Modeling multiple different DHW boilers with only one input

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I?m working on a two-building university dorm with eight and six floors respectively. (using EnergyPro 5)

There are four DHW boilers ? two of 399 MBTU capacity and two of 900 MBTU capacity.

I am only allowed to enter one DHW boiler type and can then add a multiplier.

What would be the appropriate way to model this most accurately?

I was thinking of taking a weighed average value and inputting that with a boiler multiplier of four.

This would require more than a little explanation for LEED and Plan Check


Thanks kindly,

Christopher F. Zabaneh

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Joined: 2011-10-02
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Hey Chris,

How are you doing?

There?s an EE4 modeling guide (a software in Canada that is similar to Energy Pro) that explains how to do this. I?ll give you an excerpt that may help.

4.2.2 Multiple Boilers

Multiple central boilers are often installed to provide better part load performance or back-up capacity in case of equipment failure or maintenance. Boilers that are strictly for back up, that is kept off and must be manually switched to the operating boiler, should be ignored. In all other cases (lead/lag boilers, load/peaking boilers) enter all boilers. In EE4, it is assumed that the multiple boilers have the same capacity and performance characteristics. (It is not possible to

define boilers of different sizes or efficiencies within the same heating loop). Only separate heating plants, which serve distinct parts of the building, can accommodate different boilers.

4.2.3 Multiple Boilers of Different Efficiencies

At present, EE4 cannot model multiple boilers of different operating efficiencies. If the boilers are

operated to provide uniform run hours, determine the capacity-weighted average efficiency.

Example: 2 boilers:

? boiler 1: output capacity = 450 kW, efficiency = 88%

? boiler 2: output capacity = 150 kW, efficiency = 72%


Boilers 1 and 2 would be combined as follows:

? capacity = (450 + 150) / 2 boilers = 300 kW each

? efficiency = (450 * 0.88 + 150 * 0.72) / (450 + 150) = 0.84 (84%)

Entered plant boiler multiplier = 2, each boiler: output = 300, efficiency = 0.84, sequencing = 270


This lead/lag and peaking boiler may be confusing and the exact method may not be applicable to Energy Pro, but it may give you ideas. Let me know if you have further questions.

For lead/lag and peaking boiler cases, the following procedure is to be followed:

Note: This procedure involves 2 EE4 simulations. The first is used to determine the average boiler efficiency. The second is used for determining compliance for NRCan?s validation (NRCan validation is **similar** to Title-24 compliance). **similar in the sense that you need to comply to its requirements.

1. Complete Steps 1a) to b) to create a table such as that shown below (I can send you the EE4 modeling guide if desired).

Step 1a) List the boilers in order of their firing sequence, with the thermal efficiency and output capacity of each. The example below uses three boilers: B-1 is a 500 MBH condensing boiler with rated thermal efficiency = 95%; B-2 and B-3 are 800 MBH non-condensing boilers with rated thermal efficiencies = 80%. (B-1 is the lead boiler. B-2 will fire second, and B-3 third.)

Step 1b) Determine the cumulative plant capacity at each stage of the firing sequence. In the example below, only B-1 fires during Sequence 1, so the cumulative plant capacity is 500 MBH. During Sequence 2, both B-1 and B-2 fire, so the capacity is 1,300 MBH. During Sequence 3, all three boilers fire, so the capacity is 2,100 MBH.

Step 1c) Determine the percent of the total plant capacity that can be met by each stage of

the sequence. In the example below, the total plant capacity is 2,100 MBH. So

Sequence 1 can provide 500/2100 = 24%, Sequence 2 can provide 62%, and

Sequence 3 can provide 2100/2100 = 100%.

Step 1d) Determine the Plant Thermal Efficiency of each stage of the sequence. This is done by calculating the capacity-weighted average thermal efficiency of all the boilers that fire during a sequence. In the example below, in Sequence 1, only B-1 fires, so the Plant Thermal Efficiency for that stage is 95.0%. In Sequence 2, both B-1 and B-2 fire, so the Plant Thermal Efficiency is: (95x500 + 80x800) / (500 + 800) = 85.8%.

2. In EE4, set the boiler thermal efficiency to 80%, and make sure the total boiler plant output capacity is equal to the total plant capacity calculated above. (For the example above, this would be 2,100 MBH.)

3. Set the EE4 options so that the Plant PV-A and PS-C reports are created and the DOE-2 output files are not deleted when the simulation is run.

4. Run the simulation for the proposed building.

5. Open the DOE-2 ?.sim? file for the proposed building and locate the PV-A report. Check that the total installed boiler capacity matches the Total Plant Capacity from the table above. If it is not, then return to Step 2 and correct the boiler capacity entered into EE4.

6. Complete Steps 6a) to 6e) to create a table such as that shown below.

6a) In the DOE-2 ?.sim? file, locate the PS-C report. Use the LOWER line of data for theboiler to create a table of part-load hours as shown below.

6b) For each part load range, enter the Plant Efficiency in the table below based on thecalculations in the above table. (Use the column ?Percent of Total Plant Capacity? inthe above table to determine which efficiency corresponds to each part-load range.If the ?Percent of Total Plant Capacity? falls within a Part Load Range, switch to the next higher Sequence.) Step 1a) Step 1b) Step 1c) Step 1d)

6c) In the following table, multiply the ?Hours? column by the ?Efficiency? column.

6d) Sum the Hours column and the ?Hours x Efficiency? column.

6e) Divide the sum of ?Hours x Efficiency? by the sum of ?Hours? to determinethe average plant thermal efficiency. (For the above example, it is 93.3%.)

7. In EE4, enter the boiler efficiency determined in step 6e).

8. Run the simulation for the proposed and reference buildings to determine compliance forNRCan?s validation.

I hope this is helpful.

Danielle Krauel, B.A.Sc, LEED? AP

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