A useful medium-efficiency filter RFQ fixes four things before a supplier is asked for a price: which air-handling unit and filtration stage the filter serves, the required particle-efficiency classification with the test method it refers to, the airflow per filter with the resistance data you want at that airflow, and a dimensioned drawing of the rack or holding frame the filter has to seat in. Without those, two bids can both say “medium efficiency” and describe different media grades, different depths and different pressure penalties. The exception is a controlled replacement specification that already fixes the duty, rating basis and geometry — in that case, quote its document number and revision instead of rebuilding the detail.

Define the filter duty and rating basis

Name the equipment the filter serves before naming the filter. A line that reads only “medium-efficiency filter, quantity 40” tells a supplier nothing about whether the item is a second-stage filter protecting a downstream HEPA bank, a return-air filter in a recirculating unit, or a standalone panel in a ducted branch, and each of those duties is normally built differently. Identify the air-handling unit tag, the filtration stage within that unit, the duty it performs, and the required particle-efficiency classification together with the test basis that classification refers to.

The test basis matters as much as the classification itself. ASHRAE Standard 52.2 covers laboratory testing of general-ventilation air-cleaning devices by particle-size efficiency and reports results through a minimum efficiency reporting system; its purpose and scope do not extend to HEPA or ULPA classification, to installed verification in a cleanroom, or to predicting service life. So a rating written against that standard tells you what the filter achieved in a defined laboratory test, and nothing about how long it will hold that performance in your air stream. A bare phrase such as “medium efficiency” with no stated test method leaves the supplier free to price whatever construction fits that label commercially.

A common specification mistake is to copy an efficiency grade from an old drawing without the standard it was written against. If the required stage or grade is still open — because the downstream HEPA protection target or the fan energy allowance has not been settled — resolve that first. Fixing it inside the RFQ, under commercial pressure, tends to produce bids built on different assumptions. Where a controlled replacement specification exists, reference it by number and revision and state that deviations must be declared.

State airflow and pressure-drop conditions

Give the airflow per filter and its units, not only the total airflow through the unit. A supplier who knows the unit handles a given total but not how many filters share it, or what face area each one has, cannot calculate face velocity, and face velocity is what drives both the clean resistance and the rate at which resistance climbs as the filter loads. State the number of filters and the installation space available for them in the same place.

Then request the clean pressure drop at that stated airflow, plus the changeout pressure basis the supplier proposes, so the site can review it against its own maintenance practice. Resistance quoted at a convenient airflow rather than yours is the point where comparisons quietly break down: a filter rated at a lower airflow will always look better on paper, and the difference is absorbed by the fan. If the fan pressure budget in an existing unit is already tight, a deeper or denser filter that meets the efficiency requirement can still fail to deliver design airflow once installed. That is a selection consequence, not a filter defect, and it is avoided by asking every bidder for resistance at one declared operating point.

Variable-flow service changes the data request rather than removing it. Where a unit modulates — for example an air handler serving zones with changing occupancy or load — a single nominal point hides what happens at the extremes. Identify the operating range and the airflow points that actually decide the selection, such as the maximum design airflow and whatever minimum the control scheme allows, and ask for resistance at those points. The changeout pressure basis still applies, and it still belongs to the site to review and accept rather than to the supplier to impose.

Document rack dimensions and replacement access

Attach a dimensioned drawing of the rack or holding frame. The nominal face size that appears in a catalog — the number people quote when they say “a 24 by 24” — says nothing about the seating and access dimensions that decide whether a filter fits and can be changed. The drawing should show width, height and depth, the frame or header geometry, where the seal sits and against which surface it bears, the airflow direction, and the clearance available to withdraw the filter. Where the filter is a pocket or bag type, include the expansion clearance the pockets need when inflated under airflow, because a pocket filter that fits the frame can still foul ductwork or coils downstream once the pockets open.

Seal location deserves its own note. A header-framed filter that seals on its upstream flange and a box-framed filter that seals on a gasketed face are not interchangeable in the same rack even when their nominal dimensions match, because the clamping arrangement bears on a different surface. Showing the clamping or latching hardware on the drawing lets the supplier confirm compatibility instead of assuming it.

Replacement access is part of the specification, not a site problem discovered later. Record how much room exists in front of the rack, whether the filter is withdrawn from the access side or the clean side, and whether the handling path is obstructed by coils, dampers or service platforms. A supplier-approved replacement drawing can carry all of this, provided its revision is current and the rack it applies to is clearly identified in the RFQ.

Describe the operating environment

Efficiency classification says what a filter removes; it says nothing about whether the filter survives the air stream it is placed in. Report the expected temperature and relative humidity conditions, any moisture exposure such as carryover from an upstream cooling coil or a humidifier section, the contaminants the filter will see, and any cleaning or disinfection contact the installation may expose it to. Then ask the supplier to identify suitable media, frame and seal materials and to state their operating limits.

Those limits are the part that governs the purchase. Media binders, frame adhesives, cardboard or particleboard frames, and gasket elastomers each have temperature and humidity boundaries, and a filter run outside them can lose structural integrity or shed material downstream — which matters most when the filter is protecting a terminal HEPA bank that is expensive and disruptive to replace. Where oily aerosols, solvent vapors or corrosive species are present, say so, because they affect frame and media choice rather than efficiency grade.

Do not invent conditions to complete the form. If the humidity range after a planned coil change is unknown, or the cleaning agent for the plenum has not been selected, list that condition as an open quotation assumption and ask the supplier to state what their offer assumes. An unknown recorded as an assumption stays visible through bid comparison; an unknown replaced with a plausible design value disappears, and the mismatch only shows up in service.

Build a tagged filter schedule

A filter schedule is a table of line items, one per distinct duty, that lets a bid be compared item by item rather than as a single total. Each line carries the quantity, the equipment tag the filter serves, the governing drawing and its revision, the efficiency classification with its test basis, and the airflow per filter at which resistance is to be quoted. Add a column for the supplier’s quoted clean resistance and proposed changeout basis so returned bids land in the same structure they were issued in.

A line might read: AHU-02, second-stage pocket filters, quantity six, rack drawing reference and revision, required classification and test standard, airflow per filter as stated, resistance to be quoted at that airflow. Anything about that line which differs from another — a different rack geometry, a different stage, a different air stream — makes it a separate line, even if the nominal face size is identical. Identical items may share one line, as long as every equipment tag it covers and the total quantity are both written out.

The reason for this discipline is traceability through the bid. When several duties collapse into one lump-sum price, a supplier can price the easiest construction across all of them, and the substitution is invisible until delivery. Separate, tagged lines force any substitution to appear against the line it affects.

Separate quotation scope and alternatives

Ask for a price breakdown, not a single delivered figure. State the delivery destination and the date the filters are required, then request the filters, packing, any test records you want supplied, samples, spare filters and any installation support priced as separate items. Delivery terms and inclusions routinely move more money than the unit price does, and a bid that looks cheaper per filter can arrive more expensive per installed set once freight, packing and spares are added. A budget enquiry can proceed on stated assumptions, but any scope left unresolved has to remain visible on the comparison sheet until an order is placed.

Alternatives need the same treatment as the compliant offer. If a supplier wants to propose a different construction — a different depth, media type or frame — require it as a separately identified alternative carrying its own changed dimensions, efficiency basis and test method, airflow resistance at your stated points, replacement access implications and price. A good alternative can genuinely improve fit, lead time or energy cost; what it must not do is appear unmarked inside the compliant line. Where the purchase is an exact replacement under an existing equipment specification, say in the RFQ that alternatives are not accepted, so nobody spends effort proposing one.

When bids come back with different rating systems or test standards, keep each proposed rating beside the test method and test record that support it, and have the required performance compared on the basis your specification named. There is no verified conversion between a general medium-efficiency label and a particle-size laboratory classification, so two ratings expressed differently are not equivalent until someone competent has compared them on a single stated basis. If the project has already accepted a comparison basis for earlier packages, it can govern here too, provided its applicability to the proposed filter is confirmed. Acceptance of an alternative, like acceptance of a changeout pressure, sits with the project’s approved procedure and its designated approver.

Before issuing the RFQ, have the air-handling unit tags and stage identification, the required classification with its test standard, the airflow per filter and operating range, the dimensioned rack drawings at their current revision, the temperature, humidity and contaminant conditions, and the delivery destination and date assembled in one package. Issuing with any of those missing produces bids you cannot compare, and a second round of clarifications costs more time than the first round saved.

Frequently Asked Questions

Q: Can I request a quotation before all operating conditions are confirmed?
A: Yes, for a budget enquiry, provided the open conditions and quotation assumptions remain explicit. Identify which temperature, humidity, moisture or cleaning conditions are unconfirmed, and require the proposed construction and its operating limits to be stated against those assumptions. Resolve any assumption that affects suitability or scope before placing the order.

Q: Can filters for several air-handling units share one RFQ line if they have the same nominal size?
A: Only if they are otherwise identical for the purchase. Compare the required classification, airflow, rack geometry and construction requirements before combining them, then retain the applicable equipment tags and total quantity. Use separate lines where those requirements differ so a shared face size does not conceal different duties.

Q: Do I need a new drawing every time I reorder a replacement filter?
A: An existing supplier-approved replacement drawing can be referenced when its revision and applicable rack are clear. First check whether the rack, seal arrangement or replacement access has changed. If the approved drawing no longer represents the installation, provide the changed details rather than relying on the previous order or nominal dimensions alone.

Q: Can I accept a supplier’s claim that a differently rated filter is equivalent?
A: A claimed equivalence is not enough to accept it as the specified offer. Keep the proposed rating with its test method and supporting test record, then compare it on the project’s required performance basis. If it remains an alternative, keep it separately identified with any changes to fit, resistance, replacement access and price visible for review.

Q: How should I compare a low unit price with a higher quotation that includes more items?
A: Compare both offers against the same requested delivery and supply scope before ranking their prices. Reconcile filters, packing, requested test records, samples, spare items and installation support as applicable, using the same destination and required date. Keep any unpriced or assumed item visible; an apparent saving is not a settled comparison while the included scope differs.