Common Issues when installing heat pumps

Common Issues
when installing heat pumps

Introduction

Many industry professionals believe that designing a system with air-to-water heat pumps is the same as designing a system for air-to-water chillers. Although there are similarities, there are some key differences. Please review below for common issues in the field that have caused units to break, not to perform properly, etc.

Water Volume

Scenario 1

Cause: We believe we have sufficient water volume based on the system size however, when many zones close the size of the available volume can shrink. As a result, there may not be enough water volume. When the system size with the smallest zone open is not enough volume a buffer tank must be added.

Effect: Lack of water volume may be the result of too small system piping or a result of buffer tanks being too small as they may be sized for comfort cooling in a commercial application. Commercial systems require more volume than comfort systems.

Scenario 2

Cause: Lack of water volume due to buffer tanks on the drawings being forgotten.

Effect: Lack of water volume will cause units to short cycle and eventually compressor failure. Ensure maximum and minimum loads are noted.

Scenario 3

Cause: System sized with maximum water temperatures

Effect: System may not be able to supply the maximum temperature, as the system may not be installed perfectly as well as changing conditions. Refer to the benchmarks below:

● Design the system at the sweet spot with radiant heat and stay between 95 and 105°F
● Fan coils VAV try to stay between 110-120°F

Radiant heat is very efficient at the above temperatures. It is best to size VAV / Fan coils at the lower temperature as it is not ideal to be operating the heat pumps at the maximum all the time.

Scenario 4

Cause: Operating units when there is little or no load as the system will cycle, this often occurs on startup or commissioning. If the heat pump is oversized, meaning the capacity output from the unit cannot be transferred, the unit sees it as a low load condition.

Effect: In heating, set point will be met very quickly and may even surpass it causing high temperature alarms. The compressors will short-cycle to come on again frequently.

For a chiller in cooling the set point quicker will be achieved quicker and may even surpass and hit the antifreeze alarm. The compressors will short-cycle to come on again frequently. With this happening, slowing the staging of compressors is needed.

In addition, it will be necessary to limit the unit to have limited compressors available to match the building load (only on units with multiple compressors). More water volume (for conditions of process or low load) will also be required to ensure minimum runtime is met and exceeded to avoid short cycling.

Voltage

As the units lock out to protect themselves. Ensure the system voltage is within 10% of the units design voltage or there will be electrical faults and abrupt stopping.

Scenario 1

Cause: Bad electrical system in the building

Effect: Lose compressors due to rapid restarting and stopping.

Scenario 2

Cause: Oversized pumps or hydronics are not balanced.

Effect: Includes the following:

● High water flow
● Lower Delta T leading to quick (shorter) staging of compressors
● Shorter duration of compressor run time
● Noisy check valves which may eventually break and damage other system components
● Higher pumping costs
● Heat exchanger degradation

Scenario 3

Cause: Low water flow rate on primary loop, incorrect pump size, dirty strainer, hydronic system not balanced or designed improperly.

Effect: Includes the following:

● Higher Delta T leading to slow (longer) staging of compressors
● In cooling mode: Low pressure or anti-freeze alarm
● In heating mode: High pressure or high water temperature

Correct flow rate on primary and correct pump selection and timing leads to correct Delta T with compressors staging correctly.

Flow

Flow is a major concern when it comes to heat pumps. Air to water heat pumps are required to run at constant flow on the primary side. This is especially important during defrost, when variable flow can cause serious damage to the heat exchanger.

The recommended Delta T for heat pumps 10°F. If the minimum (5.4°F) or the maximum (18°F) is used and the flow is low or high there will be problems with low Delta T or high Delta T.

Scenario 1

Cause: The heat exchanger is undersized.

Effect: When sizing heat exchangers it is vital to ensure that the heat exchanger is able to reject the maximum amount of heat that the heat pump is able to supply.

For example, if the heat pump is able to supply 500,000 Btus at 80°F and 300,000 Btus at 15°F then the heat exchanger must be sized to handle 500,000 Btus if the unit will be operating at 80°F for domestic hot water. If a smaller heat exchanger is needed, the load is 300,000 even at 80°F, please contact MitsAir as there are a few options to limit the compressors as the outdoor ambient increases.

Scenario 2

Cause: Resetting alarms.

Effect: Effect: One of the most serious problems in building management and control companies is contractors resetting an alarm. The unit alarms for a reason, it is protecting itself. The alarm gets reset and a few hours later it is back and reset again. In a few weeks or a month, there will be a failure (ex. High-pressure alarms on the heat pump, eventually compressor failure).

Scenario 3

Cause: Units installed with variable flow on the chilled water and/or hot water side.

Effect: There will be many alarms and issues as the result of compressors short cycling and operating modes constantly changing. Use constant flow only.

Scenario 4

Cause: Below minimum water flow rate for a given unit.

Effect: For example, for larger units, with 120°F LWT or higher, nominal heating flow rate must be used to prevent “low flow” issues. Other heat pumps can use any flow rate between minimum / maximum permitted in the technical manual.

Working Outside of Compressor Limits

Cause: Working at the compressor limit or outside the limits.

Effect: The limit of compressors are high compared to the operating limit of the unit because the approach of temperature (the difference between the refrigerant and water temperature) needs to be taken into consideration. This approach is usually 6-7°C and usually 2-3°C is added in order to be safe.

The compressor can work up to 65°C condensing temperature but the unit is limited at 55°C due to the approach temperature. Sometimes the unit will work up to 56°C or 57°C depending on the water flow and load. This is the reason that we take the 2°C to 3°C safety factor.

The approach is a physical part of the plate exchanger and cannot be reduced much for a few reasons. The flow of the refrigerant must reach a certain level to have proper oil return to the compressor, so if a larger plate exchanger is used then the flow will decrease, the distribution of the Freon will be low, and it may cause low oil back to the compressor. This is the reason why they cannot reach 140°F or is 60°C, just a small mistake in water flow or temperature reading will bring unit in alarm. Too many alarms cause compressor failure. Some manufacturers will allow this to happen as they only care about making it out of the one year warranty.