WSHP Diversity - Central Equipment

[ME27272727]

When applying a diversity factor to water source heat pump systems, let's assume 80% for this example, do you apply this factor to pump flow (variable primary pumping)? I've been told by a grey-haired engineer to size pumps for total connected load, and to apply diversity factor to boilers / cooling towers water temperate "range" and heating/coolingn capacity, but not the water flow rate. I can see this need for older constant speed pumping / 3-way valve controlled systems, but I don't see the need on a variable primary pumping system. If I'm missing something, I'd apprecaite your feedback. Thank you.

 

[nuuvox000]

Honestly, I would size it for full-load and then let the boilers/cooling towers turn things down to maintain target temperature. Usually this type of system is used for a large plant or campus so if your pipe runs are very long then you wouldn't want the water to move through the pipe too slowly. Also, less flow may result in less pressure at the ends of branches so it might affect the balancing.

In my opinion I would select the quantity and size of pump(s) that would be the most efficient configuration and then let it(them) run at that speed all of the time.

 

[EnergyProfessional]

Without knowing more about the specific design..... I'd size hydronics for the full load and apply variable flow. there will be a time they may start up at once adn then need full flow for a minute. Unlike a heating coil, heat pumps are sensitive to too little flow and may trip. If you have too little flow in a coil, then you just have less capacity for a minute, but for a HP, you may have to manually re-start.

 

[willard3]

You should read ASHRAE on water-source heat pump diversity factors. ASHRAE says that, after night setback, boilers and chillers will have to run at total load conditions, ie, no diversity.

 

[GT-EGR]

You have to get your diversity a little more specific in order to make an engineering decision.

There’s the diversity that represents the maximum simultaneous usage of all connected devices in the same mode (cooling or heating) peaking at the same exact time. So for example if you came to the conclusion that they realistically never have more than 90% of the spaces fully occupied in cooling mode (different sun exposures alone can give you that), then that’s the most load you’d see in the plant.

Heating diversity in these systems works differently, all the rooms that have heating would need heat at the same time, although possibly some at a lower setback temperature - but if you have interior space and you reasonably predict your minimum cooling while in full heating mode than that could technically offset your boiler.

The pump flow is specifically to satisfy opening valves, which is why the gray hair knows it’s so hard to determine that one (predicting the exact timing and duration valves open and close regardless if heating or cooling demand). Realistically let the pump VFD do that work, and turndown on a pump isn’t as complicated or cause as many issues as oversizing the chiller or boiler. But all that simultaneous max flow to every unit likely does not necessarily equate to a full delta T of cooling or heating drawn from that same equipment - based on what I typed in the previous 2 paragraphs.