Is airflow in modulating ducted heatpump lower or higher than conventional furnace?
I’m trying to square two sentiments from GBA articles pasted below (I’ve seen similar sentiments expressed numerous times and they appear to somewhat contradict):
From Jon Harrods piece (https://www.greenbuildingadvisor.com/article/191335)
Compared to oil and gas furnaces, heat pumps require more airflow per Btu of heating capacity. An older natural gas furnace might be perfectly happy with 100 cubic feet per minute (cfm) per 10,000 Btu of rated input; a high-efficiency condensing furnace needs about 150 cfm per 10,000 Btu. But heat-pump air handlers typically require 300+ cfm per 10,000 Btu of nominal output. This is because heat pumps deliver cooler air, typically around 95°F, compared to the 105°F-140°F air typical of a fossil-fuel furnace. Each cfm of air contains less heat energy, so more airflow is needed.
From Patrick McCombe’s piece (https://www.greenbuildingadvisor.com/article/high-tech-heating-and-cooling-systems)
Quieter operation. Inside, the system is running at a lower speed, so there’s no woosh of air rushing through the registers.
I have seen Patrick’s sentiment expressed elsewhere where I have interpreted it to mean that a variable capacity heat pump will have more steady airflow but less velocity*. Is that incorrect? Is he just saying that on low load days the airflow won’t be as high as on high load days… but that on high load days the airflow requirements are actually higher?
*Of course, larger ducts could reduce velocity needs while maintaining CFM, but that has nothing to do with variable speed equipment, only the duct design, so why mention it in relation to the equipment?
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The lower heat pump temps require higher airflow, all else equal. But - furnaces are frequently way oversized so furnaces can often be replaced with systems that use less or equal airflow. This is all at design temps- the rest of the time lower airflow is needed.
Conventional furnaces don't modulate, they're just on or off. When they're on, they're on at full speed. When a modulating heat pump is running full out it's going to be moving more air than a conventional furnace. But 99% of the time it's not running full out, it runs longer at lower velocity.
The historically the US manufactures requirement is you need to move 400 cubic feet of air for every 12000 BTUs of capacity. My guess is the wall mounted mini splits do the same in cooling mode but suspect the mini heads do lower their fan speeds in heating mode allowing them to make warmer air. Note getting an accurate measurement of air flow from a mini split head is no small feat.
I think you will find in cooling mode every manufacturer requirement is 400 cubic feet of air for every 12000 BTUs of cooling capacity.
The fact is HPs make cooler air than furnaces so you must move more air to deliver the same number of BTUs just no way around that fact. The next question is at what speed will the air move if you make the duct have a larger cross section you can move the required number of BTU and cubic feet of air at the same speed or velocity making the same amount of noise. It costs more to buy and install larger ducts and registers and HPs need better designed and register placement because people don’t seem to mind it when you blow 130° air at them in fact, they kind of like it but tend to get upset when you blow 90° air across bare skin.
"Quieter operation. Inside, the system is running at a lower speed, so there’s no woosh of air rushing through the registers."
To my ear this line is about the fact that ideally the variable speed indoor fan would start running at the beginning of the heating season and change speeds as needed throughout the season but not stop until the end of the heating season. As opposed to the furnace blower that will be cycling on and off several times each and every hour.
Walta
So I'm realizing I don't even know what dictates the CFM rating of a given system. In other words, if we aim to achieve 800 CFM for a 24k btu/h system, what are we tweaking in order to reach that number?
Is it a combination of the blower rating and the static pressure of the duct system? In which case, the thing we are really tweaking is the duct system for any given unit. Are people really fiddling with duct systems that carefully to accurately hit the target CFM?
As an example, the Mitsubishi SVZ-KP24NA multi position air handler specs are as follows:
Air flow rate CFM: 515 - 625 - 735
External Static Pressure in. WG: 0.3 - 0.5 - 0.8
Do the three numbers imply there are 3 blower speeds? I had thought it was variable, but it appears to be stepped with 3 distinct speeds?
In any case, if at max blower speed we have 735 CFM available prior to any duct work pressure, that's not even 400 CFM per ton pre ductwork.
If we WERE to calculate the actual CFM using the given 0.8 IWC and the 735 CFM blower speed, is there a formula to calculate actual experienced CFM for that blower/static pressure pairing?
If i'm way off the tracks hopefully someone can see where I've gone off.