the original flow rates to work well, and if it's only a few 10s of feet of PEX to hook up the loop, that may still work at 3/4" without resorting to a ridiculous sized pump. (I imagine the original pump is a pretty beefy sucker, not a Taco-007 or similar.)

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    DIY Junior Member madhavok's Avatar
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    Dana,

    Thank you for some good information. I guess I should have clarified that when I mentioned plugging the "tee's" that would only be for the tee's servicing the first floor radiators. I will retain the existing loop and the tee's that service the second floor radiators. I will then put the first floor radiators on a new loop with tee's, using pex, and If I understand right I could probably get away with 3/4" pex but 1" wouldn't cause any problems?

    Also you are correct the taco 007 isn't the original pump because I found a small B&G aqua colored pump, 1/12 hp I beleive floating around the garage. The previous owner must have had it changed for some reason.

    Lastly the reason for two boilers is so that eventually when this is two rental units they can have separate gas bills for heat & hotwater. (I was thinking two small boilers for heat & indirect hotwater?).

    I decided to make a quick sketch of the existing conditions for clarity. What I'm proposing is keep the existing loop for upstairs, & plug the tee's that feed the downstairs radiators. Install a new loop & tee's for the downstairs radiators which would be the same exact length as existing, in fact in would run right next to it.



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    Last edited by madhavok; 03-12-2013 at 01:36 PM.

  4. #4
    In the trades Dana's Avatar
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    If the individual zone heat loads are low enough and the radiation sizing large enough you may be able to make it work using a condensing hot water heaters rather than boilers + indirects. The heat load calculation is critical for making those decisions.

    Mono-flow tees have little scoops/deflectors in them that are restrictive. Rather than plugging the tees, add a short bypass pipe between them, (replacing the radiator and the runs to/from the radiator) or it will end up restricting the total flow.

    To use the same radiators with the same room-to-room heat balance you may need to plumb the upper floor loop with mono-flow tees too, but I'd strongly recommend getting competent hydronic design help with this, even if it's all sweat-equity on your part for the installation end. There are variations on the theme on monoflow tee systems and I'm the furthest thing from an expert on designing with them, but done well they deliver pretty good comfort. Last thing you want to do is to have to rip it out and start over, or suffer along with a marginally functional system that could have been working GREAT for the same or less money up front.

    Start with the room-by-room heat load calculations along with the room-by-room radiator sizes, and we'll see how it all shakes out. (With wall construction type & R-values, attic R-values, and window type & U-factors, if you have them I can show you how to ball-park the heat load well enough.)

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