Danfoss AB-QM: What It Is, How to Size It, AB-QM vs ASV

Short answer: The Danfoss AB-QM is a pressure independent control valve (PICV) that combines a differential pressure controller and a control valve in one body: it keeps the set flow constant even when system pressure fluctuates, and with an actuator it provides modulating control. It is sized by design flow, not by pipe size; you choose the AB-QM size that covers the flow and set the presetting as a percentage, calculated as design flow divided by the valve's nominal flow (Qnom).

This guide covers how the AB-QM works, the flow ranges from the Danfoss data sheet, the presetting calculation, actuator options and when an ASV differential pressure controller is the better choice. Technical values come from the Danfoss AB-QM and ASV-PV data sheets.

How does the AB-QM work?

The valve has two parts. The differential pressure controller keeps the pressure difference across the control valve constant through a balance of diaphragm and spring; whenever the available pressure changes or the control valve moves, the controller moves to a new position and restores the set difference. The control valve has a linear characteristic, and its stroke limitation sets the maximum flow. As a result the valve keeps full authority at every setting and the flow is unaffected by pressure changes.

According to Danfoss, typical applications of the AB-QM with an actuator are fan coils, air handling units, chillers, induction units, radiant panels and heat exchangers. Without an actuator the valve works as a flow limiter, for example in one-pipe systems.

AB-QM 4.0 flow ranges (DN15–DN32)

The values below are from the 2026.01 edition of the Danfoss AB-QM data sheet. Qnom is the nominal flow at 100 % setting; Δpmin is the minimum differential pressure at which the valve reaches at least 90 % of the nominal flow.

* The current data sheet gives 700 l/h for the standard DN15; earlier editions list 650 l/h, and some product titles still carry that figure. Order by code (e.g. 003Z8201). Threaded AB-QM 4.0 valves are PN25, with a maximum differential pressure of 600 kPa and a medium temperature of +2…+95 °C. For larger flows there are threaded DN40 (7,500 l/h) and DN50 (12,500 l/h) valves and flanged AB-QM from DN50 to DN250 (12,500 to 500,000 l/h, PN16).

How to size an AB-QM, step by step

• 1. Find the design flow. From the terminal unit's capacity: flow (m³/h) ≈ capacity (kW) ÷ (1.163 × ΔT). Example: a fan coil with 5 kW cooling capacity and a 5 K temperature difference needs about 0.86 m³/h, i.e. 860 l/h.
• 2. Pick a size that covers it. For 860 l/h both DN15 HF (1,200 l/h) and DN20 (1,100 l/h) work. At the same flow the HF version needs a higher Δpmin (25 kPa against 16 kPa); if pump head is limited, the size with the lower Δpmin has the edge.
• 3. Check the differential pressure at the index unit. The pump must deliver at least Δpmin plus pipe and unit losses at the most remote valve, and 600 kPa must not be exceeded across any valve.
• 4. Calculate the presetting. Presetting = design flow ÷ Qnom. In Danfoss's own example, 720 l/h on a DN15 HF gives 720 ÷ 1,200 = 60 %. In our example a DN20 gives 860 ÷ 1,100 ≈ 78 %.
• 5. Verify the flow. For DN15–32, flow measured via the test plugs is within ±10 % in the 20–100 % setting range; Danfoss advises not to readjust the setting when results fall within that tolerance.

How is the presetting adjusted?

The factory setting is 100 %. Remove the blue protective cap or the mounted actuator and turn the pointer to the new value: clockwise reduces the flow, anticlockwise increases it. On DN15–32 the scale runs from 0 to 10 (10 = 100 %), and no special tools are needed. Danfoss does not recommend modulation below 3 % of Qnom.

Choosing an actuator

DN15–32 AB-QM valves have an M30×1.5 actuator connection. The choice depends on the control signal and the building management system. The options on our site are in the valve actuators category.

AB-QM or ASV?

Both provide automatic balancing, but they control different things. The ASV-PV is a differential pressure controller: it is fitted in the return pipe, connected by an impulse tube to a partner valve in the supply pipe (ASV-BD, ASV-M or ASV-I), and keeps the differential pressure of a riser, a horizontal loop or an underfloor heating manifold constant. According to the Danfoss data sheet, ASV-PV setting ranges are 5–25 kPa or 20–60 kPa, with a factory setting of 10 or 30 kPa. Radiator flows are then limited by presettable thermostatic valves such as the Danfoss RA-N.

In short: for risers feeding many small radiators with thermostatic valves, use an ASV; for fewer, larger units under actuator control, the AB-QM is the right choice. Both families and other options are in the dynamic balancing valves category.

Installation and commissioning notes

• DN15–32 AB-QM valves can be installed in the supply or the return pipe, and the valve has a service shut-off function.
• When a DN15–32 valve is mounted with the actuator on top, air can be trapped on the control valve side; in the rare case of hammering noise at low opening, follow Danfoss's de-airing steps.
• Water quality should meet VDI 2035 or BSRIA BG29/BG50.
• The current price is shown on the product card, with the VAT-inclusive price first and the VAT-exclusive price below it. Technical support is available for project lists of Danfoss products.

Frequently Asked Questions

Is the AB-QM installed in the supply or the return?

Either. The Danfoss data sheet states that DN15–32 and DN125–250 valves can be installed in the supply or return pipe for the service shut-off function.

Can the AB-QM be used without an actuator?

Yes. Without an actuator the AB-QM acts as a flow limiter that never exceeds the set flow; one-pipe systems are a typical example. For DN40–100, a separate handle accessory is required when no actuator is fitted.

Do I need a special tool for presetting?

No. Remove the cap or actuator and turn the pointer to the required percentage. Presetting = design flow ÷ Qnom; the factory setting is 100 %.

Is the DN15 AB-QM rated 650 l/h or 700 l/h?

The 2026.01 edition of the Danfoss data sheet gives 700 l/h for the standard DN15, while earlier editions give 650 l/h. When calculating the presetting, use the Qnom marked on the valve and in the current data sheet.

Which actuator connects directly to the BMS?

The NovoCon range, which communicates over BACnet MS/TP and Modbus RTU. Modulating actuators such as the AME 110 NL use analogue 0–10 V or 4–20 mA signals.

Prepared by the Novus Mühendislik ve Enerji Sistemleri technical team · Updated: November 2026