Size the equipment correctly by performing Manual S
House Type Existing
Equipment Type Daikin VRV 4 Ton, RXSQ48TAVJU
Configuration Connected to only system FXTQ48TAVJU installed horizontally
Heat Load 38,000 BTUs
Area of the house 1800 square foot.
Location Phoenix, Arizona
Problem Feels Hot, Does not cool...
VRV or VRF systems are those groups of system where multiple coils can be connected to one condenser. But, Daikin VRV Life is the only system that allows 1-to-1 or 1-to-Multiple connection, and the only brand to allow installing VRV in existing homes. And due to this flexible piping system, Daikin VRV Life is one of the best in the market and efficient for the homeowners who are looking to control temperatures independently for each room or zones.
Even this beast has downside. Well, it turns out it is not a beast.
Let's go back to the problem conclude the important of Manual S, what assumptions HVAC contractors make during selection of equipment, and what can be done to fix the problem.
As soon as salesman hear 38,000 BTUs, many make an assumption that the house needs about 4 Ton in Phoenix, AZ. They lack effort in researching equipment’s capability to meet the load and airflow. Their only focus is to sell the best.
Well, now lets come to how did they come got 4 Ton? Traditional cube or box type systems, depending on brand, are capable of producing about 36,000 BTUs to 40,000 BTUS in Phoenix, Arizona when ambient temperature is 112-115 °F.
Knowing that, the contractor might have concluded to sell the greatest and the best - Daikin VRV Life. But, little do they know, Daikin VRV 4 Life produces only 29,000 BTUs @ 115 °F in Phoenix, AZ, as shown in Figure 1.
How do we fix the problem? Should it be replaced with 5-Ton?
Well, 5 Ton has the capacity of about 38,000 BTUs @ 115 °F as shown in Figure 2, but on an existing house, it is not recommended at all. Think about the errors in heat load. U-values and R-values plugged in the heat load, may not reflect 100% with reality. House may 15-20 year old, materials might have degraded within. So, it may not be exactly 38,000 BTU's always have some buffer when selecting the equipment for existing house.
Moreover, FXTQ48TAVJU Air Handler may de-rate up to 10% when installed horizontally (see Figure 3). Considering this, it is safe to assume that one VRV Life system is not capable to cool the house.
The best solution is to replace it with a traditional 4-ton system or add a 9000 -12000 BTUs mini-split in a zone that calls for that amount.
This is why Manual S, the equipment selection procedure is highly recommended.
Just like HVAC contractors, there are builders, architect, and even MEP engineers, who follow the old rules of thumb in HVAC industry which leads to discomfort in many homes.
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