Marstair – Cellarator / MRC+ Series

Compressor not starting at all (Evaporator fan runs, but outdoor unit silent; cellar warm)

– Power check: Ensure both indoor and outdoor units have power supply. Many Marstair systems have separate supply feeds. Verify supply voltage and that any isolation switches/contactors are on. – If power is present, check the control circuit: is the thermostat or controller calling for cooling? Mechanical stat should be set below room temp (listen for click). If electronic, measure if a 240V (or control voltage) signal is sent to the compressor contactor coil. – Inspect the timer board (if fitted, in MRC+ units). These boards provide a startup delay (~1 min) and bypass LP on startup. If the board is dead, the compressor may never get a start signal. – Check safety switches: Press the HP manual reset if it’s tripped (button popped out). Check the LP switch continuity – if open (with a charged system) and no pump-down active, either LP is tripped or the switch is bad. Also some Marstair have an oil pressure switch on larger models. – Manually try to engage the compressor contactor (using an insulated tool to push it in momentarily) – does the compressor hum or start? If it runs when forced, the issue is in the control circuit (stat, pressure switches, etc.), not the compressor itself.

– Restore control signal: If the thermostat was faulty or set wrong, replace or adjust it. E.g., a mechanical stat’s sensor bulb might have come loose – remount it properly or fit a new thermostat. Electronic controllers (if used) should be calibrated or swapped if unresponsive. – Bypass checks: If the timer board failed, a temporary bypass can be done for testing (connecting thermostat output directly to compressor contactor). Ultimately, replace the faulty timer PCB if it’s not sending voltage after the delay. – Reset/replace switches: Reset the HP switch and identify why it tripped (see HP troubleshooting below). If the LP switch is open with normal pressure, it’s likely defective – replace the LP pressure switch. (Do not leave it bypassed except for test, as it’s critical protection.) If the unit uses pump-down control, ensure the liquid line solenoid is opening (a stuck closed solenoid will mimic an LP fault). Replace any faulty solenoid coil. – If the compressor contactor coil is burnt (not pulling in despite command), replace the contactor. Likewise, check any control fuses or overloads – replace blown fuses and investigate cause (shorted wires, etc.). – Compressor issues: If pushing the contactor still doesn’t start the compressor (just hums and trips), the compressor might be seized or its start capacitor (for single-phase units) may be bad. Test the capacitor µF – if out of spec, replace it and try again. If the compressor is seized (draws locked-rotor amps), it must be replaced. Also consider the cause (loss of lubrication, overheating from previous HP trips) to avoid repeat failure.

Compressor runs but short-cycles via LP (pump-down) (Outdoor unit starts briefly then stops, repeatedly)

– Many Marstair systems use a pump-down cycle: a solenoid closes when setpoint is reached, causing LP trip and compressor off. Symptoms: compressor turns on for a few seconds/minutes, then off for a while, and you might hear a hissing as pressures equalize. If your system is supposed to be pump-down, this may be normal at end of cooling cycle. But if it’s doing it rapidly while the cellar is warm, that’s a problem. – Verify if a pump-down solenoid valve is installed on the liquid line. If yes, check if it’s energized (open) when cooling is demanded. If the solenoid isn’t opening (coil bad or wired wrong), the compressor will immediately suck the evaporator into vacuum and hit the LP cut – causing short cycling. – If no pump-down intended (some systems run direct control), ensure there’s a jumper wire across the pump-down terminals as required. Marstair diagrams often show linking certain terminals when no remote thermostat or pump-down is used – a missing link means no signal to keep liquid flowing. – Also confirm the LP switch setting: It might be set too high (cutting out prematurely). Typical LP cut-out for R407C/R448A cellars might be around 1.0–1.5 bar. If it’s cutting out at, say, 3 bar for some reason, it’s set incorrectly.

– Pump-down solenoid fix: If the solenoid coil is faulty, replace it. If it wasn’t getting power, correct the wiring from the thermostat/controller to the coil. After fix, the compressor should run continuously until setpoint is nearly reached, then do a proper pump-down off cycle. – If the system was not supposed to pump down, but was wired as such, simply install the proper link or set the controller for direct control per manual. This will keep the solenoid open all the time and control the compressor directly via thermostat. – Adjust LP control: A qualified engineer can tweak the LP cut-out a bit higher or lower. However, do not set it too low (risk running in vacuum) or too high (will nuisance trip). If the LP control is erratic or untrustworthy, replace it with a new one set to the correct cut-out (and differential if adjustable). – Check refrigerant charge as well – a borderline undercharge can cause the LP to flirt with the cut-out especially right after startup. Weighing in the correct charge as per Marstair specs ensures the pump-down has enough refrigerant to satisfy properly.

Unit trips on High Pressure (HP) (Loud click from HP switch, compressor stops; often occurs on hot days or if dirty condenser)

– Condenser fan: Confirm the outdoor fan is running whenever the compressor runs. If it’s not running or intermittent, that’s likely the cause. For fan-speed-controlled units, see if it ever ramps up. If stuck at low speed in high ambient, could be a controller fault – some Marstair condensers have optional fan speed controllers; others run full speed constantly. – Condenser coil: Is it blocked by dirt, leaves, or grease? Shine a light through – you should see through the fins. A clogged coil causes rapid pressure rise. Also check clearance – if located in an enclosed yard or near other heat sources, recirculation might be an issue. – Measure the high-side pressure with gauges (technician task) to verify it truly hit the trip point (~29.5 bar for R407C in older Marstair, ~45 bar if R410A on some units). If the pressure isn’t that high but HP trips, the switch could be faulty. – Inspect for overcharge or non-condensables: If someone added refrigerant without proper evacuation, there could be air in system raising head pressure. Sight glass showing unusual bubbles or erratic pressures can hint at this.

– Clean and cool the condenser: Remove power and thoroughly clean the condenser fins (use coil cleaner and water or air blow). After cleaning, ensure the unit has adequate ventilation – relocate or add ducting if it’s in a hot confined space. This alone often solves HP trips. – Fix the fan: Replace any non-working fan motor. If it’s a capacitor-run motor, test/replace the capacitor. For fan speed controllers, if the fan never goes high, consider adjusting the potentiometer (set to maintain ~20°C condensing above ambient) or bypassing the controller on extremely hot days. In critical cases, a tech might install a second fan or a mist system for very hot environments, but that’s seldom needed if unit is sized right. – If high ambient is unavoidable (e.g. condenser in direct sun in summer), plan preventive maintenance: keep the coil extra clean and maybe shade the unit. Marstair does have High Ambient condensing units; if yours is under-capacity for the heat, you might discuss upgrading to an HA model. – Refrigerant corrections: If suspected overcharge, recover and weigh the refrigerant, then recharge exactly to spec (or to a clear sight glass if specified, noting R407C has glide so slight bubbles can persist). If non-condensables (air) are suspected, recover, evacuate thoroughly, and recharge with virgin refrigerant. A telltale is if high pressure persists even after coil cools, or if discharge pressure doesn’t correlate with subcooling. – Reset the HP switch: After fixing cause, reset the manual HP switch by pressing its button (if auto-reset type, it will resume on its own). Verify pressures and operation on restart. If the HP switch had been tripping frequently, monitor compressor for any damage (oil discoloration, noises) – repeated trips stress it.

Evaporator icing / low cooling (LP trips)

– As with other brands, check the evaporator fan. Marstair’s evaporators (Cellarator) typically have one or two fans; if one fails, airflow drops and coil can ice. – Thermostat setting: If a mechanical de-ice thermostat is fitted to the coil, see if it’s shutting off the compressor around 0°C coil temp. If it’s mis-set or bypassed, the coil might go subzero and freeze. If no de-ice stat, consider if off-cycle defrost is sufficient. – Inspect the expansion device (capillary or TXV). A TXV stuck open could flood the coil (unlikely in these low-temp apps, more likely it would stick closed causing starving). If capillary system, frosting heavily at the inlet might indicate overfeeding or low load. – Refrigerant level: Low refrigerant will cause low pressure and coil icing from the inlet outward. Check pressures; an LP reading below normal (e.g. vacuum or near 0 bar on R407C) means undercharge or maybe a failed TXV stuck closed. – Ensure the cellar isn’t too humid (excess moisture leads to quicker icing). A very wet cellar or frequent door opening could cause more frost accumulation than the unit can naturally defrost.

– Fan repair: Replace any bad evaporator fan motor. Also clean the evaporator coil fins to improve airflow. After ice melts, run the unit and verify strong airflow through the coil. – Defrost strategy: If icing persists, you may need to introduce a regular off-cycle defrost. For example, put a timer to turn the compressor off for ~30 minutes once or twice a day (when the pub is closed) to let ice melt. Alternatively, Marstair offers optional electric heater kits – installing one can provide active defrost for very low-temperature operation (e.g. if trying to hold near 4°C constantly). – De-ice thermostat: If present, check its function. It usually clips to the coil and opens the control circuit if coil gets too cold. If it’s failed (always closed), replace it. If it’s simply set too low, adjust it to cut out ~-2°C evap temp. This prevents deep freeze. – Recharge refrigerant: For confirmed low charge, fix leak and recharge. Marstair’s guidance is to charge by sight glass and pressures; ensure no bubbles at operating condition (taking glide into account). Once charged, verify that LP stays above the cut-out during operation. – TXV issues: If a TX valve bulb lost charge or valve stuck, replace the TXV. Re-set superheat to ~5K as factory (per Marstair data). That should balance feed and prevent icing if all else is in order.

Unusual noises or vibrations (Marstair)

– Marstair units are generally quiet, but vibration issues can occur if mounting isn’t firm. Check that the outdoor unit (MRC) is on proper mounts – loose mounting can cause buzzing. The compressor itself in MRC units is often on rubber feet; if those perished, metal-to-metal contact will vibrate. – Listen for gurgling/hissing at the evaporator – a constant gurgle might indicate low charge (vapour in liquid line). Also, hammering or knocking could be liquid refrigerant floodback to compressor (for example, TXV oversized or indoor fan failed causing liquid to return). – If you hear a buzz followed by a click periodically at the outdoor unit, that could be the compressor’s internal overload kicking in (indicates either hard start or overheating). Causes could be low voltage, bad capacitor, or tight compressor.

– Secure mounting: Tighten any loose screws or bolts on the units. Add or replace vibration pads under the condensing unit. Ensure piping is clamped so it doesn’t resonate against walls. – Replace compressor mounts if cracked. That can cut down vibration drastically. – Refrigerant noise: If gurgling from low charge, obviously, recharge as mentioned. If some expansion noise is normal but bothersome, adding a liquid line muffler is possible but rarely needed – better to ensure correct charge and proper TXV operation. – Liquid floodback: If suspected, check superheat. If near zero, adjust TXV to increase superheat slightly. Also, ensure evaporator isn’t flooding because of fan failure or very low load. In some cases, adding a suction accumulator can protect the compressor if perfect tuning is difficult (like at very low cellar loads). – Electrical hum: A loud buzz could be a contactor coil issue – replace noisy contactors or relays. If the compressor struggles to start (buzz/click), consider a hard-start kit (start capacitor & relay) for single-phase units or ultimately compressor replacement if it’s indicating impending failure.