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Boiler Circulation Problems

505BEN
505BEN Member Posts: 6
Hello, been lurking and learning for a while but this is my first post.

I have a 4 zone baseboard radiant heating system in my home with a boiler that i believe is short cycling (often burn time is close to 1 min) and sometimes shutting off with an error code (ERR 65) indicating the internal water temperature reached 230F. Manual says that this is caused by air in the system or poor circulation. I purged each zone individually and bled the two upstairs zones (no air came out) so I don't think air is the issue. My belief is that when only one zone calls for heat it does not create enough flow across the boiler.

Exacerbating the problem is what I believe to be a leak in one zone that occurs only when heated. If I run that zone I can hear air moving (flowing water noise) back at the manifold, and then when the system cools to tepid I have lost pressure. If I shut that zone off and only run the other three I maintain pressure. Leak detection guys coming out next week but I thought it might be germane to this discussion.

Since history is always good info, two years ago, before I bought the house or learned anything about radiant heat, the boiler was replaced by the previous owner. I don't know anything about the old boiler. Last year around this time the pump stopped working and the expansion tank diaphragm failed and had to be replaced.

I think the boiler (78kbtu/hr net output at 5500' elevation) is oversized by about 30% for my baseboard system, but more importantly it is massively oversized for situations in which only one zone is calling for heat. I have attached a diagram of my system as it is. Using B&G software I calculate with my circulator set at Low (5' of head) the lowest flowing zone is only pulling approx 2.7 GPM. If I increase pump to mid (10' of head) then my shortest run will be flowing over 5 GPM which i keep reading is too high. Also i still get the error just less frequently. If I lock open one zone so that min two zones are running in a call for heat I don't get the error. As for heat, my smallest zone is 19' of baseboard which by my calcs only dissipates 11.4kbtu/hr.

In this situation will creating a primary loop at the boiler and a secondary loop for the zones fix my flow/overheat problem? Will doing so make the short cycling worse? Is there anything I can do about the short burn cycles?








Comments

  • hot_rod
    hot_rod Member Posts: 15,029
    What brand and model boiler? I don't recognize the SG-135 number. Many mod con type boilers have a way to electronically derate them, anti cycling settings, etc.
    In some cases adding a buffer tank is a good upgrade.
    Primary secondary alone may not fix the issue. The quick high temperature indicates a lack of flow.
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • 505BEN
    505BEN Member Posts: 6
    @hot_rod
    It is an Allied Boilers Saturn series SG-135 and I'm pretty sure it is not a mod/con.

    I am a sparky by trade and know just enough about hydronics to be dangerous (ill have a heater specialist do any work) so I apologize if I am misunderstanding a concept. Would not the primary loop increase the GPM through the boiler just with a much lower delta T ? I know this would not stop the quick burns but I imagine it would at least keep the boiler from overshooting its 180 degree target.

    As for the buffer tank, I understand how it helps the short burns but don't believe it helps the flow issue is that correct?.

    Thanks
  • hot_rod
    hot_rod Member Posts: 15,029
    Looks like a basic cast iron boiler, it really should not be that sensitive to low flows.

    Adding a pressure activated bypass valve would assure you always have flow through the boiler, and help prevent over pumping when only 1 or 2 zones are calling.

    The cycling will be based on the boilers fixed output vs the load of the zone or zones that are calling.
    A perfectly designed and sized boiler would be expected to run n on stop on a design day, and cycle on any load less than design.

    Really should not need a P/S piping on a single temperature cast boiler like that.
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
    505BEN
  • STEVEusaPA
    STEVEusaPA Member Posts: 5,312
    edited November 2020
    Here's the manual. Maybe 5' isn't cutting it.
    https://alliedboilers.com/wp-content/ProductDocuments/SG/SG Installation & Service Manual (S9361A).pdf

    Page 5 is showing 11.5 gpm for a 20 degree delta T, with a 2.8 gpm drop thru the boiler.
    steve
    Solid_Fuel_Man
  • 505BEN
    505BEN Member Posts: 6
    @hot_rod
    Wow thanks, that bypass valve is a lot simpler than what I was thinking and accomplishes what i am looking for regarding flow.

    For the short cycling it looks like a properly sized boiler may only be 50% more than the buffer tank plus plumbing work. From an ROI standpoint I'm probably better off just letting my boiler short cycle during the late fall and early spring and replace it with a mod/con when it breaks.
  • EBEBRATT-Ed
    EBEBRATT-Ed Member Posts: 10,287
    Sounds like you may need a buffer tank to reduce short cycling.

    You say there is no air in the system but then you say

    "If I run that zone I can hear air moving (flowing water noise) back at the manifold, and then when the system cools to tepid I have lost pressure."

    I would also have the expansion tank checked if the pressure is changing a lot when the water temp changes
  • Zman
    Zman Member Posts: 6,985
    You have a steel fin tub boiler that is a bit finicky about minimum flow.http://alliedboilers.com/products/saturn-series-boiler/ A differential bipass valve or the primary/secondary piping you sketched would resolve that problem.

    You have an oversized, low mass, single stage system which is a bit of a nightmare from a short cycling point of view. It looks like your controller is a rebranded Tekmar. http://alliedboilers.com/wp-content/ProductDocuments/SG/BC1-Controller_Manual.pdf
    You might try setting the on/off differential to 30 degrees or so and seeing if that helps with the cycling.
    If that doesn't work, a buffer tank would be a good next step.

    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein
  • 505BEN
    505BEN Member Posts: 6
    @STEVEusaPA
    I think you are right. I turned the pump speed up to mid which is 10’ of head. Problem is that puts my shortest zone at around 5 gpm when it is the only one running. My intent at this point is to replace the pump with an 007 and add a dpbv to the system. This should keep the flow up through the boiler and manage the flow in my short zone to below 4 gpm.
  • 505BEN
    505BEN Member Posts: 6
    @EBEBRATT-Ed
    I think I have isolated one of my zones that is allowing air intrusion when it gets hot. I stopped running that zone and purged air from a tees on my upstairs zones so now things run nice and quiet. I have a leak detection company coming out next week.

    I’ll look into the expansion tank pressure. I do see the total system pressure on the supply side gauge change from 13 psi cold to 26-27 psi hot.
  • 505BEN
    505BEN Member Posts: 6
    @Zman
    I think you and Hot_Rod are probably right about the buffer tank. The calculation I am now making is if it is better to install a buffer tank to correct a boiler that has been short cycling for three years, or if I should let the boiler run till it dies and replace it with a correctly sized mod con.