Shared Boilers with Water Loop Heat Pumps

Caution: This article describes a work-around for modeling this system which is not fully supported by Ekotrope yet. This work-around might not provide an accurate analysis for ENERGY STAR Multifamily New Construction in Ekotrope version 4.0.0 or later, but we believe it to be accurate for all other analyses in Ekotrope. Buildings permitted after January 1, 2022 should be using at least version 4.0.0. Please email support@ekotrope.com if you need to model this type of system for ENERGY STAR Multifamily programs. We will not be able to support it immediately but knowing that you need support for this system will help us prioritize its development.

If heat is distributed to the dwelling unit from a Shared Boiler using Water Loop Heat Pumps:

Model 2 separate heating systems:

1. For the Heat Pump:

   1. % Heating Load = [Percent of Load served by the combined system] / COPheat pump

   2. Equipment Type: Custom

   3. Efficiency = 1 COP

   4. Capacity = [Design Load Served by Combined System] / COPheat pump

2. For the Boiler:

   1. % Heating Load = [Percent of Load served by the combined system] x (1 - 1/COPheat pump)

   2. Equipment Type: Boiler

   3. Efficiency = AFUE as rated by manufacturer

   4. Capacity = [Design Load Served by Combined System] x (1 - 1/COPheat pump)

   5. EAE = 2080 x SPkW/Ndweq

      1. SPkW: shared pump power in kW

      2. Ndweq: number of Dwelling Units that share the pump

For example, if the system serves all the heating load for a dwelling unit with a 40 kBtu/hr heating load, having a 4 COP heat pump and 90 AFUE boiler with a 5 kW pump serving 10 units, the inputs should look like this:

Wondering why the system should be input this way?

A heat pump’s primary function is to move heat from one place to another. In this system the heat pump is moving heat from the water that has been heated by the boiler into the Dwelling Unit. Unlike more common air- or ground-source heat pumps, the heat source isn’t ‘free’, energy is being used to supply the heat pump.

A heat pump’s Coefficient of Performance (COP) is the ratio of heat output to electricity input. In the example shown above the heat pump has a COP of 4, which for every 1 BTU of electricity used, 4 BTUs of heat are produced. 3 of those BTUs are pumped from the water that’s heated by the boiler, while 1 is from the electricity consumed by the heat pump.