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System short cycling

Krash
Krash Member Posts: 17
My system is used to heat my whole, 2,000 sq. ft. home, plus domestic water. The system works fine when used to heat the whole house. However, I will be alone in the house during the winter months from now on, so I am trying to heat only two small rooms. But that causes the heater to short cycle. On for about a minute, then off for about a minute.

This is a split system with the heating side, and fresh water side separated by a heat exchanger. That heat exchanger is connected to a 199,000 A.O. Smith propane heater by about 3 feet of tubing. The heater requires about 15 degrees or more difference between the output and input temperatures or it shuts the heater off until the inlet temperature cools.

When the whole house is being heated, the return temperature of fluid is enough to keep the heat exchanger cool enough to allow the system to operate normally. But with only two small rooms connected, the heat exchanger is getting overheated allowing the return water to the heater to rise too quickly.

My thinking is that I need some way to cool the water returning to the heater, maybe by a buffer tank or cooling coil of some sort? Has anyone come across this situation, and has a fix?

Any help would be appreciated!

Comments

  • Jamie Hall
    Jamie Hall Member Posts: 23,276
    When a system has a boiler which is as wildly oversized as yours is going to be, the only real solution is a buffer tank, and the bigger the better.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
    Krash
  • hot_rod
    hot_rod Member Posts: 22,121
    So a 199,000 water heater is the hydronic heat source, via a plate HX? So 75 gallon or larger tank? That tank should buffer a small heat load.

    If the system is zoned with zone valves a delta P circulator would reduce flow, automatically through the heat exchanger as zones open and close. What size circulator is on the A and B side of the plate HX?
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • Krash
    Krash Member Posts: 17
    A 75 gallon tank would be very hard to fit in the space available.

    Could I use some sort of coil instead? The system is against a wall of an unheated garage which is very cold. If a coil could be located in that garage, would that reduce the temperature of the return to the heater?
  • Jamie Hall
    Jamie Hall Member Posts: 23,276
    Krash said:

    A 75 gallon tank would be very hard to fit in the space available.

    Could I use some sort of coil instead? The system is against a wall of an unheated garage which is very cold. If a coil could be located in that garage, would that reduce the temperature of the return to the heater?

    Ah... well, yes it probably would. You would be heating the garage, and to the extent that you did, that could reduce the return temperature. However, most times you want your hard earned dollars to be spent heating the places where you are living, no? You would be solving a pretty much non-problem (short cycling the boiler) by wasting the heat -- and money.

    Not sure that's a good bargain.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • pecmsg
    pecmsg Member Posts: 4,844
    Whats going to happen in the unheated rooms? How are you going to protect the pipes?
  • hot_rod
    hot_rod Member Posts: 22,121
    Do you have all the settings adjusted in the boiler control? There should be multiple settings to reducing cycling. What is the model of your boiler? Attach a picture 
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • Krash
    Krash Member Posts: 17
    The boiler is actually a water heater. 199,000 btu, By A.O. Smith (Takagi). I have talked to the manufacturer who assures me that the return must be 15 degrees lower than the output, or the unit stops heating. There is no adjustment.

    Perhaps the only practical solution is the go back to heating more rooms?

    The entire system uses glycol, so there is little chance of reaching it freezing point.
    flat_twin
  • hot_rod
    hot_rod Member Posts: 22,121
    Yeah, that is not the best appliance for the application. If you had and actual boiler it would have adjustments to address short cycling, derate, step fire, time delay, outdoor reset etc

    Probably the only fix is 30 gallons of buffer, or more. Or replace it with a real hydronic boiler.
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
    mattmia2
  • ryanwc
    ryanwc Member Posts: 50
    Certainly heat the house rather than heating the garage. 
    HVACNUT
  • Dave Carpentier
    Dave Carpentier Member Posts: 587
    Do the other rooms have their own thermostats and individual zone valves ?
    30+ yrs in telecom outside plant.
    Currently in building maintenance.
  • EdTheHeaterMan
    EdTheHeaterMan Member Posts: 7,832
    My thinking is that I need some way to cool the water returning to the heater, maybe by a buffer tank or cooling coil of some sort? Has anyone come across this situation, and has a fix?

    You could use the unheated rooms to cool the return water somewhat. Just spitballing here
    Edward F Young. Retired HVAC ContractorSpecialized in Residential Oil Burner and Hydronics
  • ryanwc
    ryanwc Member Posts: 50
    As long as the rest of the house is insulated and sealed, it’s your supersized, open air buffer tank. Not sure why you’d buy another one.