Adding a "Fan Coil" to a Condensing Furnace

[jimnorth]

I came across a "Warning" not to install a fan coil in a forced air system which utilizes a high efficiency condensing furnace. There was no explanation as to why. If anyone here can explain what the reasoning is behind such a warning, I'd appreciate it.

Thanks.

 

[BrownianHeatingTech]

I came across a "Warning" not to install a fan coil in a forced air system which utilizes a high efficiency condensing furnace. There was no explanation as to why. If anyone here can explain what the reasoning is behind such a warning, I'd appreciate it.

Most likely related to the static pressure in the ductwork. The fan coil is a restriction. The higher-efficiency furnaces can be more sensitive to proper airflow. Many of the lower-end of that class have regular steel primary heat exchangers, and then stainless secondary heat exchangers (where the "condensing" happens). If they don't get proper airflow, the flue gas can condense in the primary heat exchanger, resulting in corrosion and premature failure.

Lack of proper airflow can also result in short-cycling, which could cause excess wear, as well.

The solution is to make sure that the ductwork is designed correctly, so that the stating pressure of the ductwork (including the fan coil) does not exceed the furnace fan's capacity, at the airflow that it needs to produce. If it does, the fan coil might need to be replaced with a lower-restriction unit, or the blower upgraded, or the ductwork redesigned (many residential duct systems were done using whatever material was cheapest, not what was the right size, unfortunately).

Improper airflow will also be detrimental to the output of the fan coil. Coils are rated to produce a certain number of btu's at a certain water temperature and a certain entering air temperature, with a certain airflow. The manufacturer should be able to supply a set of tables that compares those three variables, and you may find that the actual rated output of the fan coil - in your application - is significantly lower than the maximum-rated-output that the seller listed it as.

Joe

 

[jimnorth]

Joe,

Thank you for your very informative reply. Now that I know what the issues are, I can make sure they are addressed properly.

I suppose the warning (part of the Garn installation manual) was inserted, without explanation, upon legal advice... Much safer, liability wise, to say "don't do it", than to explain.

Thanks again!

 

[mtfallsmikey]

Do you have central A.C. installed on this furnace?...check your manuals; the rated cfm/static pressure ratings should be there for your furnace/cooling coil application. Rule of thumb would be 400 cfm/ton of A.C. required. Hopefully, all you would have to do IF everything else jives would be to bump up the blower speed a notch.

 

[jimnorth]

Do you have central A.C. installed on this furnace?...check your manuals; the rated cfm/static pressure ratings should be there for your furnace/cooling coil application. Rule of thumb would be 400 cfm/ton of A.C. required. Hopefully, all you would have to do IF everything else jives would be to bump up the blower speed a notch.

Yes there is a central AC evaporator in the duct. I have the manuals for both, and I see that I do have a variable speed blower. I'm fairly confident that the ducts are sized properly. The contractor used HVAC design software which calculated everything from heating/cooling load, airflow and duct sizes. I have the printouts in the file.

Thanks

 

[heaterman]

You may be in trouble. Carrier for one does not recommend the addition of a hot water coil in the duct system of any of their variable speed products. (these would be Carrier, Bryant, Payne, Day-Night etc.) I've been told that their current manuals state that the warranty is void if more than one coil (your A/C coil) is installed in the duct system. Variable speed fans as a rule do not deal very well with high external static pressure and the replacement motors are very pricey, which you will probably find out, after installation of the second coil. If you proceed you should get an HVAC tech in there to measure the static pressure in your duct system and make sure it still falls within specs of your furnace.

The blower motor is one issue, the second one is the heat exchanger of your furnace. What happens in the furnace is that due to reduced air flow, the temperature of the secondary HX becomes elevated past its design. Many secondaries in high efficiency furnaces contain plastic material in the form of linings, end caps and connectors. These will not tolerate elevated flue gas temperatures as you may well expect. These pieces can deteriorate and become structurally unsafe or even melt in some cases allowing flue gas from your furnace to enter your house. .......Do you have a working CO detector?

I'm not exaggerating the seriousness of this or using scare tactics on you. It's just that with HVAC equipment, it's the things you DON'T know that will kill you. Always more to it than meets the eye my friend. Get all the ducks in a row before crossing the street.

 

[jimnorth]

You may be in trouble. Carrier for one does not recommend the addition of a hot water coil in the duct system of any of their variable speed products. (these would be Carrier, Bryant, Payne, Day-Night etc.) I've been told that their current manuals state that the warranty is void if more than one coil (your A/C coil) is installed in the duct system. Variable speed fans as a rule do not deal very well with high external static pressure and the replacement motors are very pricey, which you will probably find out, after installation of the second coil. If you proceed you should get an HVAC tech in there to measure the static pressure in your duct system and make sure it still falls within specs of your furnace.

The blower motor is one issue, the second one is the heat exchanger of your furnace. What happens in the furnace is that due to reduced air flow, the temperature of the secondary HX becomes elevated past its design. Many secondaries in high efficiency furnaces contain plastic material in the form of linings, end caps and connectors. These will not tolerate elevated flue gas temperatures as you may well expect. These pieces can deteriorate and become structurally unsafe or even melt in some cases allowing flue gas from your furnace to enter your house. .......Do you have a working CO detector?

I'm not exaggerating the seriousness of this or using scare tactics on you. It's just that with HVAC equipment, it's the things you DON'T know that will kill you. Always more to it than meets the eye my friend. Get all the ducks in a row before crossing the street.

Thanks for the warning.

The unit in question is a Goodman (Janitrol) GSM-80. It has an aluminized steel primary H/E and a stainless steel secondary H/E. There is some plastic at the tail end (draft inducing fan and stack connection), which is protected by a stack overtemp switch. I see no mention of fan coils in the manual -- including the warranty.

Based on all of the replies to my posting, it appears the primary issue to be addressed is the reduction of airflow caused by the introduction of the fan coil -- making sure the flow and static pressure remain within design limits.

 

[heaterman]

Your quote: "Based on all of the replies to my posting, it appears the primary issue to be addressed is the reduction of airflow caused by the introduction of the fan coil—making sure the flow and static pressure remain within design limits."

You got it. I wouldn't worry too much about the Goodman. They aren't a super high efficiency furnace like some of the others despite what the AFUE rating says. Just have a tech check your airflow after the coil is installed. Reduced airflow can really raise heck with the A/C side of your system too.