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Boiler Protections From Flue Gases: Valves, Piping & Circulator Hold-Offs

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D107
D107 Member Posts: 1,849
edited November 2019 in Gas Heating
Re: Atmospheric Boilers. I have heard of bypass piping, three-way valves, primary secondary pumping, buffer tanks, thermic valves, etc. I used to have a Buderus G-115 with its own Logamatic control that would hold the circulators back until the water temperature reached a certain level. I currently have a boiler with a Hydrostat that only will allow circulators to go on when water temp reaches 125º, but turns them off when it drops below 115º––but the holdoff expires after 15 minutes. I spoke with the Hydrolevel people and one tech said that their Circulator Hold-off is not a substitute for protective piping or valves. (It's described as 'enhanced condensing protection'.)

I found the attached table below online that shows the condensing temperatures of various fuels given different levels of excess air. If I measure 60% excess air in my boiler, then according to this table the flue gases will condense at 125º or lower.

So, if I'm visualizing this correctly, if 62º return water hits a firing boiler whose overall water temp is 125, then that firing may keep the boiler water temp at 125--which is on the border of condensing. But with the circ continuing to pump until water temp reaches 115, it would seem to me that the flue gases will start to condense. I understand this is normal or acceptable for the first ten minutes of a cold start heating cycle, but after that it becomes problematic.

I'm sure I've oversimplified or mis-stated some of this and would appreciate thoughts from the pros here.


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  • Jamie Hall
    Jamie Hall Member Posts: 23,286
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    First off -- the condensed water (if the boiler is condensing hot water) is on the fire side of the heat exchanger, not the water side -- so it doesn't become part of the system's water volume at all. In a boiler designed to condense, a drain is provided for it. In a non-condensing boiler, the hope is that it will be evaporated by the flue gasses. Which it usually is.

    Second, remember that the circulating water never touches the flue gas, so the temperature of concern is the temperature of the flue gas, not the water. In a properly designed boiler they won't be that different (these things are, or should be, designed as counterflow heat exchangers, so that the temperature difference between the hot side and cool side is more constant. If the boiler is designed to be condensing the presence of condensate in the flue gas is taken into account in the choice of materials to resist the corrosive attack. In a non-condensing boiler this is not true, so that one wishes to have the flue gas temperature higher than the dew point of the flue gas. By far the safest way to do that is to ensure that the returning water is above that temperature. This can't be done from a cold start, of course, but it can and should be maintained by proper design of the associated piping,, pumps, and controls.

    It really isn't hard to do this if the boiler is at least big enough to handle the load. If it is not, you can get into a low temperature return situation and ideally the circulator should shut down until the boiler can recover. Note that doing this will not affect the building heating -- it is just a reflection of the fact that the boiler just isn't big enough.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
    D107
  • hot_rod
    hot_rod Member Posts: 22,143
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    Think of the circulator turning on and of to control return conditions as driving your car and turning the key on and off to control your speed. Sure it is possible but there are much
    more sophisticated and affordable ways to accomplish the basic goal.

    There are no hard set rules for cold start conditions, 10 minutes seems to be the industry accepted time frame.

    Different boilers handle the low temperature conditions differently. Copper tube boilers generally complain quickly under low temperature operation, the copper fins plug and corrode quickly in thin materials. Cast boilers are more accepting of the abuse😉

    Unsafe operating condition can be another side effect, flame roll out, possible CO spilling, etc
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
    D107
  • D107
    D107 Member Posts: 1,849
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    @hot_rod @Jamie Hall Then operationally, until I can make a decision as to what protective piping or valve system is added, my initial instinct was to slow down the cold return water coming back into the boiler via the circuit setters. Seemed logical, just like a partial 'circulator hold.' But I was told: "Reducing the flow rate is only going to increase the delta t and return even colder water. Increasing the flow will tighten the delta t and increase the return temp."

    I do see certain circs with boiler protection mode where they say it will slow the pumping until the temperature rises to a certain point. I have since totally opened the circuit setters which probably lessens the length of the cycle which is probably a good thing.
  • hot_rod
    hot_rod Member Posts: 22,143
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    If you choke flow either supply or return side, you limit heat output to the system. the system should be allowed to flow the required gym to meet the load.
    The return temperature protection is a different function.

    There are some VS circulators being sold as return protection, they will work only if piped properly.

    If you have an hour to spare, this webinar will show you options and explain the concepts.


    https://www.youtube.com/watch?v=hzPcVeSwwVU&list=PLuuV0ELkYb5VE0I4evUZ30b5U78CRlRdg&index=66&t=0s
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
    D107
  • hot_rod
    hot_rod Member Posts: 22,143
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    notice the subtle difference in this piping, it will provide 100% return protection, the system can be decoupled from the boiler with this piping.
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
    D107
  • D107
    D107 Member Posts: 1,849
    edited November 2019
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    @Hotrod Thanks for this.

    So any restriction--including circulator hold--which is the ultimate restriction--will limit heat output, understood. But it might offer more boiler protection. On the other hand, throttling output may just prolong the time the boiler is exposed to cold return even if that return is a little warmer. Another tech advised me to take off the circ hold. Won't do that yet.

    But while these drawings and other ideas are being studied, in the short run I have bought a Honeywell T-6 Pro T-stat so I can increase the frequency of the cycles from once every three hours to maybe once an hour that should keep the return water warmer and avoid the 'cold 70' we have now. Also perhaps raising hi limit from 160 to 170 or 180. And raising indoor ambient temp from 65 to 68. This is cold start now; but I don't think making it warm start --putting in a low limit--would accomplish much. And I look forward to the Caleffi seminar--great stuff!
  • D107
    D107 Member Posts: 1,849
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    @hot_rod Very good video. Of course it gave me something else to think about: thermal shock. I don't have a good handle on how much risk there is in this setup for thermal shock with 62º return water--at the cycle outset--hitting 130 or 150º boiler water. That's why I was thinking of reducing flow.

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