A heat pump described as 100 kW may not deliver 100 kW on the night that matters most. Rated output depends on the outdoor condition and the temperature it must supply. Comparing nameplates without those conditions can leave a greenhouse with an expensive capacity gap.
Match the equipment's documented output to the greenhouse load at the same design point. For an air-to-water system, include the required water supply temperature. Then account for defrost, auxiliary equipment, backup heat and the electrical service needed when those loads coincide.
Put demand and output on the same basis
The greenhouse load should come from its envelope, leakage, ventilation strategy, indoor requirement and local weather basis. A supplier's floor-area rule cannot capture all of those variables. Assemble the heating-load design inputs before asking for machine capacity.
The US Department of Energy's cold-climate sizing resource emphasizes equipment selection against heating demand and operating conditions. Its building applications do not establish greenhouse loads, but the need to match conditions is equally important when reviewing a greenhouse proposal.
Request a performance table, not one headline number
| Comparison row | Required information | Why it matters |
|---|---|---|
| Cold design condition | Delivered heat and input power at the selected outdoor and supply temperatures | Reveals whether peak demand can be met |
| Moderate weather | Minimum and maximum output, modulation and cycling behavior | Shows whether small loads are manageable |
| Defrost operation | Capacity accounting, duration assumptions and support from storage or backup | A steady-state rating may omit interruptions |
| Backup mode | Output, fuel or electrical input and switching logic | Determines both crop protection and peak utility demand |
| Operating limit | Permitted outdoor range and supply-temperature envelope | Identifies conditions where the unit cannot provide the requested duty |
Check the proposed emitters
A heat pump connected to an existing high-temperature pipe system may need a different operating point from one serving low-temperature bench heat. Lowering water temperature without checking emitter output can reduce the heat delivered to the greenhouse, even if the heat pump itself operates efficiently.
Ask the designer to show that the emitters can transfer the required heat at the proposed supply and return temperatures. Include circulation-pump power and hydraulic balancing. A new heat source cannot correct inadequate distribution by nameplate capacity alone.
Use COP carefully in the operating-cost calculation
The coefficient of performance, or COP, is useful heat delivered divided by electrical energy consumed over a stated boundary and period. In a hypothetical operating interval, 90 kWh of heat delivered using 30 kWh of electricity gives a COP of 3. That does not establish the annual value or include equipment outside the measurement boundary.
Compare proposals using the same boundary: compressor only, complete outdoor unit or the whole heating system including pumps and backup. Apply the local tariff, demand charges where applicable and realistic operating hours. Do not multiply the best catalog COP by every winter hour.
Plan the cold-night handover
Define what happens when outdoor conditions fall beyond the selected design basis, a unit faults, or power is interrupted. Who receives an alarm? Which zones retain priority? Can the backup start without exceeding the electrical service? These questions belong in the control sequence before commissioning.
Give CFGET the local climate basis, required crop temperatures, existing distribution drawings and utility constraints. Request a point-by-point capacity comparison and a witnessed commissioning plan. Initial mild-weather operation can verify controls and flow, but cold-condition capacity remains a documented design calculation until conditions or an appropriate test actually establish it.