Supply CFM based on heating / cooling mode

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Hi,

I'm trying to specify separate, constant CFMs for heating and cooling
mode operation (Ex: 5,000 Heating & 7,500 Cooling).

The system I created is Variable Air Volume which serves multiple
thermal zones.

The Fan Flow Parameter MIN-FLOW-RATIO is set to 1.00. I read this as
the minimum supply CFM during cooling mode.

The Fan Flow Parameter HMAX-FLOW-RATIO = 5000/7000. I read this to be
the maximum supply CFM during heating mode.

The Fan Flow Parameter HMIN-FLOW-RATIO = 5000/7000 -0.01. I read this
to be the minimum supply CFM during heating mode.

But from the hourly report the CFM supplied, defined by SUPPLY-FLOW =
7,500, is the cooling quantity during heating mode when I would have
expected it to be 5,000. As far as I can tell no flows are specified at
the zone level that would override it. Any thoughts on things to look
at would be greatly appreciated.

Thanks, Todd

ToddGibson at Eaton.com's picture
Joined: 2012-06-19
Reputation: 0

I have a few thoughts/ideas, in no particular order ? maybe something here helps:

Note these flow inputs exist for the system and zone level ? you could be editing one and looking at results for the other.

I think MIN-FLOW-RATIO (minimum flow ratio) can override the (optional) heating minimum flow input. Required ventilation capacities can override either as well if I?m not mistaken ? maybe worth checking.

Just to point out: a MIN-FLOW-RATIO of 1.0 means the flow will go no lower than 100% of what?s specified (or calculated) for SUPPLY-FLOW. Consider carefully if and where that should apply to fans in the system you?re modeling.

Another thought is at least some of these zonal inputs will default to the system?s values if you aren?t specifying them separately? perhaps your central system is in cooling, using 1.00, while your terminal units are in heating and you end up with a conflict?

~Nick

[cid:489575314 at 22072009-0ABB]

NICK CATON, P.E.

Nick-Caton's picture
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Joined: 2011-09-30
Reputation: 805

Thanks for the suggestions. It appears that the MIN-FLOW-RATIO is overriding the heating flow rates even though the documentation doesn?t make that clear.

Because of the occupancy pattern of my building, the majority of the occupied hours are in the heating season so I have set the supply CFM to be the heating CFM rather than the cooling CFM as a workaround for now.

Todd Gibson

ToddGibson at Eaton.com's picture
Joined: 2012-06-19
Reputation: 0

Plumbing and heating supplies are crucial and very essential elements within any new construction or build. Whether it be a domestic or commercial property, plumbing and heating is essential to the main build and has to be accounted for on a number of levels.

Tom Plumb

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Joined: 2013-11-13
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