Bypass still needed in some circumstances even with ECM. Key element is not variable CFM but airflow to smallest zone. The only system I know of that avoids a bypass is Carrier's Infinity (Air Source). It slightly opens non-calling zones to dump the excess air.
All two stage systems using Copeland 2 stage compressors only drop 30% of system capacity in the lower stage. In other words a 4 ton drops to 3. If a small zone has just a ton or so of load and ductwork to accommodate just a ton or so of air, if that zone alone calls, there will be significant extra air and capacity. Fixes include:
1) Avoid small zones
2) reducing ECM CFM setpoints. This can help with dehu in cooling mode but can lead to evaporator icing and a loss of EER. In heating mode higher than desirable head pressures can result as well as reduced COP
3a) Use a return bypass. This essentially causes some of the air to pass through the blower twice or more. The ECM thinks it is moving the right CFM per ton but some air is recirculated from supply to return. Overdone this leads to the same issues as in #1
3b) Use a dump zone. If there is a 2 story entry, great room, or big stairwell, dump extra air into it.
4) Upsize ductwork in smaller zones so it can take a bit more air than Manual D suggests. Overdone, short-cycling results along with uncomfortable amounts of air in small zones.
Some combination of the above should yield a good system in most circumstances.
My reading and limited experience suggest barometric dampers work fine for residential tonnages and static pressures. Motorized are called for in bigger higher pressure apps. |