Is 18×12 filter size return large enough? 773 sq ft house. Im getting quotes to replace furnace and condenser w\heat pump. Several quoters have said 2 ton & that existing 18×12 is too small. I was going to cut larger the existing hole which was originally cut thru cmu structural wall. Not a task im looking forward too. I’m looking at larger return vents that id need to buy & most have grills that block up to 50% of airflow. Air handler manufacturers must consider that when specifying return size. My 18×12 has no grill. So. I think it can safely be 50% smaller then manufacturers specify.
Questions I know ill be asked:
This hole was cut thru structural cmu wall when central oil furnace heater was added to 1952 house before i bought in 1986. Sometime after that, but still before I bought, central cooling was added.
In 2002 the condenser started over heating & I was told it needed to be replaced. Contractor installed 3 ton and used that existing hole. After he left I read the install papers & it specified a larger hole. Later I learned 3 ton was too big. A few years later I removed the grill. This also makes it easier to see if it’s dirty. It’s worked fine all these years, but I know I use it much less then most Floridians.
I think im ok leaving it at 18×12 since no grating to restrict air.
Your thoughts?
Replies
Yes, the amount of air (and size of return) will scale with the capacity of your HVAC.
Replacing is a good time to look at all the options, and consider a heat pump.
The right way (best way) to size is based on a manual-J calculation. This will include the construction details of the house, the outside and inside design temperatures.
I have seen HVAC contractors err on the side of a too-big system, which costs a bit more (meaning their cut is more) and then short cycles.
The right sized system will run more consistently, and do a better job removing humidity (which I am led to believe you have plenty of in Florida)
If you want to keep your current hole, you can ask for a larger filter to be installed next to the furnace, and put any grate on that side of the hole. a 4 or 4.5 inch thick filter will do a better job of filtering. The filter and grate can be sized to allow for the net free air flow you need. will work best if the duct work from your hole tapers to where the filter is. Might mean relocating the HVAC equipment some to make it all work.
You can make a smaller capacity unit work if you improve the insulation and air sealing of your house.
You also can look at better grille options.
For example
https://drive.google.com/file/d/1CtnrfFRkq8UyFyPNjA7hm5A_I7NP3iQZ/view
https://hvacpremium.com/products/18w-x-12h-steel-return-air-grilles-sidewall-and-ceiling-hvac-duct-cover-white-outer-dimensions-1975w-x-1375h?variant=15783497465922&country=US¤cy=USD&utm_medium=product_sync&utm_source=google&utm_content=sag_organic&utm_campaign=sag_organic&utm_campaign=Finch-Google-Shopping-Access-Doors&utm_medium=ppc&utm_source=google&utm_term=&utm_content=shopify_US_1912093409346_15783497465922&hsa_mt=&hsa_net=adwords&hsa_ver=3&hsa_kw=&hsa_acc=6168230911&hsa_grp=&hsa_tgt=&hsa_src=x&hsa_ad=&hsa_cam=18136792002&gad_source=1&gclid=CjwKCAiAgoq7BhBxEiwAVcW0LBVoh1f4Q_BNWFNys5ffgQQnPFhr_I_WbQcwxk0GIbbUlSy0WiXqvRoCUPYQAvD_BwE
This place has a 12x18 grille with a .99 value for cfm compared to the opening size.
They also spec pressure drop for various cfm values.
The cost of having a too-small value of net free area of the return is that the fan will need to spend more energy to get the same air flow past the coil.
Similar effect happens when the filter has been in service too long and becomes clogged with dust and such.
Last contractor actually said he wasn't sure of size needed & will get back to me. 2 days later he said "I have an air duct calculator and a 12 x 18 opening will move 1400 CFM. A 2 ton system is 800 CFM. If you add a filter and a front grille, it drops to 900 CFM. It is fine for a 2 ton system but too small for a 3 ton system."
Your thoughts?
If your contractor would share what make and model he is planning, you can access the documents online and verify the statements.
Are you also replacing the furnace at the same time? Did you look at heat pumps?
If you get a 2 ton vs a 3 ton, it will just run longer. (and remove more moisture in the process)
If the heat load is really such that you would nominally need a 3 ton, it might not get as cool in very hot, humid weather.
You said you do not use as much as the average floridian, does that mean you only run it once in a while, or that you keep the thermostat at a higher temperature?
You have been living with a 3 ton. how does it seem to work for you?
Done that. Filter opening 16x20 & 18x18.
Both. Only use when im uncomfortable. Set it 2 deg lower or higher then ambient.
3 ton always felt fine, but ima odd animal. J method says 1.5
If your manual J result with the operating conditions (inside and outside) match your desires, I would go with a 1.5 ton unit.
This would work best if you set ahead of your needs, rather than wait till you are uncomfortable.
2 ton should be more than enough.
I want to guess a 2 ton would save you quite a bit at purchase and installation vs the 3 ton.
You might want to call around and see if another outfit wants to come give you an informed estimate and recommendation.
This AH can support internal air filter. In that case could i keep the return hole 18x12?