Two stage GFAs and inefficiencies

RnRn

I've been hearing that two stage GFAs are not all they are cracked up to be and that low fire efficiencies are much lower due to lower HE temps and blower cool down.  Is it as simple as jumping w1 and w2 to keep in high fire?  Is a delay needed? 



Reason I ask is that I am oversized after a second Man J and would be just about perfect if I could snag low fire btu outputs off two Heil GFAs, 100k and 60k input, or keep in low fire state if someone can convince me its the right thing to do. 



Thanks,  Ross

Tim McElwain

Ross the problem in most cases with staging residential systems

is the system itself is of a poor design. What I mean is in the case of warm air there is insufficient return air and blower speeds tend to be excessive. The blower speed issue has to do with the design of FWA. The expectation of an air conditioning coil in the plenum means that even on low speed the blower runs at too high a speed for heating. The new ECM variable speed blowers solve some of this. Even those however are installed as replacements with insufficient return air so the efficiency is defeated by poor system design. The National Warm Air Heat Institute in 1926 determined that return air should be 1 1/2 times greater in area than supply.



We could handle some of that when furnaces had a temperature on and temperature off fan control. We could by adjusting the on and off settings, dial in some comfort and eliminate the cold air blowing on start up and shut down. The modern furnace which uses a time on time off control (IFC's and EFT''s) makes it difficult to overcome the blowing cold air problem. In many cases the on time is fixed at either 30 or 60 seconds and is not dip switch adjustable. This is to protect against thermal stress on the heat exchanger. The furnaces in order to have higher efficiencies have reduced flue loss (holding more heat in the unit) and the trade off is higher plenum temperatures and possible thermal stress if the blower does not come on quickly enough. In fact we even put back up high limits in the blower compartments to shut the system off more quickly if the blower fails.



Two stage systems in my estimation if not properly designed are inefficient. I would prefer for example a system coming on with high fire first and then if the high fire call is not needed then cut back to low fire. Starting on low fire affects the delta T and exacerbates the blowing cold air syndrome.



I would need to have from you some information on the controls on your system numbers and make to then suggest a solution to your problem. It is possible in some cases to simply jump out W1 to W2 which now would mean on every call for heat it would go directly to high fire. That is not what it seems you are looking to do. It sounds like you want to cut back and only run of low fire so you can more closely adjust to your Manual J calculations relative to heat loss.



The problem with firing at less than full input is that design parameters of the furnace are violated. The combustion chamber is designed to be fired to maximum in order to obtain full combustion efficiency. If I run a staging setup then I may loose some of the overall efficiency if the system itself is not designed to handle that setup. I prefer firing a system at maximum designed firing rate as determined by a combustion test conducted by a professional who understand combustion principles.



I hope that helps.