Chimney condensation after Utica TriFire boiler install

[Ron Read]

I had a Heating contractor install a Utica Trifire 4 section three pass boiler and Superstor 45 gallon DHW over the summer. The boiler AHRI rating is 79 MBH
The Boiler vents into a tile lined 8X8 chimney that is on an exterior wall and is in perfect condition. Old Dunkirk boiler had a stack temp of nearly 450 degrees and the chimney never had condensation issues.
Now the heating season has arrived I notice condensation plumes leaving the chimney any time the boiler is running. The other day when it was 35 degrees out I went on the roof and removed the chimney cap and noticed that the top clay tile liner was completly wet and condensation was dripping back into the chimney.
The combustion report listed the Gross stack temp of the new boiler at 380 degrees. Should I have the contractor remove the 3rd pass baffles and re adjust combustion to try to get the stack temp up another 60 to 80 degrees to limit or stop the amount of condensation in chimney? I understand that I will loose about 2% efficiency but I would we willing to if it will save a perfectly good chimney and avoid a Stainless steel liner install this year.
Boiler has a Taco PC 700-2 outdoor reset and I have the boiler MIN set to 150 degrees and Design set to 180 degrees. This time of year the boiler seems to be short cycling with a 2.5 minute run time with a 5.5 minute idle time during a continuous call for heat from thermostat. The Differential on the PC700 is set to auto and when Boil target is 161 degrees the controller swings the boiler from 170 to 151 I am sure some of the condensation issue is with the short run time followed by the 5 minute idle time allowing the chimney to never really reach a good operating temperature but hope an extra 60 to 80 degrees in stack temp will help.

[Dana]

Are you SERIOUSLY considering killing the efficiency and voiding the warranty on the boiler to save the cost of the ss liner?

At $4/gallon oil you'd pay for the liner in one heating season on the difference in efficiency!

To get better burn times either raise the min to 170F and design to 200F- whatever it takes, or increase the hysteresis (differential setting) around the setpoint (I haven't looked at that controller) boost it to the max. You apparently don't have enough radiation to emit anything close to the full boiler output at 150F, and the short-cycling at 2.5 minute burns pretty much kills the efficiency benefit of the outdoor reset. Ideally you'd get at least 10 minute burns out of it, but even 5 minutes might be enough to bring the chimney up to temp.

But get a stainless steel liner and crank the temps back down when you can.

[Ron Read]

I will try the higher min and max boiler temps on the reset and set the differential to manual and a number around 25 to start. Thank you for the input. This is the first boiler I have owned with an outdoor reset and I was not sure on settings but have been monitoring cycle times. The only reason I thought the baffles would be a temporary fix until I can get a liner was that the I&O manual for the boiler mentions removal to increase stack temperature is allowed with a loss of efficiency to be expected but since the Manufacturer mentions it as an option I did not see the potential for voiding the warranty but as only a loss of efficiency. I have not investigated Liner costs but I expect a 30 foot long 6 inch diameter insulated stainless liner would be at a minimum $2000 for a PRO to install since a DYI kit like Flex King PRO runs about $900.00 for the Kit alone. I will need to start the search for a reputable installer. Thank you.

[Dana]

I had misread the 2% as 20% (mea culpa!) But even 2% has a cash value, and getting longer burn times is imperative to getting anything like the labeled AFUE out of this boiler.

The problem with using outdoor reset (ODR) with a fixed-output boiler is that the radiation emits less heat a lower rate at a lower temp, but the boiler's burner is putting the same 79MBH in no matter how much the radiation is taking out.

When the heat emitted by the radiation is lower than the boiler's output, the temperature inevitably rises. How fast the temp rises depends on the thermal mass of the system and the difference between radiation-output and burner-output. When the temp hits the top end of the programmed differential it turns the burner off, and re-ignites when the system cools off to the bottom end of the differential. You could add thermal mass to fix the short-cycling by slowing the rate at which the limits are hit, but if you can get there by programming the controls it's a lot cheaper & easier.

Setting the differential higher is the best place to start- if the auto-differential was setting it to 10F and you were getting 2.5 minute burns out of it you can program it to 30F and probably get 10 minute burns. The 30F differential might be enough, since as the temp rises the radiation will be emitting far more heat at the high end of the range, slowing the rate at which the temperature rises. Start there, if that stops the short cycling behavior, then it's time to tune in the ODR settings.

Look up the manufacturer's minimum recommended return water temp (or call them), since that will determine just how low you can go. Some newer boilers have internal controls to take arbitrarily low water temps returning from radiation, but most have a lower limit of 140F. Without a stainless flue liner you should keep the return water temps at 140F or higher or you'll likely run into the same flue condensation issues even on longer burn, and this might be in play in your current situation even more than the burn times if the boiler output is running 150F, with return water at 130-135F. (Even if the boiler is designed to be low-temp tolerant, that doesn't save the flue from having condensation issues.) Raising the output temp 10F-20F will increase the radiation output and may increase the return water temp to where the exhaust stays above the flue-condensing range for most of the burn time.

If the venting has a barometric air damper that mixes dilution air into the vent (or if it doesn't have one, but needs one) that too will affect the amount of flue condensation. A properly adjusted burner minimizes the excess combustion air to an optimum point for combustion efficiency, but lower excess combustion air results in a higher dew point for the exhaust. The dew point of the exhaust goes way down with even a modest amount of mixing with normal room air, so it takes an even colder chimney to get it to condense. Read the installation manual- some require a barometric damper, others say "never".

Edited to add: The manual shows a barometric damper see figure 32, p37. Minimum return water temp is 130F- see p.18. That temp may good enough protect the boiler, but not necessarily protective a terra-cotta lined chimney.

[Ron Read]

Tonight I reconfigured the Reset controller with a curve more appropriate for the fin tube baseboard in my home. To steepen the curve to get the higher target temp while allowing a 35 degree differential to work while keeping boiler min at 140 I had to set the outdoor start at 80 degrees instead of 70 and I set Boil Design to 180 degrees.
Now with an outdoor temp of 45 degrees I get a boiler target of 160 that climbs to 178 during burn and drops to 144 I have black tape on the boiler return line and during the new boiler cycles I have not seen return temps drop below 140 using an infrared thermometer. I now have burn times of 8 minutes with off times of 11 minutes and it now takes less time to satisfy the call for heat. As soon as the overnight temps get back to around 30 I will re examine the chimney flu to see if any progress has be made with condensation but I am sure I will need to reline the chimney. Thank you for your suggestions.