[MEP Center Engineering]

Starter question — a frequently asked question posted by the MEP Center team to open this forum. Share your own experience below.

I am laying out a VRF heat pump system for a six-storey office. The outdoor units sit on the roof and the farthest indoor unit is in the ground-floor reception. The catalogue lists several limits: total pipe length, longest pipe (actual and equivalent), length after the first branch, height difference between outdoor and indoor units, and height difference between indoor units.

• Which of these limits actually govern the layout?
• How do I calculate equivalent length?
• What do I do when one limit is exceeded, and does long piping reduce the capacity I selected?

[MEP Center Engineering]

All of the listed limits govern at the same time, and they are manufacturer- and model-specific: check every one against the data book of the exact outdoor unit, then apply the length and height capacity correction before you confirm the selection.

The limits to check

1. Longest pipe, actual and equivalent – from the outdoor unit to the farthest indoor unit. Typical published values are in the range of 150-200 m actual and somewhat more as equivalent length.
2. Length after the first branch – often about 40 m as standard, extendable (commonly up to about 90 m) only if the manufacturer’s conditions are met, such as upsizing certain pipes or limiting the difference between the nearest and farthest unit.
3. Total piping length – sum of all liquid (or gas) pipe runs, typically several hundred metres up to about 1000 m on large systems.
4. Height difference outdoor-indoor – usually different for outdoor unit above (often 50 m, more with options) and below (often 40 m).
5. Height difference between indoor units – commonly 15-30 m. This is the limit most often forgotten on multi-storey buildings.

Equivalent length

Equivalent length = actual length + (number of elbows x equivalent length per elbow) + (branch kits x their equivalent length) + any other fittings listed by the manufacturer. Per-fitting values depend on pipe size; take them from the data book, not from water-pipe tables.

Worked example

Roof outdoor unit, 56 kW nominal (two equal 28 kW modules). Indoor units total 54 kW nominal, so the connection ratio is 54 / 56 = 96 %, within the 100 % maximum. Assumed data-book values: 0.5 m per elbow, 0.5 m per branch kit.

Check	Design	Example limit	Result
Longest equivalent: 118 m + 22 x 0.5 + 5 x 0.5	131.5 m	190 m	OK
After first branch	58 m	40 m (90 m with conditions)	Check conditions
Total piping	640 m	1000 m	OK
Outdoor above indoor	26 m	50 m	OK
Indoor to indoor	22 m	15 m	Fails

Two fixes are needed. The 58 m after the first branch is acceptable only if the extension conditions are satisfied; otherwise move the first branch closer to the far end of the system. The 22 m indoor-to-indoor height exceeds this model’s 15 m, so split the building into two systems by floor zone (or choose a model with a higher allowance).

Capacity: if the manufacturer’s correction chart gives, say, 0.90 for 131.5 m equivalent and 26 m height, the available cooling capacity is 56 x 0.90 = 50.4 kW. Compare that with the diversified block load of the zone, not with the sum of indoor nominal capacities. Do not fix a shortfall by raising the connection ratio above 100 %; select the next outdoor unit size, keeping the modules in a group equal or at most one step apart.

Common mistakes

• Checking only the longest run and ignoring indoor-to-indoor height.
• Using plan lengths without risers and drops.
• Forgetting that additional refrigerant charge grows with liquid pipe length, which then affects the room leak-concentration check.
• Applying one manufacturer’s limits to another manufacturer’s equipment at tender stage.

Related: VRF branch kit selection, 10 common VRF design mistakes. For deeper study: VRF/VRV system design course.

Check the edition of the code that applies to your project.

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