DDC Workbench Air handler

Read the economizer permit first, because it is two conditions and not one. A fixed high limit asks whether the outdoor air is cool enough to be worth using at all; a differential comparison asks whether it is cooler than the air already coming back from the building. Either one alone gets a unit into trouble — a fixed limit on its own will open the dampers on a mild humid morning when the return is cooler still, and a differential on its own will open them at ninety degrees just because the building is hotter. Both have to agree before the damper leaves its minimum position.

The minimum position is the other half of that block. When the economizer is not permitted the damper does not go shut — it goes to the ventilation minimum, because the building still needs outdoor air. That is what the select block below the permit is doing, and deleting it is a fault worth trying: the damper drops to zero, the unit stops ventilating, and absolutely nothing on the graphic goes red.

Airflow proof gates both coils, and the ordering is the field's. A discharge sensor sitting in still air reads the room, not the coil — so a low limit behind no proof does not merely fail to trip, it goes blind, reporting a comfortable number while the coil behind it could be anything at all. Break the belt from the scenario row and watch it: the fan command stays on, the blades stop, the chevrons freeze, and the DAT chip climbs to the zone temperature. What is deliberately NOT gated is the fan itself — gate a fan on the proof its own airflow produces and nothing could ever start. Command, status and proof are three different claims, and Status & Proof works through the difference. On a real job you rarely get all three — status is usually the only claim on the screen, so the work becomes deciding whether to believe it. That is what the ΔT well beside the discharge reading is for: a fan reporting ON with no split across a loaded coil is a status that is lying, and the temperatures are what catch it.

The heating valve is a proportional loop, not a stage: the error between the heating setpoint and the space runs through a gain and a limit block, so the valve sits wherever it has to sit to hold the space rather than banging open and shut. Watch the authority, with every NULL box checked so the program is in control: driving the outdoor-air slider down opens the valve further to hold the space, and far enough down is where it reaches a hundred percent and the room starts falling anyway. The scenario buttons will not show you this — each one puts its hand on all six outputs, so the valve holds wherever the scenario put it until you write NULL back.

The low-limits sample splices two winter protections into the same sequence, in series and in different styles: a mixed-air low limit that modulates, and a discharge low-limit stat that trips. The PID watches the mixed-air well against the Min MAT constant — warm mixed air parks its output high, and as the mixing box drags down toward the constant the output winds down with it. The MIN block after the economizer select then takes whichever is less: the position the economizer wants, or the most the low limit will allow. Drive it: outdoor air cold, cooling setpoint pulled below the space — drag it down in the rail — so the economizer opens hard, and the damper command settles well short of open while the MAT chip rides the constant. Colder still and the clamp passes below the ventilation minimum, because the MIN sits downstream of the minimum-position select on purpose. When freeze protection and ventilation argue, this sequence has already picked freeze protection — a real sequence writes that trade down, because someone will eventually ask where the minimum went.

There are two low-limit stats on this machine, and only one of them is on the sheet. The hardwired one is a manual-reset element strapped across the coil face and landed in the fan starter circuit; this unit carries one, set at thirty-eight. The comparator here is the software limit, and it sits three degrees above that, at forty-one — so the program is what acts first, in a way you can watch on the sheet and clear from it, and the hardware element stays the backstop it is meant to be. That pair comes from one engineer’s commissioning practice in the Northeast: the physical stat around thirty-eight, the limit in the program somewhere between forty and forty-two. Treat it as a worked example and not as a rule, because freeze protection is a climate question before it is a setpoint — what drives the number is the design outdoor-air temperature, how much outdoor air the unit takes, and whether there is a wet coil sitting in it. Somewhere warm enough, the argument is whether the stat goes on at all. What travels is the shape: know what your machine can legitimately produce, put the program’s limit above the hardware’s, and set neither by habit. Set this one too low and it becomes decoration — this unit’s two compressor stages bottom the discharge at thirty-five, and nothing in the program would ever see a colder number than that.

The reason a number down there matters at all is what it says about the rest of the machine, and that part is not regional. If the discharge really is riding the stat, the ductwork is sweating badly and the pressures on that circuit are not pretty — the stat is not trimming a setpoint, it is catching a unit that is already in trouble. Which is the argument for the spread: you would rather the program stopped the machine on a sheet you can read than have somebody find a popped button on a wet air handler.

A trip is the classic freezestat response, all at once: damper driven shut, hot-water valve driven open — water moving through the coil is the protection when nothing else is — and the fan stopped, with the compressor stages falling out on the airflow-proof interlock they already answer to. Then the trap this page keeps circling: the moment the fan stops, the discharge probe reads the room. Watch the DAT chip after a trip — it climbs to a perfectly comfortable number while the machine sits dead, which is exactly why the trip is held by a latch and not by the comparison alone. An unlatched stat would clear itself on its own blindness and bang the fan straight back on. The LLS Reset source is the reset button on the device: click it true and back once the cause is fixed and the machine restarts in order — fan first, proof made, then the loads. Hold it true instead and you have wired the button down: the latch is defeated, and the unit short-cycles on and off exactly as fast as each trip can clear itself.

There are two ways to defeat a low limit on this machine and they are not the same defeat, which is worth being exact about, because they read alike on a work order and look nothing alike on the roof. Wiring that reset input down defeats the latch: the trip still happens, it just cannot hold, so the unit short-cycles — down, clear, back on, down again, all morning. The jumper on the device face defeats the contacts: the trip still happens and still holds, and the fan never drops at all. One machine bangs on and off where anyone can hear it. The other runs straight through a freeze point looking perfectly normal from the front end, which is the more expensive of the two.

The cleanest way to see all of it is one lie. Open the wiresheet before you start — its first open loads the sheet into the editor the way a download loads a controller, and a download hands a software latch back cleared. Then, on the Unit tab, force the DAT sensor low — the picker option already names what watches it — and the stat takes the machine down the same tick. Now release the force, and nothing restarts: the discharge reads the still-air room number, the set side of the latch is long clear, and the machine stays down waiting for the button. A software stat trusts its sensor with no questions asked, and that cuts both ways. Watch what the hardwired element does during that same experiment: nothing. It is a capillary in the airstream, not a transmitter, so a lie told to the controller never reaches it — and by the same token it would never miss a freeze the transmitter failed to report. Running both is not redundancy for show; it is two different failure stories covering each other.