As part of a renovation I'll be upgrading my system to the following dual fuel setup:
Furnace: 59TN6B080
Heat pump: 25VNA436
I solicited 3 bids from 3 contractor. One of them recommended by my architect, one by a highly reputable contractor but can't take the job now, and one I found on the neighborhood listserv. All 3 of them gave me bids within 2% price difference (amazing!). I personally like the 3rd one because the contractor is a small family owned business and the person I'm speaking to spends a lot of time answering all my questions and writing very thoughtful emails to help me make an informed decision.
I'm right know leaning towards the last contractor but his configuration varies from the two other when it comes to equipment width. Two of them are proposing I go with a 21" wide furnace and coil. The third contractor is saying I should go with 17".
Here's his rationale which I wanted to run by you:
The minimum airflow is too high on the 21" wide furnace. It has a minimum airflow of 750 CFM, vs 500 for the 17" wide. We're putting in a 3-ton inverter heat pump, so 750 CFM is too much airflow for proper performance (350 CFM/ton). The inverter lowest output is 25% (i.e. 0.75 ton) so realistically stage 1 / 2 at the 750 CFM minimum of the 21" furnace is going to defeat the purpose of those stages, which is mostly humidity control.
As a general principle, lower cooling airflow improves latent heat removal (humidity), so the higher the airflow, the poorer the humidity control. In an ideal world, the system wants to move as little air as it can across the coil when it's dehumidifying, basically if it could move exactly enough air to avoid freezing up the coil, that would be our target.
At a certain point in the extreme, too much airflow results in a loss of sensible (temperature) heat removal too - there is so much air moving across the coil that the coil can no longer absorb a meaningful amount of heat from the house.You can overshoot it by some factor, but it needs to stay within reason.
The maximum airflows will also never come into play in this configuration. The 21" wide furnace is meant to support up to a 5-ton A/C. It's basically complete overkill for this configuration, for what the ductwork can support, etc. Its airflow range is 2.5-5 tons, vs 1.5-3.5 on the 17" wide.
Finally, in order to properly install the 21" wide unit, I've got to get a box underneath it to give it return air from below as well as the side, which eats up height and limits coil selection, etc. It's doable but every inch is going to count for getting the ductwork to fit and work properly in that space. Space is a premium in the room, and with the duct layout, so the narrower width helps make everything work.
I suggest you go with the following configuration:
59TN6B080C17--14
25VNA436A003
CNPVP4217
The other contractor is recommending the 21" furnace with the 21" coil (CNPVP3621) while the last one is recommending the 21" furnace with the 24" coil (CNPVP4324ALA).
Would love to hear your perspectives before making a decision.
Furnace: 59TN6B080
Heat pump: 25VNA436
I solicited 3 bids from 3 contractor. One of them recommended by my architect, one by a highly reputable contractor but can't take the job now, and one I found on the neighborhood listserv. All 3 of them gave me bids within 2% price difference (amazing!). I personally like the 3rd one because the contractor is a small family owned business and the person I'm speaking to spends a lot of time answering all my questions and writing very thoughtful emails to help me make an informed decision.
I'm right know leaning towards the last contractor but his configuration varies from the two other when it comes to equipment width. Two of them are proposing I go with a 21" wide furnace and coil. The third contractor is saying I should go with 17".
Here's his rationale which I wanted to run by you:
The minimum airflow is too high on the 21" wide furnace. It has a minimum airflow of 750 CFM, vs 500 for the 17" wide. We're putting in a 3-ton inverter heat pump, so 750 CFM is too much airflow for proper performance (350 CFM/ton). The inverter lowest output is 25% (i.e. 0.75 ton) so realistically stage 1 / 2 at the 750 CFM minimum of the 21" furnace is going to defeat the purpose of those stages, which is mostly humidity control.
As a general principle, lower cooling airflow improves latent heat removal (humidity), so the higher the airflow, the poorer the humidity control. In an ideal world, the system wants to move as little air as it can across the coil when it's dehumidifying, basically if it could move exactly enough air to avoid freezing up the coil, that would be our target.
At a certain point in the extreme, too much airflow results in a loss of sensible (temperature) heat removal too - there is so much air moving across the coil that the coil can no longer absorb a meaningful amount of heat from the house.You can overshoot it by some factor, but it needs to stay within reason.
The maximum airflows will also never come into play in this configuration. The 21" wide furnace is meant to support up to a 5-ton A/C. It's basically complete overkill for this configuration, for what the ductwork can support, etc. Its airflow range is 2.5-5 tons, vs 1.5-3.5 on the 17" wide.
Finally, in order to properly install the 21" wide unit, I've got to get a box underneath it to give it return air from below as well as the side, which eats up height and limits coil selection, etc. It's doable but every inch is going to count for getting the ductwork to fit and work properly in that space. Space is a premium in the room, and with the duct layout, so the narrower width helps make everything work.
I suggest you go with the following configuration:
59TN6B080C17--14
25VNA436A003
CNPVP4217
The other contractor is recommending the 21" furnace with the 21" coil (CNPVP3621) while the last one is recommending the 21" furnace with the 24" coil (CNPVP4324ALA).
Would love to hear your perspectives before making a decision.
source https://hvac-talk.com/vbb/threads/2227342-Proper-size-coil-for-furnace-and-HP?goto=newpost
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