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boiler bypass

If I install a boiler by pass on a ci rad system and size boiler to building heat loss(152,000), how much will it effect the net output of the boiler rating or is it negligible. Assume at 10* design conditions.


  • Mitch

    Provided you size the boiler to the building heat loss and establish a water temperature needed based on the ratio of boiler btu's (from heat loss) to actual connected load of the system, the effects will be negligable because the AFUE ratings are pretty much established with water temperatures reduced somewhat. What will change is that you will be running longer thermostat cycles with cooler water to establish more system comfort. The boiler will remain somewhat safe from the effects of large volumes of cool return water and will remain above the condensing point or dew point. There is a chart on or near Page 23 in The Heating Helper to help you establish this system temperature. There are also instructions on how to use it. Another option would be to use a boiler such as the Burnham Revolution which has a modulating bypass system already built in utilizing a variable speed pump control and injection pump. Hope this helps.

  • mitch moore
    mitch moore Member Posts: 32
    connected load

    I should determine the edr of the connected load and deliver a water temp that will satisfy the heat loss for the designed conditons while at the same time insuring that the boiler bypass is balanced so that the boiler water temp does not fall below mfg specs.If the edr is low enough and still able to to meet the heat loss at design contions I would run higher water temps therefore eliminating the need for boiler bypass. Correct?
  • Right

    Perfect description of what you are trying to accomplish. The magic number to watch for is an average water teperature through the boiler of 140°F minimum. If your return temperature is 120°F, you would want to see about 160°F at the boiler outlet to ensure an average of 140°F through the boiler. Thermometers take the guesswork away. One on the return, one on the system supply and you already have one monitoring the boiler outlet temperature. Also remember that the system pump needs to be on the system side of the bypass to make it a boiler bypass.

  • Boilerpro_3
    Boilerpro_3 Member Posts: 1,231
    One other thing

    With a cast iron system you are almost always dealing with high mass. Under typical conditions, even if you need 180F water at design, the boiler and system will not get to that temperature because the thermostat will be satisfied before the system gets to that temperature. It takes alot of btu's to get 160 gallons of water and a couple tons of steel and iron to change temp.
    On a converted gravity system (high mass) that had a 180F design temp and a properly sized boiler, the temp would rise only about 20F on each 20 min burner cycle during typical winter weather. The rads would be running only about 120F on average. So you end up with a boiler still running at condensing temp for long periods of time if you don't use a bypass or mixing devuce to provide boiler protection.
  • mitch moore
    mitch moore Member Posts: 32
    What about cold starts

    I see most single zone res boilers set up without bypass. What effect does a setback stat or leaving boilers off for long periods of time (work hours) have. What is considered long run times at low temp conditions?
  • The lower the system water temperature

    the longer the system (thermostat) run time. The bypass allows the burner to cycle on and off as it would as if a low mass system were attached to it without a bypass. Get it set correctly and the boiler as well as the homeowners will be comfortable and happy. Hope this helps.

  • mitch moore
    mitch moore Member Posts: 32
    what about?

    Yes, but what about systems with no bypass. I see systems run at 140*+ but it take long periods of time to get at that temp. What,s considered a long run time to achieve the 140* minimum before it becomes harmful.Such as in the cases where there are set back stats or persons who set stats to 50* while at work for 10hours a day.I recall a boiler taking 1 hr to achieve temp of 140*.If a bypass is not in the system would it be wise to install a aqustat to control the circulator should cold water return below 140*. Many of the systems I,ve seen turn the circ on immediately on a call for heat.Is it harmful to the boiler for the low temp water returning after a cold start up.
  • If the boiler

    is sized properly and the system is matched to the heat loss requirements, cold start conditions should be brief enough so as not to condense flue gasses for long periods of time. A boiler bypass on every installation is not a bad thing! An oil-fired boiler with tankless heater will perform better and produce more consistant domestic hot water if a bypass is installed. The shorter the period that the boiler operates below 140°F, the better.

  • mitch moore
    mitch moore Member Posts: 32
    can you give me an idea

    what would an acceptable/unacceptable time frame be?
  • Generally

    if that period were about 5 minutes or less, then the boiler will have a better chance to burn off and evaporate any condensation that has occured during the ramp up cycle without discernable harm to the boiler. This will vary with boiler size and fuel type.

  • mitch moore
    mitch moore Member Posts: 32
    thanks in other words

    before the acid gets a chance to do it's thing
  • Joe_30
    Joe_30 Member Posts: 85

    Glenn, Does a SYSTEM bypass provide equal boiler protection.? Picture a single circulator driving a five zone supply manifold with zone valves, and off one end of the manifold [therefore after the circulator] a small pipe runs right back to the return manifold, controlled in the simplest version by a globe valve. The boiler heats up quickly.

    Your interesting advice is that the BOILER bypass lengthens the cycles,. Thanks.
  • A system bypass

    will provide better boiler protection, but at a sacrifice to available system flow. In a commercial application, a system bypass would be a better choice. A better choice yet would be a modulating system bypass with a 3-way motorized diverter valve, boiler pump, return sensor and primary-secondary piping to the system. This pretty much ensures boiler protection in a commercial application. Hope this helps.

  • Mark J Strawcutter
    Mark J Strawcutter Member Posts: 625
    system bypass

    also makes sense on a residential gravity conversion - high water volume, cast-iron radiators.

    Doing it with motorized diverter has the advantage of potentially (althought probably not very frequently) allowing for full flow to the system and none thru the bypass.

    A lower-cost alternative would be to use a thermic type 3-way like the Danfoss/ESBE TV where the bypass and return connect (acting as a mixing valve). This type of valve however never totally closes off the bypass.

    Next level would be to pipe the boiler as a primary loop and the system as a secondary. Run the secondary constant circulation with outdoor reset.

    Finally, put TRVs on the radiators and a differential pressure bypass on the secondary and you have the ultimate for a gravity conversion.

  • Larry (from OSHA)
    Larry (from OSHA) Member Posts: 716
    would bypass be needed or wanted if

    situation was low water content system such as 3/4" fin baseboard with 3 series loops total emitter of approx. 120 ft. and total of all loops less than 500 ft. and approx. 2.7 gal. water content of boiler. wouldn't short cycling occur? As always, your expertise is greatly appreciated.

  • Probably not

    but it is always wise to consult the nauual for that particular boiler.

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