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Weil-Mclain Ultra Series 3 Boiler Cycles on and Off Repeatedly

rdmingordmingo Posts: 9Member
My three-year-old Ultra Series 3 boiler never reaches its target temperatures on either the DHW or heat priorities. It ignites, runs for a minute or two at its maximum modulation (set to 70%), goes down to its minimum (set to 30%), then shuts off for a minute or two. It then repeats the process over and over.

Meanwhile the input and output water temperatures never get above 110 degrees even though they are set for 160 and 140, respectively. Is there any other possiblility than that the control module has gone bad?


  • GordyGordy Posts: 9,264Member
    Pics of the system.

    Has this always been an issue, or is it a recent problem?

  • rdmingordmingo Posts: 9Member
    Recent, I think. At least to this severity. It used to reach target temperatures, but it may have cycled more than I noticed on the way to reaching them. The unit was serviced and checked in November.
  • Jean-David BeyerJean-David Beyer Posts: 2,609Member
    I have an Ultra-3 from 2009, and it does not do this. It used to do this in my baseboard zone (that has two sections of baseboard: 14 feet of baseboard in each section). So it does not take a lot of heat (it takes enough to keep the zone warm enough). My Ultra-3 is the 80K BTU/hour model and is way too big for just that zone. The damping ratio of the controller is way too low in that situation, so it is really unstable. When it notices the supply temperature is way too low (as it is when that zone firsts requests heat), it sets a pretty high firing rate and it rushes up to the maximum. When it gets close to the maximum temperature, it lowers the firing rate, but not quickly enough. Consequently, it shuts down by exceeding the high limit. Then it cools down, and the cycle repeats.

    I did three main things to fix this. 1.) I lowered the maximum firing rate for that zone to 55% (default is 94%). That way it took more time to reach the high limit, giving the controller more time to lower the firing rate.
    2.) I set the minimum supply temperature to 120F instead of the 100F I wanted to use, so the baseboard would get rid of the heat faster.
    3.) I set the range from +|- 5F (default) to +|- 7F
    No more rapid cycling.

    This just might be your problem.

    Thinking more about it, though, it may be that the supply temperature and the return temperature sensors my not be making good thermal contact with the boiler loop (I assume it is piped primary-secondary), and the boiler sensors are taking over. Or worse, they may be reversed. If you operate the controller just right (see the manual), it will give you the error messages in plain English and these should help you quite quickly.
  • rdmingordmingo Posts: 9Member
    Thanks for your help. My system never gets close to the high limit and does not shut down or produce any error codes (it did the first day it was first installed, shortly after the professional installers left, before I lowered the default target zone 1 temperature from 190 to 170).

    My 155 system is similarly oversized for my DHW and primary heat system in my semi-detached rowhouse. I fairly quickly learned the system worked better if I lowered the maximum firing rate from 94% to 50% for both DHW (priority 1) and space heat (priority 2), and further lowered the max supply temperatures to 160 and 140, respectively.

    The two reasons for lowering the maximum firing rates were (1) higher efficiency at lower firing rates, and (2) the gas burners on my stove would stay lit when the boiler was on. I have very low dynamic gas pressure in the house that pretty much goes to zero when the furnace is on.

    The temperature sensors are new and seem to be tightly installed. I believe they are accurate, or at least not far off. On a recent cold day, the return and supply temps read 108 and 110 while both the DHW and heat systems had been demanding heat for more than an hour, I could comfortably keep my hands on the pipes. Neither pipe was anywhere near the the 140 or 160 cutoff temperatures, and yet the firing rate was repeatedly fluctuating. That is when I increased the maximum firing rates up to 70% for both systems to see if that would help. It did not help at all. The rate never went above 50%, and would only do that very briefly before modulating down, then going off, then immediately reigniting and going through the same cycle.

    I do not understand why the system doesn't just come on, fire at the maximum rate, and then cycle down as the system nears the target temperature. Is it possible the gas pressure is the problem? Nothing on the system indicates that is an issue.
  • GordyGordy Posts: 9,264Member
    Uhm yeah that’s not a good thing with your gas pressure.

    You had this looked at, it’s been a problem since installed? no one figured this out?
  • rdmingordmingo Posts: 9Member
    Gordy, the gas company came out and insists the pressure is fine where the line enters the house, but installed a new meter anyway. They only checked the static pressure. The HVAC professionals who gave the system the annyual checkup in November said the static pressure at the heating unit is fine, but the pressure at the boiler drops to nearly zero when the boiler is at its maximum firing rate and to a very low but measurable value when it is operating at 50% of maximum. The gas com[pany says the problem must be bad gas pipes inside the house but would not measure the pressure at the meter when the furnace is operating, so it is kind of a catch 22.
  • DZoroDZoro Posts: 752Member
    Your gas pressure must be proper at all times. Get a tech to check and confirm.
    1- static gas pressure
    2- 100% operating gas pressure
    3- High, and Low fire O2, CO2, CO levels
    Write down his results of all the above.
    Once that has been done and you still have issues, one can go from there.
  • GordyGordy Posts: 9,264Member
    Sounds like a piping issue after the meter, pipe size. It also doesn’t mean that it’s not the service. They won’t measure pressure while system is running?? This is something the installers should have addressed. The fix is not reducing the input to the boiler, and walking away. Shame on them!
  • GordyGordy Posts: 9,264Member
    Do you know the pipe size coming in from meter? Pipe size to boiler, and any other gas appliances. Their respecting lengths also? This is not a good scenario.
  • rdmingordmingo Posts: 9Member
    DZoro, in November, the static was 9 or 10, I believe, whatever units they are, and was rated as in the very good range, if that makes sense. The 94% operating pressure was very neaer zero, barely registering, and only at 1 or 2 units, as I recall, at 50% firing rate. I am pretty sure he checked the O2, CO2, and CO levels and thety were within range. The tech seemed pretty competent and concluded that the problem was with the gas supply outside the house. I got the gas company out here after that and they finger-pointed inside the house.
  • rdmingordmingo Posts: 9Member
    Gordy, at the time the bolier was installed, three years ago, gas pressure was not a problem. The pressure was reduced rather suddenly in the middle of last winter after the gas company had been doing a lot of repair work a couple blocks away. They have insisted all along that the pressure drop was not related. I am the one that reduced the firing rate so that I could continue to use my stove burners.

    From the meter, we have new (15-year-old) 1 inch black iron pipe for about 15 feet to a tee that goes to the only two gas appliances in the house-- a hybrid range (gas burners, electric oven), and the Weil McClain boiler. The two output pipes from the tee are 3/4 inch black iron. From the tee to the range connection is less than two feet, and to the boiler is about eight feet. The first Weil McLain unit lasted only a dozen years, and the second one is three years old.
  • DZoroDZoro Posts: 752Member
    Natural gas?
  • DZoroDZoro Posts: 752Member
    The pressure drop should be less than 1" at high fire. Dropping out to nothing is unacceptable. If you are on natural gas, and depending on how many btu's your gas range is, you are close to minimum with the gas piping. How large is your gas range?
  • rdmingordmingo Posts: 9Member
    The range is standard residential-- no large btu burners. I can't even keep a single burner lit when the boiler is going.

    I persuaded the gas company to come out again tomorrow , and asked them this time to bring the necessary fittings to check pressure at the meter when the boiler is firing (last time they came without the fittings they needed). As I understand it, that should settle whether the problem is in their pipes or mine.

    What pressure drop would be acceptable, from 9 inches to 3 or 4 inches, or do I need more than that to keep my range burners on?
  • rdmingordmingo Posts: 9Member
    It is natural gas, and the range is a hybrid-- gsa stove top but electric oven, so it has fairly low gas needs.
  • DZoroDZoro Posts: 752Member
    Where are you located?
    I would expect 6"-7" stay constant from 0-100% fire with boiler and stove on at the same time. That is the normal for my area, would think your area be no different. 3-4" gas line pressure is not going to work.
  • HVACNUTHVACNUT Posts: 2,041Member
    8 or 9 inches static pressure seems high for nat gas, low for LP.
    Dropping to near 0 at 75% input is a little funky.
  • icy78icy78 Posts: 251Member
    7 in is pretty typical. It should stay very close to 7 with your appliances online.
    Whatever they have it set at 7 8 9 it should still stay there when your appliances are online.
    It would be good to have a manometer on the inlet gas pressure at your boiler, at the same time as the gas company has a manometer on the meter.
  • rdmingordmingo Posts: 9Member
    Thanks to everyone for their help. On their fifth (5th) visit to my house, the gas company finally sent someone with the correct fittings to measure the working pressure at the meter. The results were so alarming (zero perssure under load) that they sent an emergency crew to dig up the street and replace the servie line. I can finally use my gas range burners safely again!
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