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  1. #1
    Join Date
    Jan 2010
    Posts
    6

    Default New help with Pex...

    110 year-old house, 13 solid water radiators, 2" and larger black pipe, head-high in the basement. I'm going to replace my old boiler with a high-efficiency condensing model; tear out all the horizontal black pipe and run 1/2" pex for the radiators, which I've been told is sufficient for the system. But if I'm going to zone 2 or 3 radiators with a single feed, what size will I need to be coming from the manifold? 3/4"?

    Thanks in advance, they'll be plenty more questions a'coming!

    Ziffy

  2. #2
    Join Date
    Jun 2007
    Location
    Coventry, RI
    Posts
    340

    Default Re: New help with Pex...

    Your best bet is to figure out all the dimensions of your house, type and amount of insulation, type of windows and doors (i.e. single pane, double pane, low E, etc.). Bring this info to a plumbing and heating supply house not the big box store. They should be able to figure out what you will need to purchase to get the most efficient system. They will also be able to tell you the size of pex you need to run to supply enough hot water to the radiators. Hope this helps you out.

    Mike

  3. #3

    Default Re: New help with Pex...

    Pipe size depends on the heatloss calc's for the zones and pex pipe is great for lots of things but I would use copper for rads.
    Plumbing and Heating Tech.
    My plumbing website Hidden Content

  4. #4
    Join Date
    Feb 2008
    Posts
    556

    Default Re: New help with Pex...

    Ziffy,

    I would tend to agree with sabo---you'll have to get someone to give you a solid plan of how the convector/PEX layout will be set up---also consider consulting the Yellow Pages under "Heating Contractors" to have a pro come over & give you an estimate of what needs to be done, so you can have some idea of which way to go.

    For example, cast iron rads are sized and designed to operate at 180 degrees water temp & if you want to keep them & use them you would be better off with a non-condensing boiler (less expensive) that operates at those temps.

    A condensing boiler is designed to operate at approx 90 degrees water temp & is designed primarily for radiant floor heat, where many hundreds of feet of PEX are stapled up in loops to the undersides of the flooring (radiant floor heat) to provide approx. 25 btu/sq.ft. throughout the floor plan, where the entire floor becomes a giant radiator.

    If you decide to use just the radiators, 3/4" PEX is often used to zone several rads together in a loop using a monoflow piping arrangement, or even a series,loop one-pipe piping arrangement with the supply/return manifolds.

    Also Google "Garybk radiant retrofit" (without the quotation marks).

    Also click onto the "heating help" site, then onto "ask questions" then scroll down to "radiant heat" for a radiant heat forum.

    http://www.houseneeds.com/shop/atop/...atingindex.asp
    http://highperformancehvac.com/boile...ter-loops.html
    http://www.heatinghelp.com
    Last edited by NashuaTech; 01-28-2010 at 08:33 AM.

  5. #5
    Join Date
    Jan 2010
    Posts
    1

    Default Pex & high Efficiency heater retrofits on iron pipes & radiators

    I have a 1925 triple decker and did two different retrofits:
    2nd floor, I retained the ancient boiler (40+ years) that works very well and most of the cast iron pipes EXCEPT those to the living-room and dining-room that I replaced with 1"PEX (white - oxygen barrier necessary!): boiler circulates by CONVECTION (no pump or circulator) but the pipes MUST maintain a positive slope on the feed in order for the hot water to "push" its way up and generate the circulation. The 1" pipe ensured that I had an adequate distribution from the rest of the network (2x3/4" pipes feeding two other radiators). The PEX feed then splits into one very large cast iron radiator (living room) and one regular baseboard (20ft long 3/4"). Everything works like a charm!

    3rd floor unit I installed a high-efficiency gas-fired hot water boiler (Munchkin). I connected everything as spec'ed by the manufacturer, using copper 1 1/4" splitting into 2 zones 1" each and then sub-splitting into 3 networks each, two of which with PEX 3/4". It all works great (but the old boiler heats much faster and does not cost more to run than the super-duper modern high-efficiency one with its 3 circulators (2 zones + 1 common return)!)

    Depending on the length of pipe you plan to run, I would pay close attention to the friction factor (I believe PEX has slightly more friction than does Cu, but the angles in cu may offset that... ). 3/4" will also be easier to fit onto your cast iron radiators (1/2" valves are not as common). Lastly, if you are running great lengths of feeding pipe, you might consider buying the PEX that has an insulating layer built onto it.

    Good luck - and DON'T just discard your old boiler or radiators: the key to reducing our Carbon footprint begins with reduced consumption!

    NC

  6. #6
    Join Date
    Mar 2010
    Posts
    8

    Default Re: Pex & high Efficiency heater retrofits on iron pipes & radiators

    [QUOTE=HouseTechnic;124227]I have a 1925 triple decker and did two different retrofits:
    2nd floor, I retained the ancient boiler (40+ years) that works very well and most of the cast iron pipes EXCEPT those to the living-room and dining-room that I replaced with 1"PEX (white - oxygen barrier necessary!):

    Sorry to have to say it but WRONG!! You only want oxygen barrier if it is for a heating system. If it is for Potable hot water you want non-oxygen barrier.

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