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zoning new oil boiler system

getheheat Member Posts: 3
This is my first post so here goes. I am currently heating with a coal boiler 2 pipe gravity system and a oil boiler backup that I haven’t used in years, that looks to be plumbed in such a way that circulates the heated water into the coal boiler and gravity takes over to move the hot water through the system to the radiators. The flue is in need of repair so I have been thinking about removing both boilers and installing a new flue sized for a new oil boiler. If I do this I'm thinking about going to a micro zone system. I'll remove the gravity piping (to remove all that unnecessary mass and water) and replace it with pex to a zoning manifold with thermo electric actuators and install thermostats in the rooms according. I would have 5 zones on the first floor (approximately 1200 sg ft) and 3 on the second floor (590 sq ft) and possibly add heat to the basement (1200 sq ft). I’m thinking and Buderus G115WS 85K BTU DOE boiler supplying a 50 or 75 gallon buffer tank and using outdoor reset mixing valve on system loop and possibility ODR on the boiler also if there is any energy savings in doing so? I have many days that heating with the coal boiler only requires 100°F water and I’m thinking 150° for “design day” weather. I’ll use the ODR valve to set the best water temp for the radiators (sort of what building the correct coal sized fire does now when you think about it) and the boiler ODR to set the best water temp for fuel savings. Does all of this sound doable? I’ve done some rough heat load collections and the max btu of boiler is so close when figuring in the basement it sort of make my wonder if I shouldn’t go bigger? Currently the foundation walls are not insulated and should I heat this space I would do so, when I did the calculations I did them w/out insulation so that would help bring the heat loss down and when doing a system like this where every rad is practically its own zone and can the boiler output be reduced by a certain percentage? Thoughts?


  • STEVEusaPA
    STEVEusaPA Member Posts: 6,505
    edited April 2016
    My first piping job on my oil fired baseboard and radiant system I followed the micro zone concept. Thought it was the way to go...total comfort, right? Uh....not the way to go (for me).

    First, of course, do the heat loss. This will tell you eveything you need to know about flow rates, supply water temps.
    An oversized oil boiler will not be efficient, will short cycle a lot, creating wear and tear on the boiler and components. The micro zones (for me) mean a properly sized or oversized boiler is just too big. Unless you're going for deep setbacks, or are prone to power failures with long off times when you need heat.

    The buffer tank is pretty much mandatory. But if you have to stick to micro zones, make sure it's big enough. Bob Boan once posted a formula for sizing a buffer, and Idronics by Caleffi has one too.

    I think you want the massive pipes. You need mass. If you switched to a mod/con gas boiler, you would definately want/like/need the mass. But the near boiler piping is critical, especially protecting the boiler from low return water temperatures.
    I'm assuming you have radiators.

    My initial design thoughts would be P/S, boiler protection, delta P circ (continuous) and TRV's on the radiators, and ODR. Use the buffer as hydraulic separation and buffer. If you use a reverse indirect or a 'solar type' tank with 2 coils, you could also use this for domestic hot water.

    Caleffi Idronic series has lots of design ideas, with pictures. Plus all the hydronic people on this sight like to weigh in with design ideas, based on their experiences.

    I plan on a re-pipe of my system this spring/summer. Combining zones, adding buffer tank, etc.

    There was an error rendering this rich post.

  • SWEI
    SWEI Member Posts: 7,356
    This all starts with a heat loss calculation and a radiation survey. From those comes a proper boiler size and flow rates, which dictate pipe and valve sizing. Old gravity systems are wonderful things, but there are some tricks to getting them right. Changing out some of the large pipes and leaving others can often result in an unbalanced system.

    BTW, micro-zoning may not be the best term to describe your plan. It's the cause of a lot of grief we see.

    Partial boiler reset paired with full system reset (mixed) is the best way to run an oil boiler for sure, though you may need some additional thermal mass depending on the system specifics. That's one of the primary reasons to consider keeping the old piping in place.