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Cast iron to PEX repipe for more even heating

tazman1937 Member Posts: 8
edited January 2017 in Domestic Hot Water
Hello All- I've been reading through your forums for a few weeks now and finally I think I know enough about what I don't know to ask some questions.

I'm trying to resolve a problem that my ancient rowhouse has in extremely slow and uneven heating from my hot water baseboards. The house heats oppositely to what one would like: the 1st floor is warm and cozy, and the 2nd floor is usually very cold. A big part of the problem is improper insulation in the 2nd floor bedrooms, which I'm working on taking care by renovation. However, it seems that the baseboards in some of the rooms only ever get warm, and they are relatively uneven in how much heat they throw.

My system uses a NG boiler, which is probably about 8-14 years old, which is connected by 2 1/4 copper into some 3" gravity cast iron pipes which hang down ~1-2 feet from the basement ceiling - a huge problem for us, and part of the motivation for changing it. There is a circulator pump on the return side of the boiler, which only runs while the boiler is fired, not before or after. My observation is that for about the first 20 minutes the boiler is running, none of the baseboards are very warm except the shortest runs from the boiler, and most of the heat goes to heating up those giant cast iron pipes. My intuition is that you'd want the circulator to run well after the boiler has stopped firing to keep moving heat through the baseboards, but alas, the aquastat only has a 2-wire on-off to the thermostat (which is a Nest, and has no idea what to do with this).

My plan is to repipe the basement with aluminum barrier PEX, and sharkbite onto the 1" copper runs which go from the cast irons to the baseboard through the walls. This in theory will solve my slow heat up time, and save my headspace in the basement, and leave me some flexibility for renovation. The second part of my plan is to either install zone valves and a new aquastat w/ zone controller, or instead for cost savings, simply attach thermostatic valves on the baseboards. The wife wants to just install mini-split heat/AC units in each room, which will run us about $5000. I'm hoping to do this work myself, but I'm a DIY-er (although with some skill), and I figured I need to ask the pros before I do anything.

My questions are:
1.) what calculations do I need to determine the sizing of the PEX? Is that a terrible idea?
2.) Should I zone my system? How would I go about getting thermostats to each zone? Will that make better use of my circulator pump? Should I just go with thermostatic valves?

Any and all help is appreciated. I can provide photos and a schematic of my system in a few hours.


    JUGHNE Member Posts: 10,562
    Where are you located?
    There have been several postings here about mini split heat pumps not providing enough heat for the house at lower temperatures. They provide very economical heating above 25 to 30 degrees. Any air source heat pump will need supplemental heat when the temp drops into the 10-20's.

    As for your existing circulator control, you can add a simple strap on adjustable aquastat that will keep the pump running after the burner shut down. This milks all the residual heat out of the piping.
  • tazman1937
    tazman1937 Member Posts: 8
    edit: some of the measurements on the pipes were updated.

    We are in Pittsburgh, PA. It usually stays above 25F here, with only occasional dips into the teens abot 2-3 times over the course of a winter. The hot water system doesn't do a great job even in the 30's; some of the bedrooms are cold while some rooms on the first floor are fine.
  • Tinman
    Tinman Member Posts: 2,804
    The Mitsubishi Hyper-Heat systems have some incredible heat outputs in lower temp areas. Model# MXZ is one of them.
    Steve Minnich
    JUGHNE Member Posts: 10,562
    I suggest asking site admin to move your posting to the Main Wall category.....this is somewhat the hot water heater tank site.

    Pictures and plan would be good.
  • tazman1937
    tazman1937 Member Posts: 8
    attached are some photos to give a sense of the mess of giant pipes in my basement. Whomever lugged in and threaded those 3" cast irons was a man's man.

    http://s84.photobucket.com/user/tazman1937/slideshow/Heating system

    That's a nice idea about the aftermarket adjustable aquastat. I'll look into it.
  • Jamie Hall
    Jamie Hall Member Posts: 21,093
    The thing to remember about gravity hot water systems is that the greater the vertical distance, the more flow you have. Therefore, before someone put in the pump, the second floor would have gotten more flow than the first -- and would have been warmer. Now that there is a pump in there, the water -- being lazy -- takes the loop with the least head loss (the first floor) and you freeze upstairs.

    There is no harm to replacing the big cast iron with PEX. However, any way you repipe the system you need to be able to control the heat individually on the two floors -- so I would suggest two thermostats and two circulating pumps, and the boiler pump as primary. Pipe it primary/secondary. Use outdoor reset to control a mixing valve to control the temperature in the secondary side. What you want to do is to keep the secondary side running as much as possible -- all the time if you can! -- and a properly set up outdoor reset will do that.

    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • kcopp
    kcopp Member Posts: 4,188
    Someone got in there and made a bit of a mess of things. Gravity HW systems can really act completely wacky if not piped right. You need to find someone who can either follow some direction well or who has dome this set up before.
    The boiler prob it at near or fully condensing as it runs now. That will kill its lifespan if not properly piped.
  • tazman1937
    tazman1937 Member Posts: 8
    agreed- the copper-to-cast connections are a joke.
    The boiler ran pretty constantly yesterday because it was ~30F outside.

    After your suggestions I did more reading and I'm pretty convinced that a repipe is in order. The primary-secondary piping seems to be a good solution for me compared to a zone-valve manifold because I will have only 3 zones total. I'm curious- as opposed to achieving hydraulic separation of the secondary loops through "closely spaced tees", does anyone have experience with a hydraulic separator? It seems it could save me money on circulator electricity usage.