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re-piping an old rad system...

hwshws Member Posts: 43
I'm looking at a re-pipe job involving an old Victorian-era home heated with cast-iron rads. The system is currently all one zone. I'm re-zoning so that the system has either:

-one zone per room


-one zone per floor (3-5 rads per zone)

I'd like the zoning to be a bit more user-friendly.

Has anyone re-piped a system this way before? I'm looking at a one-zone-per-rad scenario using 1/2" wirsbo HePex to/from each rad, controlled by either a zone valve or a dedicated small circulator.

I wonder, though, will such small zones cause short-cycling of the boiler? It is a non-condensing, cast iron oil fired boiler with domestic immersion coil.

If short cycling could be an issue, I will instead pipe it with one zone per floor (that's 3-5 rads per zone).

In this case I'll pipe reverse/return, or maybe get my hands on some monoflo tees. Not really common around here

My main question is, will the one-zone-per rad setup work, or will there be problems?? Haven't done it before with old cast-iron radiators. Panels yes, cast rads no.


  • bob eckbob eck Member Posts: 564
    Repiping old radiator system

    You could do three zones one for each floor. You could use a radiant manifold one from legend hydronics. Using the radiant manifold you can control the flow to each radiator and balance each radiator on each zone so one is not to hot or to cold. I would use Viega fostapex tubing that is Pex with a aluminum for the oxygen barrier plus this tubing does not droop or will not grow in length when hot boiler water is run through it. Great tubing.
  • hwshws Member Posts: 43
    Bob that's a pretty good idea...

    ..the way I see it, with a premade manifold for each zone (I prefer Uponor myself)and a zone circ for each manifold.

    Have you used this set-up before? Do you find that 1/2" tubing provides adequate flow to each rad.
  • ChrisChris Member Posts: 3,056
    Heat Loss

    First thing is a room by room heat loss. Second is to measure radiator sqft of EDR and calculate the rads capable btu/hr output with 180 degree water and compare that to each rooms heat loss. Do you need 180 degree water? Can I use 3/8" pex?

    I'd be installing thermostatic mixing valves on the rads to allow each room to be tweaked to each rooms heat loss. I'd also look at doing a 4-way mixing valve with outdoor reset such as a Taco I-Valve. I'd be using a radiant style manifold to feed my rads, a Taco Bumblee Bee as a system pump, let if fly on constant circulation and call it a day. The 4-Way will provide you boiler protection so deep reset can be done on the system side.
    "The bitter taste of a poor installation remains much longer than the sweet taste of the lowest price."
  • EricEric Member Posts: 261
    I have also done this...

    using frosta pex for each radiator back to zone valves off a manifold at the boiler.  The 2nd floor had a zone for each bedroom and the 1st floor was one zone.

    I used 3/4" frostapex for the tubing, and used a drop ear 90°, screwed to blocking to transition back to a 3/4" black steel nipple thru the floor to the radiator. The drop ear protects the pex from moving when tightening/ removing the radiator valve or 90, and you can't tell from above it's fed with plastic.
  • SteamheadSteamhead Member Posts: 11,384
    Why not

    simply put TRVs on all or most of the rads and go to constant circulation and outdoor reset? Seems to me this would do what you want it to...........
    All Steamed Up, Inc.
    "Reducing our country's energy consumption, one system at a time"
    Steam, Vapor & Hot-Water Heating Specialists
    Oil & Gas Burner Service
    Baltimore, MD (USA) and consulting anywhere.
  • ChrisChris Member Posts: 3,056
    Gave Him

    The road map. Question is, will he play the pirate looking

    for the treasure.
    "The bitter taste of a poor installation remains much longer than the sweet taste of the lowest price."
  • zacmobilezacmobile Member Posts: 211
    I concur

    This is pretty much the only way to fly with cast iron rads.
  • hwshws Member Posts: 43

    ...everyone for your replies.

    I've looked through a few threads on this topic (there are a few here) and it seems like a few have used radiant manifolds to successfully zone radiators. I'm pretty sure I'm going with this idea, using one manifold per zone. More than likely going to zone with circulators, so  one circulator per manifold. As for the sizing of the home-run circuits off each rad, I'll be doing a heat loss calc soon...waiting on the owner to decide what type of insulation & windows he will be putting in.

    Steamhead, to answer your question TRVs are an excellent idea, but not for this particular system. The original installer tried to do a monoflo system using regular tees,  and thus very out of balance. Subsequent homeowners have put up with it because "those rooms are cold, that's the way the heat is". This house will be strip-to-the-studs renovated, so I am re-piping it entirely.

    Chris- I like your suggestion but not familiar with systems using a 4-way mixing valve. How would one arrange the near-boiler piping on this type of system.

    Eric - Love the drop-ear idea. Will be using it. Neatness definetly counts.
  • Paul48Paul48 Member Posts: 4,258

    What type of thermostatic mixing valves are those? The ones at the radiators?
  • Paul48Paul48 Member Posts: 4,258

    If gas is coming in the foresee-able future, you might want to take that into consideration.Keep in mind, the original system was probably a gravity system and worked very well. It was only after it was bastardized that things took a turn for the worse. A mod/con would prefer seeing larger loads, and the load of the whole structure if possible. Balancing the heat loss on a room by room basis, as Chris stated, does as much to make the home comfortable, as a complicated system does.
  • ChrisChris Member Posts: 3,056
    Read This

    It will help you out for the 4-way.
    "The bitter taste of a poor installation remains much longer than the sweet taste of the lowest price."
  • SteamheadSteamhead Member Posts: 11,384
    I think Paul got it

    this was probably a one-pipe gravity system. This system will work well if you don't over-pump it. But it won't lend itself to TRVs since the water circulates from the main to the rads pretty much by gravity- unless you can find TRVs with very low resistance to flow. 
    All Steamed Up, Inc.
    "Reducing our country's energy consumption, one system at a time"
    Steam, Vapor & Hot-Water Heating Specialists
    Oil & Gas Burner Service
    Baltimore, MD (USA) and consulting anywhere.
  • ivanatorivanator Member Posts: 39
    1/2 inch pex to cast iron radiators

    I have a Victorian with large rooms - 16 x 16, 18 x 18, etc. I have the first floor piped with 1/2 inch pex to large cast iron radiators. It's more than adequate. I did a heat loss for each room.

    I don't understand why you'd want to zone for each room. I plan on having two zones in a 4,500 sq ft house.

    My goal is to balance the flow at the manifold to the targeted heat loss, once I have the entire system running.

    For example, the kitchen has a heat loss of 18,000 btu's, so flow needs to be about 1.8 GPM with a delta-t of 20. Flow is about 1.2 GPM with a delta-t of 30.

    I'm a homeowner, so the above are my calcs - could be wrong.
  • hwshws Member Posts: 43
    Thank you Chris

    much appreciated
  • hwshws Member Posts: 43
    The radiant manifold has me thinking.

    It seems to me, in theory at least, that with a home-run loop from a radiant manifold to each radiator, it could be possible to zone room-by-room with an actuator snapped onto the head of each balancing valve. Once the flow is properly balanced to each rad, install the actuator and set up the controls.Or, at least pre-wire it for future room-by-room zoning as an upsell.

    I'm aware of course that a TRV would do the same job a lot easier fom an installation perspective. Just thinking "out loud".


    Ivanator, your idea sounds exactly like what I am trying to accomplish. You asked why zone room-by-room. It has a lot to do with culture and construction around where I live

    We are experiencing the 10th year of a housing boom/suburban sprawl in which every new home is a turn-key job with electric baseboard heat. Hot water heating is perceived, sadly, as either something from a bygone era (older Victorian homes like these, with poorly-operating heating systems) or very expensive/only suitable for large custom homes (in-floor jobs).

    In cases like this job, where a major renovation is taking place, I'm bidding against an electrical contractor who wants to put in electric heat. To win the job I have to offer the same features of electric heat, the most common of which is room-by-room zoning. I'm quoting for people in their 40's who grew up with oil-fired hot water heat, single zone, badly-running system, and is swayed to electric for that reason. I have to give them all the features of electric. A common sentiment is expressed "I like those big old cast iron rads, but oil heat (sic) is always so uneven, the thermostat is downstairs and upstairs gets too hot etc etc"
  • hwshws Member Posts: 43

    natural gas maybe in about 20 years. Electric baseboard heat is king here. A large portion of my work is converting systems from oil-fired to electric boilers.
  • Paul48Paul48 Member Posts: 4,258

    With all due respect......If they're not worried about what it will cost to heat the home with electric, it will be impossible to win that bid.
  • ivanatorivanator Member Posts: 39
    replacing cast iron radiators with electric baseboard in a

    Victorian house is like replacing double-hung windows with casement windows - doesn't look right.

    If it's a true Victorian, with Victorian elements, such as millwork, flooring, large windows, the cast iron radiators are an important feature - a future buyer of that house may be turned off by electric baseboard. I know I would. Around here, the only older homes with electric baseboard are shoddy slumlord properties.
  • Paul48Paul48 Member Posts: 4,258
    edited February 2013
    Old Girl

    The Old Girl is being violated to no end.She's being gutted.
  • hwshws Member Posts: 43
    Paul, fear not

    She will get the respect she deserves.

    Gutted, yes, but restored to her former glory. The GC on this job is some sort of "heritage home" specialist. First time working for him but apparently this is what he does. I looked at 2 other jobs with him today, similar heating situation. Won the bid on this one BTW, and looks like there might be more in the future.

    Thanks everyone for your help. I love the Wall.
  • Paul48Paul48 Member Posts: 4,258
    Glad to Hear

    It's good that she'll be given the respect due her. Some folks don't really appreciate just how beautiful the old Victorians are.
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