For a fuel-fired greenhouse unit heater, ask two questions separately: where do the combustion products go, and where does the combustion air come from? An exhaust fan or an outdoor flue answers only part of that comparison. A quotation saying “power vented” does not, by itself, document an outdoor combustion-air supply.
Before comparing price, have the supplier identify the exact model and its air-path architecture. The greenhouse's circulating-air fan is a third path and should not be mistaken for either the combustion intake or the exhaust.
Trace three air paths through the offer
A conventional vented unit heater transfers heat through a heat exchanger and sends combustion products outdoors through its approved exhaust arrangement. Power venting describes mechanical assistance in moving the combustion gases. Separated-combustion equipment additionally uses a manufacturer-designed separate combustion-air intake.
Wisconsin Extension's revised 2011 unit-heater publication explains these distinctions. The architecture remains useful for reading offers; its historical prices and efficiency examples are not current purchasing estimates.
| Air path | Question for the exact model | Insufficient evidence |
|---|---|---|
| Combustion-air supply | Does the approved design take it from inside the greenhouse or separately from outdoors? | A fan icon or the words “gas heater” |
| Combustion-product exhaust | Where does the approved exhaust arrangement discharge? | A statement about intake alone |
| Heat-distribution air | How does greenhouse air pass across the heat exchanger and return to the growing space? | Assuming this circulation supplies outdoor ventilation |
The heat-distribution path moves the air the crop experiences. In a vented heat-exchanger design, it is distinct from the intended combustion circuit. A separate outdoor combustion intake serves combustion; it is not a substitute for the greenhouse ventilation design.
Read the manufacturer's architecture statement
As a named commercial example, Modine describes its North American HDS/HDC separated-combustion range as drawing outside air for combustion, and lists greenhouses and nurseries among its applications. The same page distinguishes propeller-fan and blower models. This shows why “it has a fan” is too vague to classify the heater. See the HDS/HDC product description.
That description does not select a model for a particular greenhouse or establish its suitability in another market. Request the current documentation for the offered model, fuel and destination. Have the supplier identify which vent and intake components are included and which remain the responsibility of the qualified local designer or installer.
If the quotation names only the heater body, leave the package-completeness question open. A documented architecture and a complete, locally assessed system are different purchasing milestones.
Resolve two similar-looking quotes
In a fictional tender, Quote A and Quote B both advertise fan-assisted exhaust. A confirms discharge outdoors but says nothing about the combustion-air source. B supplies a model-specific statement showing a separate outdoor combustion-air path.
Record A's intake as unknown. Record B's intake as documented, with the relevant manufacturer reference. Do not infer that A has the same intake because its casing or circulating fan resembles B. Nor does B's clearer documentation prove that the proposed site package is complete.
The next action for A is a written model-specific clarification. For B, send the documented architecture and package scope for qualified site review. Neither quotation supports a do-it-yourself intake retrofit.
Describe the greenhouse conditions that matter
Give the heating professional a realistic operating description: closed-house periods, use of exhaust fans, and humid, dusty or potentially corrosive conditions. These affect the review of combustion-air supply and equipment suitability. Do not resolve them by improvising openings or assuming that a power exhauster compensates for every pressure condition.
Exhaust destination also matters for exposure. UMass discusses water vapour and potentially harmful combustion by-products from unvented heaters in its unvented-heater guidance. Treat that as a reason to settle the intended air paths before purchase, not as a way to obtain “free” crop carbon dioxide. No air-path label removes all combustion risk.
This comparison is for selection. Vent and intake sizing, terminations, gas work, wiring and commissioning require the actual manufacturer instructions and locally authorized professionals. If the issue is an operating fault, do not run the unit to test whether plants react.
Download the three-path offer map
Download the unit-heater air-path comparison (CSV). The file is a manual record with no formulas. Import as UTF-8, comma-separated text, fill the two offer columns, and use “unknown” where the supplier has not supplied evidence. Keep the three air-path rows separate. Enter the model-document reference beside each conclusion and name the qualified site reviewer. Adjust widths and wrapping, then save a native spreadsheet copy.
A useful completed comparison makes every connection to greenhouse air explicit and identifies the package questions still preventing an informed order. It is not a code-compliance certificate.
Heating capacity is a separate selection step. See commercial greenhouse heating-load design inputs.