Two greenhouses of the same size can behave like different buildings. In the first, the shutters sit at one end and the exhaust fan at the other; in winter the plants nearest the inlet take a cold blast, and in summer the far end of the house stays stale while the fan runs. In the second, the intake sits high in the gable, baffles bend the incoming air, and a thermostat or humidistat decides when the fan stages step up. Same square footage, opposite complaints.

In a retrofit, inlet height and control logic should be settled before anyone quotes a fan size. A bigger or more efficient fan moves more air, but it does not decide where that air lands or when it runs; the inlet path and the controller do. Treat fans, inlets and controls as one system and the shopping list gets shorter, not longer.

Map the Air Path on Day One

Start with a walkthrough and no tools. Find the exhaust fan wall and note which wall it is on. Walk the opposite wall and mark every shutter, louver and inlet, then note how high each one sits relative to the crop, whether it opens into the gable or at bench level, and whether a baffle already deflects the incoming air. Note the sensor location too, because a thermostat hanging near the covering is reading the building's weather rather than the crop's.

Sketch it on one page: fan wall, inlet wall, an arrow for the air path, marks for baffles and sensors. Skip that page and every later purchase is made blind, because you will compare fans without knowing where the complaint actually happens. The sketch is not a heat-load calculation and it sizes nothing; it fixes the geography you are trying to change.

Write the Selection Brief on Day One

Before you open a catalog, write one page of facts about your own house. Copy the make and model off the label of the existing fan and the existing control, since both will be asked about. Record the inlet type and its height, the crop you grow, and the season when airflow is worst. Name the complaint precisely, whether that is heat, humidity, condensation or cold draft on leaves, because each points at a different part of the system. Add a budget ceiling so the brief stays a brief.

Record the greenhouse dimensions if a vendor asks. Do not turn those dimensions into a fan size yourself. The brief is a purchasing document, not a load calculation, and the difference shows up in the conversation: a supplier who hears only a floor area will quote a fan, while a supplier who hears the complaint, the inlet height and the control model has to talk about the air path. Without the brief, the discussion slides straight to a replacement fan and the real problem never gets named.

Upgrades to Schedule in the First Week

Order of work matters more than the parts list.

Three upgrades fit into almost any existing house in the first week, and each has a specific effect. Raising the intake into the gable and adding baffles to deflect the incoming air reduces cold drafts on plants; that is the remedy extension guidance gives for houses with shutters at one end and an exhaust fan at the other. Putting fan operation under thermostat or humidistat control through the greenhouse control system makes the fans respond to measured temperature and humidity instead of a hand switch or a fixed timer. Swapping a cheapest-tier fan for a higher-efficiency model may cut energy use, but the result depends on the facility and the climate, and the case has to rest on the exact model's ventilation efficiency ratio rather than its diameter. Do these in reverse order, new fan first and inlet and control later, and you can finish the week with the same draft and the same humidity swing, having spent the money.

Correct the Inlet Path in Week Two

The failure mode is well documented: shutters at one end of a house and an exhaust fan at the other can produce cold drafts on plants during winter ventilation. The mechanism is plain. Air enters as a narrow, fast jet at whatever height the opening sits. If that height is at or near the canopy, the jet crosses the plants before it has mixed with warmer house air, and the leaves on that side feel the outside temperature. Placing the intake high in the gable and using baffles to deflect the incoming air gives the jet room to blend with house air above the crop, so what reaches the canopy is a mixed stream rather than a shaft of cold outside air.

Treat that as a design principle, not a promise. Gable height, roof pitch, house width and fan position all vary, and a tall narrow house mixes differently from a wide low one. What the principle buys you is a question worth asking before another fan is ordered: where will the replacement pull its makeup air from, and at what height does that air enter? If the answer is a low shutter at the far end, the fan is not the problem.

Compare Fan Efficiency by Week Three

Fan catalogs are written to make comparison hard. Two units with the same diameter and the same horsepower can perform differently, and the cheapest fans on the market are often the least energy efficient. Larger fans tend to be more efficient than smaller ones, but the spread between individual models is wide enough that size alone tells you little.

So ask for the number, not the description. Request the ventilation efficiency ratio for the exact model you are considering, from the manufacturer or from published product data, and compare ratios instead of diameters. Two fans that clear the same ratio can then be weighed on price, warranty and service access. No brand or model recommendation comes out of this exercise; the point is that the ratio exists, that it is model-specific, and that it is where the real differences sit.

Give Controls a Real Sequence by Week Four

A control that is wired but not sequenced behaves like a switch with extra steps. The working arrangement is simple: fans are activated by a thermostat or humidistat and managed by the greenhouse control system to hold proper temperature and humidity. In warm periods the system has to pull enough air through the house to complete an air exchange within a short interval, and fans may be combined with a cooling pad for evaporative cooling, with or without water running over the pad.

Use that description as a checklist against your controller's manual. Confirm that a temperature signal and a humidity signal both reach the controller, that fan stages are assigned in a deliberate order, and that pad operation is part of the same sequence rather than a separate manual switch. Check the air-exchange interval against your own house before designing around it; the figure quoted in guidance comes from specific conditions and should not be copied onto a different structure without checking. Without a written sequence you cannot tell whether the fan or the controller is causing a temperature swing, and you will replace the wrong part.

Review the Results One Month Later

Suppose you made the inlet and control changes and let the house run for a month. What would tell you whether they worked? Walk the canopy on the inlet side on a cold morning and look for leaf edge burn, cupping or condensation on upper leaves, signs that a draft may still be reaching the crop. Check the far end of the house on a warm afternoon for still air and humid pockets. Compare fan run time with the controller's log, and watch whether humidity climbs before the fans react or only after it has already peaked.

If cold-draft complaints persist, go back to inlet height and baffles before adding fan capacity. If the house stays humid while temperature looks fine, revisit the humidistat setting and the staging rather than the fan. Judging the retrofit on temperature alone is the usual mistake, because a humidity problem that a humidistat is meant to catch never shows up on a thermometer. This is an observation plan, not a measured performance claim; the readings are for your own decisions.

Choose the Next Purchase One Month Later

By this point the air path and the controls are settled, and the question is what to buy next. The answer is not automatically a larger fan. Check the existing unit first: find its model-specific ventilation efficiency ratio and confirm it runs in the condition it was designed for, with a clean shutter, an unobstructed outlet and a belt that is not slipping. Fan performance varies between models, and any savings from a swap depend on the facility and the climate, so the ratio plus the unit's actual condition is the material you need.

If the ratio is poor and the unit is one of the cheapest models on the market, replacement is a reasonable buy. If the ratio is respectable, keep the fan and put the next dollar into inlet hardware or a controller with proper staging. Replacing a fan before the air path is corrected buys airflow that still lands in the wrong place.

Can you just replace the fan? You can, and a higher-efficiency model may well use less energy doing it. But if the intake still sits low and the whole house still runs off one thermostat, the cold draft on the inlet side and the humidity swing at the far end will still be there once the new fan is bolted in. Fix the inlet and the control sequence first, then let the ratio pick the fan.