Oversizeing the coils in your furnace?

[Sawyer]

As not to hijack a thread, it has been mentioned in "Gonna Take the Plunge" https://www.hearth.com/econtent/index.php/forums/viewthread/74084/ that over-sizing the furnace coil will allow for lower water temperature operation.

I am presently sized for 100MBTU @ 180 degrees, would adding a second similarly sized coil allow me to operate at temperatures similar to radiant? Perhaps I am over-simplifying the solution to a problem? It may be much easier for me to double the coil than to install radiant under the hardwood floor in the lining room.

 

[Sawyer]

Heaterman has made a thorough reply to my question in the above thread. No need for him to repeat it here.

Fan Coils, Heat Exchangers and Water Temperature......Oh My!! (with apologies to the Wizard of OZ people)........

There are indeed a host of variables that will ultimately determine the lowest temp you can run your storage at. These are impossible to determine here and can only be done with an accurate heat loss calc of the structure, a duct sizing/cfm calculation based on the heat loss and evaluation of what the air handling device/furnace can put out (cfm) in the first place.

BTU transfered into the air stream is based on this simple formula. (Temp rise across the heat exchanger x cfm x 1.08) = btu into the airstream.

Obviously a hotter coil will transfer more btu's but also tightly limit the operating range of your storage or boiler. Running a gasser with no storage on a system that is strictly forced air in nature has been a poor application in my humble experience. Many times on a call for heat, the air handler will drain the btu's from the boiler creating a low temp condition that activates boiler protection before the fire can "respond" and drive heat back into the water. Meanwhile the fan blows cold air into your house which leads to very low WAF.

You might want to consider adding a few panel radiators to the system to augment the forced air side of things. I guarantee you would be amazed at the comfort level difference between rads and a forced air system.

 

[heaterman]

As not to hijack a thread, it has been mentioned in "Gonna Take the Plunge" https://www.hearth.com/econtent/index.php/forums/viewthread/74084/ that over-sizing the furnace coil will allow for lower water temperature operation.

I am presently sized for 100MBTU @ 180 degrees, would adding a second similarly sized coil allow me to operate at temperatures similar to radiant? Perhaps I am over-simplifying the solution to a problem? It may be much easier for me to double the coil than to install radiant under the hardwood floor in the lining room.

If you are talking about adding a second coil after the first one, that would be a no no for two reasons. 1st. You will double the static pressure in your duct system and which will greatly shorten blower motor life as well as reduce your airflow. (remember that cfm is a component of how much heat you can transfer. reduce the cfm=reduce the btu's)
2nd. Adding a second coil downstream of the first will not boost your heat output any appreciable degree. You have already raised the temperature in the airstream with the first coil which lowers the delta T you can work with on the second. You would probably pick up less than 30% of the rated output of the second coil.

 

[ewdudley]

As not to hijack a thread, it has been mentioned in "Gonna Take the Plunge" https://www.hearth.com/econtent/index.php/forums/viewthread/74084/ that over-sizing the furnace coil will allow for lower water temperature operation.

I am presently sized for 100MBTU @ 180 degrees, would adding a second similarly sized coil allow me to operate at temperatures similar to radiant? Perhaps I am over-simplifying the solution to a problem? It may be much easier for me to double the coil than to install radiant under the hardwood floor in the lining room.

If you are talking about adding a second coil after the first one, that would be a no no for two reasons. 1st. You will double the static pressure in your duct system and which will greatly shorten blower motor life as well as reduce your airflow. (remember that cfm is a component of how much heat you can transfer. reduce the cfm=reduce the btu's)

Adding a coil would double the amount of resistance attributable to coils, but unless all other resistances add up to zero obviously the static pressure of the duct system would not be doubled.

So system static pressure coefficient would increase somewhat and system flow coefficient would decrease somewhat, but not in direct proportion; the loss of flow would be percent-wise less than the increase in static pressure.

The fan motor would run faster and work less, so no problem there.

2nd. Adding a second coil downstream of the first will not boost your heat output any appreciable degree. You have already raised the temperature in the airstream with the first coil which lowers the delta T you can work with on the second. You would probably pick up less than 30% of the rated output of the second coil.

By this logic swapping out the first coil with one half as large will not appreciably reduce the heat output, which might or might not be true. A useful answer can't be provided without more complete information.

--ewd

 

[nt30410]

As not to hijack a thread, it has been mentioned in "Gonna Take the Plunge" https://www.hearth.com/econtent/index.php/forums/viewthread/74084/ that over-sizing the furnace coil will allow for lower water temperature operation.

I am presently sized for 100MBTU @ 180 degrees, would adding a second similarly sized coil allow me to operate at temperatures similar to radiant? Perhaps I am over-simplifying the solution to a problem? It may be much easier for me to double the coil than to install radiant under the hardwood floor in the lining room.

Within reason, you can't be too thin, too rich or have too big a heat emitter. Go for the largest single Heat Exchanger that will fit....and be sure to pipe it in a counter-flo fashion ie. the air leaving should "see the hottest part of the heat exchanger.

Best of luck!

 

[heaterman]

Within reason, you can’t be too thin, too rich or have too big a heat emitter That is crackin me up!! Can I use that line?


And E.W. .... I was just trying to give Sawyer a general description of why stacking a couple HX's will not produce what many people think it should. Not write a technical white paper on the subject. Hope I did not lead anyone astray here.

 

[Frozen Canuck]

George: Heaterman makes valid points. Your forced air system will place limitations on what can be achieved baring a reno of course. No point adding more resistance to airflow without putting in a fan motor in the furnace that can handle it. Even then it will need to fall within a comfort range for the occupants, as too many cfm's will have things flying all over. Part of the price we pay for living with a forced air system, not nearly the variability or comfort of a hydronic system. If your like me thats a good reason to put more thought into how to make a hydronic system work in the home. Plenty of good options out there, HR, Heaterman & other members have posted quite a few for us to ponder, just a question of what fits your situation. Maybe ask members for sites with decent "how to install" videos, that may make the vast forest of choices a little clearer. Worth a try IMO.

 

[Sawyer]

Thanks all!

I am going to dump the idea of expanding the coil or adding another coil and leave "as is." no sense in sticking money and time into it if results are less than optimum.

I will go back to radiant plan for the living room which has been my only less than perfect area since I started the Gar. The irritations will be solved with controls and switches this summer which sounds like more fun than cranking pipe. ;-)

 

[ewdudley]

Thanks all!

I am going to dump the idea of expanding the coil or adding another coil and leave "as is." no sense in sticking money and time into it if results are less than optimum.

I see your point, since the coil is you have now must already be the optimum size and could never be improved upon. Changing anything would most definitely be a no-no.

I will go back to radiant plan for the living room which has been my only less than perfect area since I started the Gar. The irritations will be solved with controls and switches this summer which sounds like more fun than cranking pipe. ;-)

With home-run small-diameter pex piping, radiant panels are hard to beat, much easier to get great performance.

 

[heaterman]

Thanks all!

I am going to dump the idea of expanding the coil or adding another coil and leave "as is." no sense in sticking money and time into it if results are less than optimum.

I see your point, since the coil is you have now must already be the optimum size and could never be improved upon. Changing anything would most definitely be a no-no.

I will go back to radiant plan for the living room which has been my only less than perfect area since I started the Gar. The irritations will be solved with controls and switches this summer which sounds like more fun than cranking pipe. ;-)

With home-run small-diameter pex piping, radiant panels are hard to beat, much easier to get great performance.

I will second, third and fourth that statement EW. There is little else in the heating world that is as simple to pipe up, easy to control and offers the level of comfort as a panel rad system. There are a number of very good reasons panel rads are found in nearly every European heating system.

My favorite method lately is tapping a Wilo VS circ into the heat source of choice and controlling it via an outdoor temp sensor. Set the speed on the Wilo to provide a 30-40* drop and size the rads for 140-160* water at design temps. The TRV's on each rad provide temperature control for each room and do it proportionally rather than on/off. (that in itself makes an amazing difference in comfort) Run each rad off from a manifold just like you would use for a radiant floor and let 'em rip.

 

[Sawyer]

Perhaps I am missing something. If I size for 140-160* water, am I not ignoring the 400MBTU stored in the Garn usable for temps 110-140* that I could use if I install radiant floor loops?

 

[heaterman]

Perhaps I am missing something. If I size for 140-160* water, am I not ignoring the 400MBTU stored in the Garn usable for temps 110-140* that I could use if I install radiant floor loops?

Everything is a trade off. If you want to wring every last btu out of the water in any storage system the emitters have to be sized accordingly. I sized the rads in my house to work with variable water temp down to about 95* at 50* outside. My system (natural gas fired) will bump the water temp up accordingly as the outside temp drops, reaching a maximum of about 170* when it's -10* or less.
Now, if you want to use 110* water to heat your house at "design" conditions (-10 to -20* for your area?) you will be using some pretty good size rads. I would suggest a compromise that would allow using very low temps (<130*)when the weather is in the shoulder seasons and figuring on a little higher(>140*) during January/February weather.

Good grief............6" of new snow on April 20th is mighty ugly.........

 

[Sawyer]

Thanks Steve, I get it! ;-) Compromise, does that mean the snow we are getting could be 12"? :shut:

Thanks again.

 

[Tennman]

HM, A few months ago (Jan 2011) the 1/2 Hp fan motor went out in the propane furnace that drives the air movement downstairs in our home. Several posts up you commented about increasing static pressure with the consequence of shortening motor life..... Huh.... A light went on about the blower that I had never considered. I installed the boiler HX back in Fall 09 and the fan has been fat dumb and happy blowing with the boiler HX, occasionally propane heat, and air conditioning over last Summer. My HVAC contractor showed up accidentally with a 3/4 hp instead of the 1/2 for the last Jan replacement.... Well.... about a month later my blower started making a racket and the welds were broke on the squirrel cage fan.... At the time going to the 3/4 hp seemed fine to me because I figured the current draw would be the same because the 3/4 motor would just use what fraction of whatever the 1/2 used.

SO the fact that I significantly increased the static pressure with a HX suitable for a 180 kbtu boiler am I seeing the consequences of that big HX in the ductwork? I never considered that the HX would dramatically increase the workload of the fan system. AND I've never read here before about the increase in static pressure having any consequence. The fact that my HVAC guy screwed up and brought a 3/4 instead of a 1/2 hp back when the motor fried, I sorta think that was a good thing now because I've never seen a discussion of how a boiler install increases fan horsepower requirements.

Until this moment, I just thot that for the motor then the fan cage it was just time to die.... Sorry for totally hijacking this thread, but I've never heard of any of us worrying about adding to the stock fan's workload.

 

[dogwood]

Heaterman and ewdudley, I am putting in an oversized w/a heat exchanger over my forced hot air furnace. Might it be workable to avoid impeding the air flow too much and overtaxing the blower motor, to find a way to switch out the w/a hx out seasonally with the a/c cooling coils over the furnace. The blower motor on my Lennox Pulse furnace is only 1/3 horsepower and my w/a hx is just like the one pybyr pictured in previous posts. I'd appreciate any thoughts on the matter.

Mike

 

[ewdudley]

Heaterman and ewdudley, I am putting in an oversized w/a heat exchanger over my forced hot air furnace. Might it be workable to avoid impeding the air flow too much and overtaxing the blower motor, to find a way to switch out the w/a hx out seasonally with the a/c cooling coils over the furnace. The blower motor on my Lennox Pulse furnace is only 1/3 horsepower and my w/a hx is just like the one pyrbyr pictured in previous posts. I'd appreciate any thoughts on the matter.

Mike

It makes perfect sense that adding restrictions to the air ducting system would add load to the fan motor, but please consider that it's not true.

Adding restriction to the system will simply move the operating point of the fan further to the left on its fan curve. This means that as restriction is increased, flow decreases, and the pressure drop across the fan increases. Here's a nice short article on the topic:

http://www.jeacoustics.com/library/pdf/72-Evans.pdf

There's two ways you can get in trouble with the fan motor.

If the motor is too small for the fan itself then it will work too hard and overheat. This is a design issue. The motor could only be too small if the unit was badly designed or if somebody swapped-in an unsuitably dinky motor. If the motor is too small then you'd likely be in trouble at any point along the fan curve.

The other would be a situation where air flowing through the duct is required to cool the fan motor. If this is the case then restricting flow way too much would allow the van motor to overheat for lack of ventilation to the fan motor itself. And again, only if the motor needs airflow from the fan it is driving to keep itself cool.

So having too much restriction -- or too little restriction, for that matter -- is an efficiency problem, not a motor overloading problem. If your air handler is designed for a certain flow at a certain pressure drop at a certain RPM, then adding restriction would cause the fan to operate more into its stall region, resulting in somewhat more noise, somewhat less air flow, and -- counter intuitively -- somewhat less power input from the motor.

You can prove this to yourself by putting an ampere meter on your fan motor and record current draw as you increase restrictions in the system by closing dampers or stacking stuff on top of the registers. Or place your hand over the outlet on your shop vac and listen to the motor speed increasing towards synchronous speed, which shows that motor load is decreasing. Or note that the engine RPM decreases at constant throttle as a light airplane takes off down the runway.

--ewd

 

[Tennman]

Thanks for the reply ewd. I will go read that thread about fans. Sounds to me then that the death of my motor and fan cage was just due to age. Independent of the big, new HX. Good news. Thanks.... April in Tennessee.... 60F nites, 75F days.... lovely. Except for the occassional tornado!

 

[dogwood]

Thanks ewdudley. I'll have to think over what you're saying so I understand better what I should do.

Mike

 

[ewdudley]

Thanks for the reply ewd. I will go read that thread about fans. Sounds to me then that the death of my motor and fan cage was just due to age. Independent of the big, new HX. Good news. Thanks.... April in Tennessee.... 60F nites, 75F days.... lovely. Except for the occassional tornado!

I guess I should clarify that all this means is that you shouldn't be hurting the fan motor by adding restriction to the air flow, it doesn't mean everything will work out just fine.

If the fan was already working near its optimum efficiency point, then adding restriction will give you more noise and more agitation, less air flow, and less fan efficiency. Even though the motor power draw will drop off somewhat, the amount of electrical energy converted into moving air will drop off even more.

To return to optimum efficiency you'd need to reduce the fan RPM with a smaller pulley on the motor or whatever and live with less air flow, or go with a different fan that can efficiently deliver the required airflow with the increased restriction because it is designed to work at a higher static pressure.

--ewd