HVAC help! Velocity and duct sizing.

[tuskaudio]

Hey all. I've been reading and reading about how to do calculations for velocity and air ducts, and my head is spinning. The heat pump that I have is a Mitsubishi PEAD-A15AA7, a ducted horizontal heat pump. The specs say that that on the low setting, for heat, the fan runs at 424 CFM on the low setting.

I'm trying to figure out what size ducts/registers I need to keep the velocity below 150, but when I put 424 CFM into a velocity calculator online, it comes back with very large ducts. I would need 14x14 or equivalent ducts to start at 311 FPM, and then a 14x28 register to halve the velocity down to a manageable range. Am I missing something? That seems too big.

Can anyone shed some clarity on this? It is possible that I've oversized the heat pump for my space, but let's set that aside for now and just make sure I'm not missing some other factor.

Thanks for any help!

Roy Goodale in Auburn, Maine

[endorka]

I've only worked with fans, not heat pumps, so perhaps take this with a pinch of salt!

Is it possible you haven't taken into account the static pressure exerted by the ducts, registers, silencers, grilles etc. in your system? All of these will increase the static pressure of it. The fan in the heat pump has to push against this, and it will reduce the flow of air through the entire system.

For example, the fan I bought comes with the following chart showing the airflow of the three different fan speeds (LS, MS, HS) at different static pressures.

You can see that as the static pressure increases, the airflow decreases. So low speed can deliver 350 m3/h with no static pressure "load" attached. But with a system static pressure of 100 Pa it goes down to about 160 m3/h.

Does your heat pump come with similar documentation?

Cheers,
Jennifer

[gullfo]

sorry for the imperial measures:

when you look at the specs, 424cfm means that a 1ft2 duct (12" x 12") would have (approx - as Jenn noted - resistance has impact) 424fpm velocity. so to get that to say 212fpm, we would need 2ft2 duct (12" x 24"). or a plenum on the far end to expand it and slow it down before we vent it into the space.

however, there are dampers which can reduce the amount of flow - you need to then recirculate the difference so the system remains balanced. for example if i damp down the level to 212cfm, my 12x12 duct becomes 212fpm. but now i need to direct 212cfm BACK into the intake of the unit. NOTE: these are all approximations, and balancing a system should be done by proper engineering and installers

then once you're down to the 212cfm/fpm to get lower, you now vent into a plenum with 2x the volume to reduce it to 106fpm...