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Advice on adding zones

HoraceHeat Member Posts: 6
Background: Old (1922) 3-story rowhouse, ~3500sq ft, solid brick, limited insulation. Original coal/gravity fed system converted to nat gas boiler (150,000 BTU input). The pipes are original (~4" cast iron) and 20 big radiators throughout house. Single zone. The boiler temp is set pretty low (pipes only warm to the touch) but it works great because there is so much water.

Problem/Question: Sunroom built off the side of the house. Cold in the winter (lots of old single pane glass doors). It has two radiators that are on a separate supply/return that are just tee'd in to the main system (see pic). They work but I would have to crank the heat in the main house to get it to a normal temp out there.

I was all geared up to install two zone valves to put the main house on one zone and the sunroom on the other (see pic). I got Honeywell zone valves and am ready to go at it but then realized the zone valves are only 3.5CV flow models which worries me now because I've got 1 1/4" flow return to the boiler. I don't know what the flowrate on this old Grundfos 20-42 circulator pump.

MY QUESTION: Will two 3.5CV zone valves restrict this system too much (especially the main house). Or, do I need to get different valves. I will be going from 20 radiators returning through a 1 1/4" copper to 2 and 18 radiators returning via two zone valves. Hope this makes sense

Help or additional questions appreciated! Thanks.


  • hot_rod
    hot_rod Member Posts: 15,029
    edited July 2014
    how much?

    flow will you be trying to move.

    Assume you boiler runs 80% efficient, you have around 125,000 BTU available to move.

    A heat load calc would determine what the home actually needs for BTU/hr on design condition. Could be that boiler is oversized for the load to begin with.

    Assume the main area needs 90,000, 9 gpm at 20° ∆T

    Here is what the pressure drop with a 3.5, and a 7.5 Cv valve, looks like. The third box shows pressure drop with a known Cv and flow rate.

    So you need to do some number crunching to get an exact answer to your question, a load calc to start. It's good info to have, an accurate load calc, everything sizes around that critical number.

    Those old, large pipe conversions generally do not need a lot of flow, or present much pressure drop.
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • HoraceHeat
    HoraceHeat Member Posts: 6
    circulator flow rate?

    In terms of the heating load I think everything is well-sized currently. The boiler does not cycle too much and I like the fact that the radiators are always warm (not hot).

    I guess what I don't know is the current flow rate of the pump (UPS-20-42). The whole system works great now and I don't want to throw things off. Would a pressure drop of 6.6psi on a 3.5CV valve be too much?

    My sense is I need higher flow zone valves than the ones I have but I don't fancy returning them...
  • RobG
    RobG Member Posts: 1,850
    Thermal Bypass

    You really need to install a bypass valve to protect you boiler from condensing. If your radiators are not getting hot, it probably means that the boiler is in condensing mode most of the time. A sure way to kill a boiler.

    See the link:
  • Ironman
    Ironman Member Posts: 6,555
    Separate Circulator

    Why not add a separate cic to create the additional zone? Of course, you'll need a check valve on each one to prevent backflow between zones. The simplest approach would be to put a Grundfos ups58-58fc on each zone rather than piping in a flow check on the existing zone. I'd try setting the main zone to medium speed and the new one to low for starters and measure the Delta T on each. Like HR said, a load calc is the only way to calculate what flow characteristics are needed.

    A word of caution: creating a small zone may cause the boiler to short cycle which will greatly shorten its life. If it's already condensing - and it probably is - then you would do well to heed Rob's advice. The thing that you have in your favor is the high thermal mass of the boiler and water in the system help prevent short cycling. Creating one small zone will work against that and could cause increased condensation. If the boiler won't run for a minimum of ten minutes on the small zone under normal conditions, then more buffer should be added.
    Bob Boan
    You can choose to do what you want, but you cannot choose the consequences.
  • icesailor
    icesailor Member Posts: 7,265
    FLowing Difficulties:

    You can make this far more complicated than it needs to be.

    IMO (worth nothing)

    The circulator you are using is sufficient, no matter what it is. If all the radiators from the original install are heating to the same temperature, you have the perfectly balanced gravity system. Your problem is that the added radiators added to the Sun Room and piping aren't balanced to the rest of the system. If they are heating up to the same temperature as the rest of the radiators when the circulator is on a long run, the radiators are too small for the heat loss of the Sub Room. Sunrooms are notoriously difficult to heat and should be on their own zone. But, if you decrease the heat loss with insulation and better windows. Maybe storm panels, you might get the Sun Room into the losses of the rest of the house. You can spend a lot of money trying to heat a space that needs more heat loss resistance. You are better served to spend that same money on decreasing the heat loss. If the room is cold, and you spend money on trying to get more heat into the cold space, you just pay more to heat it. The loss stays the same, the cost of the heat goes up. If you spend the money on stopping heat loss, you might not need to spend it on revamping the heat system. Most of the Sun Rooms I have seen have been built outside the main foundation and are basically an unconditioned space. They often leak air where they meet the foundation. If it is below 32 degrees outside and the floor of the Sunroom is 40 degrees or lower, you might need to insulate the floor and stop the infiltration. The same applies to the ceiling.

    As far as changing the piping, there are other ways of doing it that will give you what you want and will work well for you. That's another subject.
  • HoraceHeat
    HoraceHeat Member Posts: 6
    great info


    Great info thanks. This sunroom is a major heat loss (brick/plaster walls, 6 large single pane glass doors, tile floor on concrete slab, 12 ft ceilings. However I can't do anything to insulate better (long story that involves historical district restrictions). An online calculator tells me I need ~20KBtus assuming all of this.

    We only use the sunroom from about 7pm-11pm and weekends. My goal in this was to heat it a bit more than the house during those times and then set low (like 50 degrees during the day). My hope is that it would make it more comfortable while lowering our heating bill (because we aren't wasting heat when not in use).

    Anyway, sounds like I will move forward with some higher flow zone valves and if it screws things up I'll put it back the way it was...?
  • icesailor
    icesailor Member Posts: 7,265

    If it was mine, and all the radiators heat evenly in the main house, and the rooms are relatively equal in temperature. I would install a 4-way Mixer on the boiler which makes it a primary/secondary piping arrangement when you use something like a Taco "I" Series 4-Way. A 4- Way, NOT a 3-way. The boiler is the primary side, and the radiator side is the secondary side. Take the Sun-Room pipes off from the rest of the first floor zone. They get connected into the primary side of the 4-way with a positive closing zone valve. You get high temperature boiler water to the Sun Room and you can control it with a thermostat. The radiator side gets controlled by a Out Door Reset control. If it is an oil boiler, the I Series has boiler protection designed into the DIP Switch settings.

    The radiators in the main part of the house will be just as warm as needed by the control valve. The Sun Room, because it is using hotter water, will be able to heat the space with hotter water in the radiators. You end up running two separate temperatures in the system. You will need two additional circulators.

    When the thermostat is satisfied for the main house, the 4-way is totally closed.

    If you ever wanted to connect an Indirect Hot Water Heater, this is where you would put it.  There are wiring issues but a cleaver Sparky can figure out a way to make anything work.

    There are other ways to do it, but that will probably be the cheapest with the best results.