I have a GMP150 gas heating unit working at home. I now want to add the cooling unit, but due to the small size of the house (about 70m2) I want nothing more than 3 ton unit. Given this requirement, all local Goodman people I contacted told me that I have to install the 5 ton unit because the 3 ton unit will give 'no cool at all' in the house. I understand the air speed using GMP150 can be faster than expected by the 3 ton unit, so, I may expect some loss in its cooling capacity. However, is hard to me to believe that such loss would be of 100% ('no cool at all'). The question is: Can you give me an estimate of the loss of coolig capacity if I install a 3 ton unit (e.g. Designed for GMP100) on top of my GMP150?

It would help me, maybe someone else knows,, but I`m in USA,, we measure homes in square feet, so unless someone comes along maybe you can convert 70m2 to something I can understand

I think what they are trying to tell you is you need to match the outside unit to the indoor coil, the furnace shouldn't make any difference. You can adjust fan speeds to allow for different applications. If you have a 5ton coil on top of the furnace, you need to match it with a five ton unit outside. Either that, or take out the coil and install a three ton coil and condenser. Then adjust airspeed in furnace accordingly through testing. If the house is small, why such a large furnace? If the house is big, go with the five ton as suggested by installer. If they did a manual J on the home, they most likely sized everything correctly. Why are you not agreeing with them? Or maybe you need to call in a different company and have them tell you what they think, heck even two or three more companys and get their advice.

I will tell you that if you undersize your a/c, you will regret forever. However, if you extremely oversize it, you won't be happy either. Im still trying to figure out why you have such a large BTU furnace, but say the house is small? Confused. Can you clear this up for me?

mygirlsdad77,, now I`ll never find out what 7m2 means

Hi, 70m2 are 753 square feet. The furnace is so bit because the house was going to be bigger, but it won't.

Okay, so you had the sizing of the furnace done for future addition, which you have now decided not to do, correct? For that footage, even three ton seems oversized. Just make sure to match the evap coil and the condenser, and have it properly sized for the home. The only real problem I see is if the evap coil is to small and will restrict the airflow of the furnace, but most newer coils of any BTU are large enough to make your system work correctly. Leave this one up to the pros you have in your area. I hope our furnace is a two stage furnace, or you may find that is will cause problems due to the extreme oversize. I think you would have been fine with a 90,000 BTU furnace, and that may even be a bit oversized. Good luck on your project, and please do let us know how things work out.

wmproop, looks like we both learned something today,, but I'm betting we will never have to remember it? 70m2, is that metric? How would you calculate that to square feet?

As advised before,, someone should do a manuel J calculation on your house,, as you know the furnace is to big, but also I`m guessing a 3 ton air is also to big,, that size house 2 ton at the most