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Why is my boiler using almost twice as much gas as expected?

asked 2016-01-26 08:32:32 -0500

I have a gas boiler with the efficiency set to 100%, the efficiency curve set to 100% (flat curve), and two zones each with baseboard water heaters.

My problem is that the building is using 8690 kWh/year, yet the baseboard water heaters are only emitting 4785 kWh/year between them (an effective efficiency of 55%). What could be causing so much extra gas to be used? There's no storage tank and no DHW use.

Looking at some of the node temperatures it seems like my delta-T is very low. Could that lead to a near-doubling of annual fuel use? Where else should I look to diagnose the problem?

Boiler:HotWater,
    PrimaryHeater,           !- Name
    NaturalGas,              !- Fuel Type
    4000,                    !- Nominal Capacity {W}
    1.0,                     !- Nominal Thermal Efficiency
    LeavingBoiler,           !- Efficiency Curve Temperature Evaluation Variable
    100% efficient,          !- Normalized Boiler Efficiency Curve Name
    75.00,                   !- Design Water Outlet Temperature {C}
    autosize,                !- Design Water Flow Rate {m3/s}
    0.00,                    !- Minimum Part Load Ratio
    1.00,                    !- Maximum Part Load Ratio
    1.00,                    !- Optimum Part Load Ratio
    Boiler Water Inlet Node, !- Boiler Water Inlet Node Name
    Boiler Water Outlet Node,!- Boiler Water Outlet Node Name
    100.0,                   !- Water Outlet Upper Temperature Limit {C}
    NotModulated,            !- Boiler Flow Mode
    25.00,                   !- Parasitic Electric Load {W}
    1.00;                    !- Sizing Factor

Curve:Linear,
   100% efficient,                                !- name
   1,                                             !- Coefficient 1
   0,                                             !- Coefficient 2
   0,                                             !- Minimum X
   1,                                             !- Maximum X
   ,                                              !- Minimum Curve Output
   ,                                              !- Maximum Curve Output
   ,                                              !- Input Units X
   ;                                              !- Output Units
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Comments

Have you tried to replace the boiler object with the district heating object, just to confirm that the issue is on the primary side?

Ivan Korolija gravatar image Ivan Korolija  ( 2016-01-26 15:25:56 -0500 )edit

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answered 2016-01-26 08:51:52 -0500

updated 2016-01-26 09:01:24 -0500

Would need more info to really determine what's going on. Could be a number of issues:

Sizing:

  • 4000 W boiler capacity is too low, so boiler operates almost all the time
  • inlet and outlet temperature fields in Sizing:Plant, Boiler, and baseboard HW coil objects don't match
  • you autosized flow rate for boiler, but may have hard-set flow rate for baseboard heaters

Baseboard Heaters

  • UA calculations for coils may yield low heat transfer efficiency
  • Availability schedule for baseboard and/or HW coils limits time of operation

My guess is that the baseboard inputs are the culprit, but all are worth checking out.

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Thanks for the pointers. I'll check those out and see if it helps. One question though - why would the boiler running constantly affect the apparent efficiency if it is a 100% efficient boiler with a flat efficiency curve?

Jamie Bull gravatar image Jamie Bull  ( 2016-01-26 09:04:41 -0500 )edit

@Jamie Bull good point, I forgot about the constant 100% efficiency

Aaron Boranian gravatar image Aaron Boranian  ( 2016-01-26 09:08:28 -0500 )edit
2

answered 2016-01-26 09:37:52 -0500

Low delta-T is nearly impossible to avoid without pressure-independent valves. It can definitely lead to increased gas usage and excess pumping energy.

I would take a look at your pumping configuration. Do you have a primary/secondary loop design or primary only? If primary only, do you have a constant speed pump or is it variable speed?

The specified boiler doesn't have any dedicated capacity control so you're relying on your pump(s) to control the boiler plant's effective capacity.

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It's a primary/secondary loop with a variable speed pump.

Jamie Bull gravatar image Jamie Bull  ( 2016-01-26 12:00:31 -0500 )edit

Depending how far you want to go into it there're two options:

The first option would be to use the EMS feature of EnergyPlus to control the secondary pumps so the secondary loop always has the desired delta-T (30F). Basically, if the delta-T started to fall the pumps would reduce the flow through the loop. This option is highly recommended if modeling condensing boilers

The other option would be to accept the reality of low delta-T at low part load conditions and set the primary pump to constant speed while making sure it's flow rate is sized to be greater than the secondary pump flow rate.

Lincoln gravatar image Lincoln  ( 2016-01-26 12:58:55 -0500 )edit

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Asked: 2016-01-26 08:32:32 -0500

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Last updated: Jan 26 '16