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1/2 inch pex for baseboards?

I had a Munchkin boiler installed last year with 3 zone valves. Only one zone was initially hooked up to slant fin baseboards on my main floor. This was plumbed with 3/4-inch wirsbo pex. The other 2 zones were for a future finished basement and a future 2nd floor addition.

The 2nd floor has been framed and I'm ready to add the zone. My question is: for ease of installation, to save on costs, etc. I'd like to use 1/2" barrier pex (instead of 3/4"). Is this okay?

I've actually already purchased 300' of 1/2" pex based on an okay I got from the sales guy who sold me the original system (Munchkin and baseboards). Now I've heard from another guy at this company who recommended sticking with 3/4" piping.

Other info:

- The boiler is located in the basement

- Size of the 2nd floor addition is approx 800 sf

- The addition (and rest of the house) has new energy efficient windows and will be insulated to code.

- The boiler is set at 180 degrees

- The house is in Seattle

Any thoughts?


  • zacmobile
    zacmobile Member Posts: 211

    1/2" pex can carry about 22,000 btuh @ 20deg delta T, so unless you are heating a hay barn 1/2" pex would be fine.

    why are you running your boiler @180?? you would benefit from munchkins Vision control with outdoor reset, its an add-on your wholesaler should be able to hook you up with and you will save 10-30% on your gas bill.
  • Brad White
    Brad White Member Posts: 2,393
    edited September 2010
    Can you

    either split the zone or use home-runs? Divide and Conquer, get less pressure drop overall and finer control.

    My initial concern is that you are using zone valves and if a single run, this circuit will define your system pressure drop and unless you throttle back the other zones, this one will get less flow. As forgiving as flow can be vs. capacity, there are limits.

    To play with some numbers, say your heat loss is 16 MBH, 20 Btuh per SF as a place to start. 

    You could get 1.6 gpm through 1/2" PEX, but that is about twice what I would design for and would have about 3.4 times the pressure drop.  OK, I am conservative, but you own the PEX already and I do have my reasons.

    Split that into equal circuits, cutting the flow in half, your pressure drop would be 30% of the single circuit. Could be a foot difference, but if that defines your circulator, why not reduce it? And if you are using an ECM circulator on your distribution, what a waste to tax it when it can cruise more slowly?

    Lastly, I am totally in agreement with zacmobile regarding water temperature.

    Firstly, are you SURE you need 180F even on the coldest day?

    Secondly, outdoor reset is a Mitzvah. You just do it, a good thing.

    But once you have it, tweak your curve and your highest design temperature. Take that 180 down to 160 to start. If that works on the coldest day, take it down another five degrees on your coldest day until someone puts another layer on. OK, half-kidding, but you get my point, take it down so that you still have comfort and let THAT be your new high limit. Let the hot water supply temperature be regulated cooler below that as the outside temperature warms up. Less baseboard creaking and actually, better comfort with fewer detectable "cycles" or bursts of heat. Let her cruise.
    "If you do not know the answer, say, "I do not know the answer", and you will be correct!"

    -Ernie White, my Dad
  • Jr_12
    Jr_12 Member Posts: 38
    outdoor reset

    Your boiler came with vision 1 and should have an outdoor sensor laying around somewhere. Use it! I agree the temp seems pretty high.
  • seattle_steve
    seattle_steve Member Posts: 5
    Thanks for the responses.

    The Vision 1 outdoor reset was not included in my installation (the Munchkin manual lists it as an "option"). However, the service tech who came out recently told me about it as well and I plan on getting one. However, my primary goal at the moment is to get the rest of the zones set up and functioning.

    @Brad: The existing main floor zone is one big loop. I was planning to do the same thing for the other 2 loops. If I were to split it into 2 circuits as you suggest, would that just be adding a couple tees or would it be adding a 4th zone valve?

    Also, you mentioned a circulator (pump?). I'm pretty sure there isn't a circulator in my system -- just the boiler plus the manifold and zone valves. Does that sound right?

    And, I can see the temp and pressure gauges, but how can I tell what my flow rate is (gpm)?

    Do I need to increase the water pressure in the system when I add the second floor zone (it's roughly 9 feet higher than the existing zone/loop)?


  • Brad White
    Brad White Member Posts: 2,393
    edited October 2010

    There HAS to be a circulator in your system, likely two. One for the boiler, one for the radiation.

    Because your new space is one big room, a single zone is just fine. I did not know the layout, so the multiple zones was just a suggestion. So yes, a single control valve and split the flow. I would avoid bull-head tees to avoid turbulence.

    Ideally, the flow goes straight, a tee takes one branch off the side, the remainder goes straight and turns with an elbow. On the return side, the branch that left out the tee becomes the "elbow" entry on the other side and vice versa, so that each have about the same pressure drops, same fittings in series. I hope that makes sense.

    For static (fill) pressure, for a two story house, 12 psig ought to do it, but 18 is better. That will give you a good margin to your 30 psig relief valve setting, presuming your expansion tank is the correct size. (It probably is if you have all small tubing.)

    Without flow meters or better instruments, it is difficult to determine flow rate. The easiest way to get a range, an idea, is to monitor the temperature drop on the coldest day if you know your heat loss. Still this is highly inaccurate.

    The best way for the cost is to install a wye-pattern balancing valve in the return. I am a fan of Macon "STV" valves which take a full ten turns open to closed. Very precise control. These valves have pressure and temperature ports on them to take, well, pressure and temperature readings. There are other manufacturers of course.

    There are charts and if you know your valve position (how many turns open to closed) and the pressure drop across the valve at that point, you can determine your flow within 5% or 2% if you have a really good digital gauge.  And with these valves you can fine-tune your flow to both temperature and pressure.

    I use a Dwyer digital pressure gauge for field readings where not critical, ran about $100.

    Keep in mind though that you are not dealing with huge flow rates. The valves will be 1/2" size, likely and are best, in your case, put on all of your circuits so you know your total. Short money in the long run especially if you are a curious type.

    "If you do not know the answer, say, "I do not know the answer", and you will be correct!"

    -Ernie White, my Dad
  • seattle_steve
    seattle_steve Member Posts: 5
    Ok, now I see the circulator

    Yeah, there's a small black box with an armored cable running into it attached to the inflow pipe beneath the Munchkin. It's labeled "Grundfos". Never really noticed it before -- I guess I just thought it was part of the boiler.

    Also, thanks for the tee design suggestions. The second floor will be one zone, but it's not open space -- it's 3-bedrooms and a full bath.

    The pressure gauge on my system reads 18.

  • Brad White
    Brad White Member Posts: 2,393

    goodness you found that! I was getting worried.  I confused "single loop" for "single room" apparently. No reason you cannot get zoning within that, or at least split the zones to best advantage, by exposure or use, bedroom vs. bath or hall. You do have options. But at least you can split them.

    If you have any existing MonoFlow or diverter tees, be certain which is the correct flow direction.
    "If you do not know the answer, say, "I do not know the answer", and you will be correct!"

    -Ernie White, my Dad
  • tim smith
    tim smith Member Posts: 2,398
    Re: 1/2" pex and zoning

    Steve, why dont you run 3/4 pex from boiler up to centrally located closet wall on 2nd floor. Then install simple copper manifold with balance valves on it in closet and run out the individual branch runs to radiators from there. Nice neat way to do it and balance will be a snap. Not that much extra $$ either.
  • seattle_steve
    seattle_steve Member Posts: 5
    Is there a good book for this stuff?

    I'd love to make all these efficiency improvements you guys are suggesting, but I think it's a bit beyond my knowledge. Reducing to 1/2" pex and splitting the loop is about all I feel comfortable doing at the moment. Unless there's a good book or online resource for learning how to do some of this stuff?

  • Jean-David Beyer
    Jean-David Beyer Member Posts: 2,666
    Good book...

    Depending on just what you want to do, for hydronic heating, a really good book is John Siegenthaler's one, obtainable right here:

    It will not answer every possible  question, but it goes in great detail into designing such systems, and the understanding you get from that will be a big help.
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