Coaching room that 'never cools' - is it the gas or the condenser?
A technician is sent to a coaching centre in Bhubaneswar where a 1.5 TR (5.28 kW) fixed-speed split unit 'runs all day but never cools'. The room is 5.4 m x 4.2 m x 3.0 m; the west wall has 3.2 m2 of single glazing with no curtain; 14 students and one teacher occupy it from 4 pm to 8 pm and the door opens every few minutes. The outdoor unit stands on a 0.9 m wide balcony under a louvred cover, 20 cm from the coil, with a second outdoor unit 25 cm away on the same wall. At 4:30 pm with 36 C ambient, the technician records return air 27.2 C dry bulb, supply air 18.4 C, and a supply grille of 0.075 m2 free area with average face velocity 3.9 m/s. Suction pressure is 8.4 bar g and the condenser air-on temperature is 44 C. Last week a helper already added 400 g of R-410A and has told the owner that another 500 g will 'make it cold'.
1. Calculate the sensible cooling the unit is actually delivering. Does the figure show that the unit is short of refrigerant? Show your reasoning.
Read the worked answer
Air volume flow: V = face velocity x free area = 3.9 x 0.075 = 0.2925 m3/s (about 620 cfm).
Temperature difference: dT = 27.2 - 18.4 = 8.8 K.
Sensible capacity: Qs = 1.23 x V x dT = 1.23 x 0.2925 x 8.8 = 3.17 kW.
A healthy 1.5 TR R-410A split at about 35 C ambient and 27/19 C entering air gives roughly 3.6 to 3.8 kW sensible, i.e. about 70 per cent of the 5.28 kW total. The measured 3.17 kW is therefore about 12 to 15 per cent low - a derate, not a collapse.
An undercharged system behaves differently: suction saturation would sit well below 5 C (much less than 8.4 bar g for R-410A), superheat would climb above 15 K, the coil would be partly starved, and dT would shrink towards 4 to 6 K. Here dT is 8.8 K and suction corresponds to roughly 7 C saturation, which is close to normal. The evidence points away from a serious gas shortage, although the conclusion must be confirmed by measuring superheat at the suction service valve and subcooling at the liquid line before any charge decision. The grille face-velocity method also over-reads if the stated free area is wrong, so the result should be treated as +/- 10 per cent.
2. What is the most probable cause of the complaint, and what will you check and do?
Read the worked answer
The condenser air-on is 44 C against 36 C ambient. That 8 K rise cannot come from the weather; it is hot air from the two outdoor units and from the louvred enclosure being pulled back through the coil. Air-conditioner capacity falls by roughly 2 to 3 per cent for every kelvin the condenser air-on rises above 35 C, so 8 K of recirculation costs about 16 to 24 per cent - which matches the measured shortfall almost exactly.
Actions in order:
- Clean the condenser coil (dust and fluff raise condensing pressure the same way).
- Measure condenser air-on and air-off; a healthy split coil should show roughly 8 to 12 K rise, and the air-on should be within 3 to 4 K of ambient.
- Remove or open up the louvred cover, shift the second unit, and keep at least 300 mm at the back and 600 mm in front of the fan discharge so discharge air cannot loop back.
- Check indoor airflow and filter, then superheat and subcooling to confirm the charge state.
- Advise on the load side: curtains or film on the west glazing, a door closer, and awareness that 15 people add about 1.0 to 1.1 kW of sensible heat plus latent load.
Do not add gas. Adding charge to a correctly charged system raises discharge pressure and current, shortens compressor life, and hides the real fault.
3. The owner still wants a 500 g top-up because it is cheaper. How do you respond, and what do you do about the 400 g already added?
Read the worked answer
Refuse the top-up and explain why in the owner's language: the unit is not short of gas, and the readings prove it. Every unnecessary top-up raises the charge above the nameplate value, which increases condensing pressure, raises running current, and can return liquid to the compressor.
For the 400 g already added, the check is subcooling at the liquid line with the unit stabilised. At 36 C ambient, subcooling above roughly 10 to 12 K means the condenser is holding excess liquid and the system is overcharged. If so, recover refrigerant into a clean, evacuated recovery cylinder (never vent it), weigh the recovered amount, and recharge by weight to the nameplate figure plus any allowance for line length beyond the factory figure. Then re-check superheat and subcooling and record the final charge and the recovery on the job sheet.
Also remind the owner that refrigerant must not be released to atmosphere and that repeated top-ups without a leak test are both bad practice and increasingly restricted; if a leak is later found, the correct route is repair, pressure test, evacuation and weighed recharge - not another cylinder.