Seer Rating and A Coils

[j-race]

Could some one please explain to me if an why you have to match The condenser units Seer rating to the coil I thought that that was strictly based on Tonage. In that we have 2.5 ton coil we need a 2.5 ton condenser how would the seer rating effect that

[dash]

New minimum SEER is 13.The new outdoor unit,13SEER ,has a different design then an old 8-10 SEER,it has more refrigerent,for one thing.

An old coil 8-10 SEER will not work ,in most cases ,with the new 13SEER outdoor unit.

However a new coil Designed for 13SEER ,can work with the old 8-10SEER outdoor unit,may need to add a hard start kit to the outdoor unit ,as the indoor could have a TXV .

[j-race]

Im sorry but could you explain exactly why it wont work nothing Iv read or seen says why it works less efficiantly like this is the A coil smaller if it was made before the standard changed is there a PSI difference is it the refrigerant used?

[j-race]

also The coil is about a year old but never been used when we installed our new furnace we had them install the a coil so we could purchase the condenser later.

[Irascible]

Actually there is a PSI difference.

The simplest way to explain is this: A brand new 10 SEER 4 ton air conditioner might have a 4 horsepower compressor. A brand new 4 ton 13 SEER air conditioner might have a 3.5 horsepower compressor. So one has a weaker motor yet both push 4 tons worth of Freon. The only way to get a compressor to push the same amount of Freon with less power is to "feed" it Freon at a higher pressure. The compressor is fed by the cooling coil. The bigger the cooling coil, the more heat the Freon can absorb. The more heat in the Freon, the higher the Freon's pressure back to the compressor will be.

That's a very non-technical explanation that I'm sure my peers can shoot holes in. But it's essentially correct. If you match a 13 SEER unit to too small a coil, the Freon will not flow back to the compressor quickly enough for that smaller compressor to do the job. You'll get a reduction in cooling capacity. You'll also lose energy efficiency and the compressor's discharge temperature will be higher. But personally it's the loss of capacity that I'm most concerned about.

[comfortdoc]

Indoor coils technically do not have a SEER rating. SEER is based on manufacturers testing of different combinations of indoor and outdoor units. Most manufacturers nomenclature of coil "sizes" is based on matchups with 10 SEER outdoor units. The tonage designation on the coil is usually larger when matched to a higher SEER condenser. The higher SEER condenser usually requires a larger indoor coil surface area to obtain it's advertised efficiency ratings.

[iheatncoolinnc]

An older system probably has a piston type coil. To small of a piston will cause starving and to large of a piston will cause flooding. System performance and reliability will be unacceptable. Failre to have correct piston (metering device) will cause improper performamnce. All 2 1/2 ton systems do not use the same size piston. Most new 13 SEER systems are using TXV on the indoor coil and not a piston. Why not change the indoor coil and get the efficiency out of the new outdoor unit and save some money on operating cost.

[BaldLoonie]

Makes sense to me.

As SEER goes up, compression ratio goes down. That means lower head, higher suction. A "10 SEER" coil may be looking for a liquid pressure say 250 psi on a 95 degree day. If it gets that, it will deliver the rated capacity and EER as that is ARI's EER rating point. But say you put a 13 SEER outdoor unit on the same coil. To get the SEER up, they've used a slightly smaller comp and real big condenser. Leaving liquid pressure is now 220 psi on a 95 degree day. What happens when the lower pressure liquid hits the coil? Lower refrigerant flow means less cooling happens. The liquid refrigerant boils off quicker, superheat is higher, coil is starved for refrigerant. High superheat could mean poor compressor cooling, lower capacity and insufficient cooling. The way around it is overcharge the system to get head up like a lower SEER unit. So we've defeated the purpose of having a high SEER outdoor unit. And the overcharge means more strain on the compressor and more electrical usage.

Now, in some brands - Trane for example, you can often take a TXV kit, which regulates the flow of refrigerant based on load not head pressure, and get 13 of the older "10 SEER" coils. With Rheem, a 13 SEER outdoor unit and TXV coil AND variable speed blower runs about 13.75 SEER or probably 12.75 without VS blower.

I've always maintained that a coil shouldn't be changed or installed with the furnace. This thread is perfect example of why! And with the refrigerant change in the future, even more important.

[j-race]

thank you

[j-race]

Originally posted by comfortdoc
Indoor coils technically do not have a SEER rating.

Thats what brought about my question. Which has now been very thoroughly answered.

[dash]

Originally posted by j-race
also The coil is about a year old but never been used when we installed our new furnace we had them install the a coil so we could purchase the condenser later.

Depending on the brand and model ,a one year old coil,may be a good match by adding a TXV.Can you post the model numbers?