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  • Primary/Secondary loop needed for boiler?

    I've been saving up the credits with my plumber-in-law, so its time to treat myself. My current boiler is a Trianco Heatmaker oil boiler in decent condition but its not my dream machine. Eventually I want to change the boiler, but right now I'd like to straighten out the pluming so a quick change out can be achieved in the future.

    System is 5 baseboard zones w/ 1 DHW feeding an indirect. Zones are fed with Circulators and DHW has priority. Original install was 3 zones and the last 3 zones have been hacked in wherever someone could cut in a tee. Its almost impossible to get around the boiler to clean it. I'd like to re-pipe the supply and return to the wall behind the boiler, nice and neat with all the circulators in a row instead of hanging in space in the way of service.

    My question is about primary/secondary piping. All zones are fhw baseboard except the DHW, but not all zones have the same radiant load, so return temp can vary quite a bit. Does EVERY system benefit from a primary/secondary set up, or is it really about mixing radiation types or looping heat sources. Largest zone is about 180' with 40' of radiation, so the return temp can be pretty chilly for the first few minutes.

    Maybe a "passive" loop back where the supply and return headers are looped together? As I said before, I'm looking towards the day I replace this with a 3 pass CI oil boiler.

  • #2
    Re: Primary/Secondary loop needed for boiler?

    There is no point is primary-secondary piping if you have baseboard. Supply temps need to be around 160-180 and it would be nice if you got a 20 degree drop across the loops but some of that will depend on the amount of baseboard and piping per loop. I don't even bother to do primary secondary on radiant anymore and have gone with a hydronic separator and variable speed circulators to simplify piping. Primary secondary piping in your case will do nothing for the system except add another circulator and the electrical expense of running it. The best thing to do is straighten out the piping that you already have, keeping the loops. Next, install an outdoor temperature re-set like the Taco PC700, Beckett, Tekmar to modulate temperatures. Guaranteed to save at least 10%, probably closer to 20%. When it's time to change boilers, look into the Buderus 115 series with the blue flame burner.

    Very good hydronic site www.caleffi.com download the Idronics series.
    sigpic

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    • #3
      Re: Primary/Secondary loop needed for boiler?

      That's what I hoped you'd say. Extra parts is almost always a bad thing. I was also thinking it would lead to short-cycling of the boiler because the primary loop would heat up so fast, leaving the secondary loops to pull heat. I guess when I do the math there's only about 20 gallons on the largest loop, and about a 30 deg drop on the the return.

      Question on setbacks: Is there a model out there that will anticipate the load of a loop, or is it only concerned with the boiler temp? I ask this because one of my loops (800sqft in-law over unheated garage) has a huge load compared to others on my system, and I'm worried that the setback will not realize that this zone needs to run hotter. I guess this is one of those thing I won't know until I've tried it, but it takes a while to catch up now even at 180 degrees.

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      • #4
        Re: Primary/Secondary loop needed for boiler?

        Re-set controls sense outside temperature and boiler temperature and adjust the aquastat to maintain whatever your thermostat is set at. This keeps the circulator running longer which is more efficient and makes your burner run cycle longer which is also more efficient. In a perfect world with perfect equipment, if the house was loosing say 30,000 but per hour, you would want your heating system to output 30,000 but per hour constantly. As the load changes the burner output changes also. this can be acheived with gas equipment but is a bit harder with oil though we are getting closer to modulating oil all the time.
        sigpic

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        • #5
          Re: Primary/Secondary loop needed for boiler?

          Originally posted by NHMaster3015 View Post
          Re-set controls sense outside temperature and boiler temperature and adjust the aquastat to maintain whatever your thermostat is set at. This keeps the circulator running longer which is more efficient and makes your burner run cycle longer which is also more efficient. In a perfect world with perfect equipment, if the house was loosing say 30,000 but per hour, you would want your heating system to output 30,000 but per hour constantly. As the load changes the burner output changes also. this can be acheived with gas equipment but is a bit harder with oil though we are getting closer to modulating oil all the time.
          Agreed about the primary secondary not nessary in your suituation.But if your boiler is oil then cool water comeing back in spring and fall can help plug up the boiler due to condensing. soot tends to enjoy moisture on the inside of a boiler. My install would have a tekmar outdoor reset . this can be set up for a minium return. also a set of two tees one on the supply and one on the return with a valve in the middle for bypass of the hot outlet directly to the return will stop this and save many headaches
          Chris

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          • #6
            Re: Primary/Secondary loop needed for boiler?

            I am going to throw a wrench in the mustard as my grandpa used to say. I would pipe it p/s especially with oil. The burner inputs the same btu's all the time, not a bad thing if controlled properly, but the flow varies as zones kick on and off. I feel the flow should always be the same in the cast iron boilers as the input is not changing. Thermal stress is caused by rapid temperature changes in the boiler. Thermal stress and flue gas condensation are caused by extremely cold water entering the boiler or cool water at a high flow rate. Keep the flow rate the same and work on a 20-40ยบ delta tee. With p/s all you need is one valve on the outlet side of the boiler to throttle back to get the proper delta T.
            The mosty important thing about boiler installations is proper sizing of the boiler. Too big of a boiler lowers effciiency and increases maintenance. The next most important thing is proper near boiler piping. I would not use p/s loop unless I had multiple temperature zones. I would use the moose antler idea with same temperature zones.
            For mor info see the link below.
            http://www.comfort-calc.net/tech_area_index.htm#HWB

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            • #7
              Re: Primary/Secondary loop needed for boiler?

              Originally posted by nhmatt View Post
              I've been saving up the credits with my plumber-in-law, so its time to treat myself. My current boiler is a Trianco Heatmaker oil boiler in decent condition but its not my dream machine. Eventually I want to change the boiler, but right now I'd like to straighten out the pluming so a quick change out can be achieved in the future.

              System is 5 baseboard zones w/ 1 DHW feeding an indirect. Zones are fed with Circulators and DHW has priority. Original install was 3 zones and the last 3 zones have been hacked in wherever someone could cut in a tee. Its almost impossible to get around the boiler to clean it. I'd like to re-pipe the supply and return to the wall behind the boiler, nice and neat with all the circulators in a row instead of hanging in space in the way of service.

              My question is about primary/secondary piping. All zones are fhw baseboard except the DHW, but not all zones have the same radiant load, so return temp can vary quite a bit. Does EVERY system benefit from a primary/secondary set up, or is it really about mixing radiation types or looping heat sources. Largest zone is about 180' with 40' of radiation, so the return temp can be pretty chilly for the first few minutes.

              Maybe a "passive" loop back where the supply and return headers are looped together? As I said before, I'm looking towards the day I replace this with a 3 pass CI oil boiler.
              If I am understanding your system correctly, you have a pump on each zone. I am assuming 1/2 or 3/4 basboard with 3 original zones and 3 hacked. Depending on the hacking, you are only look ing to flow at most 9 gpm. Get rid of the zone pumps and go single pump zone valve. For secondary pump use a Grundfos Alpha and install the primary loop as well. Now you are down to 2 pumps with variable secondary flow and doing the best for your boiler.

              As for outdoor reset be very careful. With baseboard there is not a lot of room for modulation to heat output unless you have high output baseboard. You will want the sensor to regulate the return temp of the boiler and keep it above 140F to prevent condensation and there by destroying the boiler and the venting in time.

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