VRF, but only where the zoning genuinely earns it

Heat recovery VRF costs meaningfully more than a heat pump VRF, and it only pays back where heating and cooling are genuinely called for at the same time. On the Boulder laboratory we counted about 5.5 hours a day of simultaneous demand across the shoulder seasons, driven by instrument rooms rejecting heat while perimeter offices asked for it.

That is a clear case. A speculative office floor with a uniform load profile is not, and we have talked clients out of three-pipe systems more than once.

The zone-by-zone load calculation is what settles it. It is the same work either way, so we do it before recommending the system rather than after.

The most common VRF design mistake we see is using the indoor units to bring in outdoor air. It couples ventilation to a thermostat call, which means a zone that needs air only gets it when it also wants conditioning.

We put ventilation on a dedicated outdoor air unit with energy recovery, delivering neutral air at the ASHRAE 62.1 ventilation rate procedure quantity. The VRF then does space conditioning only, which is what it is good at, and the outdoor air rate can be trended and proved.

In Denver the energy recovery wheel or core also does useful work on humidity, in the opposite direction to most of the country: our air is dry enough that winter humidification is often the real problem.

A VRF system holds a very large refrigerant charge, and ASHRAE 15 sets a limit on how much of it may be released into the smallest occupied space a branch serves. With R-454B, an A2L, the limits and the mitigation requirements under UL 60335-2-40 are stricter again.

We calculate the refrigerant concentration limit for every occupied space against the charge of the branch that serves it. On the Boulder job two small instrument rooms failed on the first pass, so we resplit the branch. The alternative, reinterpreting the calculation, is not an engineering decision.

Where mitigation is needed, it gets designed in: detection, interlocks and the ventilation rate to clear the space, all on the drawings rather than added on site.

A VRF outdoor unit rated at 30 tons is rated at 47 °F ambient. What matters in Colorado is what it produces at the 99.6 percent heating design condition, which is about 1 °F on the Front Range.

Modern cold-climate VRF holds roughly 70 to 80 percent of nominal heating capacity at that temperature, with defrost losses on top. We select on that figure, calculate the balance point where output crosses the building load, and size any supplement to the gap rather than installing a full backup plant out of caution.