HVAC and DHW: Common Questions

1. Multiple Thermostats in HPXML
Issue:

🔹 OptiMiser does not support specifying multiple thermostat improvements in HPXML.
🔹 If you need to use HPXML and are installing multiple thermostats, you must manually edit the exported HPXML file.

Solution (Editing HPXML File):

1️⃣ Open the HPXML file in a text editor (like Notepad).
2️⃣ Find this block of code:

<Measure> <MeasureSystemIdentifiers> <SystemIdentifiersInfo id="HVACControl" /> </MeasureSystemIdentifiers> <MeasureDescription>Install a timed thermostat or increase your temperature setback to save energy.</MeasureDescription> <Quantity> <Units>Setback thermostats installed</Units> <Value>0</Value> </Quantity> </Measure>

3️⃣ Modify the <Value> field to represent the correct number of thermostats, e.g.:

<Value>3</Value>

4️⃣ Save and use the modified HPXML file.

2. Modeling Multiple HVAC Systems in OptiMiser Issue:

🔹 How do I model a home with two or more HVAC systems and thermostats?
🔹 What if one system has a programmable thermostat, but the other does not?

Solution:

OptiMiser is limited to single-zone modeling, so you have two options:

1️⃣ Model each zone as a separate unit in a multi-unit building:

• Requires multiple project files.
• Use Multi-Family controls to remove the boundary between zones from the thermal boundary.

2️⃣ Use weighted averages for thermostat setpoints:

• Calculate weighted averages using the % Load for each system as the weighting factor.
• This method still specifies a setback thermostat, but the setback reduction is adjusted accordingly.

3. Dual Fuel Heat Pumps in OptiMiser Issue:

🔹 How do I model a dual fuel heat pump (gas furnace + electric heat pump)?

Solution:

1️⃣ Specify two separate systems in the main interface:

• Assign the load distribution between the heat pump and furnace.
• Ensure the total load matches the dual fuel system's combined load.
• If an existing fuel-fired system is used as backup, set the improved system to No Improvement.
• Use COP and EER values for the heat pump efficiency if available. Otherwise, use HSPF and SEER.

2️⃣ Use the Advanced Heat Pump Model:

• Click the Advanced button in the heating system tab.
• Change Model Setup to Live unless you need the model for multiple heat pumps.
• Set the Backup control to the correct heating system and define the Heating Threshold (temperature at which backup heating kicks in).
• Click Copy Result and Close Window to apply the correct % Load split between the heat pump and backup system.

📌 Note: If modeling more than one heat pump, set Model Setup to Calculator mode. This method calculates annual efficiency values instead of dynamically linking them to the energy model.

4. Modeling a Boiler with Fan Coil & Indirect DHW Issue:

🔹 How do I model a boiler with a fan coil for ducted heating and an indirect tank for DHW?

Solution:

Use a workaround by treating the boiler as two separate systems:
1️⃣ Heating System Setup:

• Heat 1: 99% load on a Fuel-fired Furnace with ducted distribution.
• Heat 2: 1% load on a Fuel-fired Boiler.

2️⃣ DHW System Setup:

• DHW 1: 100% load on an Indirect Tank connected to Heat 2 (Boiler).

📌 Alternative Approach: Use the detailed interface to add a third system.

5. Adding a Mini-Split Heat Pump Issue:

🔹 How do I add a mini-split heat pump to an existing system without removing or changing the other heating systems?

Solution:

• Enter the mini-split as an additional heating system.
• Adjust the % Total-Fuel controls to distribute the load correctly between the systems.
• Use the detailed interface to modify settings accordingly.

📌 Boiler inside a garage?
If moving a propane boiler into conditioned space, adjust the boiler AFUE to reflect reduced standby losses. OptiMiser does not currently have a specific setting for this.

6. Modeling More Than 6 HVAC Systems Issue:

🔹 A home has 7 HVAC systems, but OptiMiser allows a maximum of six. What should I do?

Solution:

1️⃣ Combine systems with similar fuel types & distribution:

• Set % Total Load to the proportion covered by combined systems.
• Set Capacity to the total capacity of all systems combined.
• Use a weighted average for AFUE:AFUE_avg = (AFUE1 × Cap1 + AFUE2 × Cap2) ÷ (Cap1 + Cap2)
• Use total duct leakage for combined systems.

📌 If flexibility allows, combine systems with the most similar distribution types.

7. Modeling Ducted Wood-Burning Furnaces Issue:

🔹 How do I model a home that uses a wood-burning furnace connected to ducts for heating and also heats water?

Solution (Workaround for Wood as Fuel):

1️⃣ Select a heating system fuel type that is not used elsewhere.
2️⃣ Choose a system type matching delivery method (Boiler, Furnace, or Direct Heater).
3️⃣ Set efficiency:

• If unknown, check EPA Wood Stove Database or RESNET Standards.
  4️⃣ Modify fuel pricing:
• Set fuel units to $/MBtu.
• Convert wood fuel as follows:
  • 1 Cord = 20 MBtu
  • 1 lb = 0.008 MBtu

📌 Sidearm Water Heater Workaround:

• Use the same fuel type as the furnace.
• Estimate annual load split between the wood furnace and backup system.