BTU of HX?

[jcappe]

For those of you heating with a forced air furnace and OWB if you would please put the sq. ft you are heating and the size of your Heat Exchanger.

I'm heating 4,000 sq.ft with a 100,000 BTU HX. I'm second guessing the dealer on the fact that I should have put a bigger one in. It may be that it is just so dambed cold but I would be interested to see your answers. Thanks.

 

[dirtyedge]

I am heating 4,000 sq ft with two furnaces one has a 140k btu HX and the other has a 100k btu HX in it. If there was only one furnace I doubt it would be able to keep the house 72 like it is now with the outside air temp at -10f.

I'm guessing that your dealer sold you the 100 because that is the size that would fit in you plenum? Do you have AC, and is your HX above or below your ac coil?
When we were installing our hx's we had to move the acoils up, and were able to cut larger openings in the duct to allow more air to flow past the HX's.

IF you have room for a larger hx I would definately install the largest possible as there isn't much price difference between them.

 

[jcappe]

Here's a pic of my setup

As you can see my HX is set right at the end of my cold air return duct work. My dealer told me I wouldn't be able to get it in above but I think I could have raised the acoil and got it in there. Was told by a guy after the install that I shouldn't put it in the cold ari side because it is hard on the furnace blower, don't know if I'll change it or not as I know several other people that have done it this way with no problems. You are probably right about the 100,000 being the only that will fit in the duct work.

 

[Slick]

I'm heating 1250sq foot house with over 100k...it's a 16x18,can't remember what the actual number is will have to check. I know when I put the gas furnace in two years ago I did the heat loss numbers and needed at least 75k...can't imagine heating 4000sq ft on 100K unless it's crazy insulated...ICF or SIPS or something.

 

[mtfallsmikey]

What exactly is the heat loss of your house?....

IMHO, a HX needs to be 20-30% larger than what yo need...why you ask?
HX's are designed to put out XXX BTU at XXX water temp. when your OWB water temp runs down to/below the turn-on setpoint, BTU output is derated. Instead of the HX capacity of lets say....100K BTU @ 180 deg., it may only do 60K BTU @ 170. That's why I say DO YOUR HOMEWORK when designing these systems!!:deadhorse:

 

[John D]

What exactly is the heat loss of your house?....

That's why I say DO YOUR HOMEWORK when designing these systems!!:deadhorse:

Exactly,and then when you've taken everything into account,go up one size larger anyway. Esp with the coils,it will allow you to heat well with cooler water.I think you are right,you didnt do your blower any favors by putting the coil on the suction side.

 

[mtfallsmikey]

:agree2: :agree2: :agree2:

 

[Marklambert61]

HX problem

Are you sure its the HX?

If your feeding it with a 1" pipe at a normal run length you will never get more than 80,000 btu's normally less based on restriction like fittings.

Water flow is the more common issue a 1" pipe will flow 8GPM rule of thumb with a 30 degree temp drop you will yield about 10K per gal therefore 80K max.

Mark

 

[jcappe]

What exactly is the heat loss of your house?....

IMHO, a HX needs to be 20-30% larger than what yo need...why you ask?
HX's are designed to put out XXX BTU at XXX water temp. when your OWB water temp runs down to/below the turn-on setpoint, BTU output is derated. Instead of the HX capacity of lets say....100K BTU @ 180 deg., it may only do 60K BTU @ 170. That's why I say DO YOUR HOMEWORK when designing these systems!!:deadhorse:

Not sure on the heat loss of my house. I did plenty of homework but when it came to size of hx and such in which I had no previous experience with I relied on my dealer for help, he saw my house, my furnace set up and told me to go with a 100,000 rated HX so I did. Your theory of 20-30% larger makes perfect sense to me. I will see what sizes I can find in the 125's and see if I can make it fit somehow, and probably move it in the process if I can make it work. Thanks for the replies guys, this site rocks

 

[Slick]

http://www.taco-hvac.com/uploads/FileLibrary/SelectingCirculators.pdf

If your curious, run the formula on this link from Taco on your system, I did when selecting my pump and it really helped me grasp the concept of flow and BTU relationships etc.

 

[mtfallsmikey]

We've had lots of threads since this past fall on this subject...maybe the Mod Gods will let some of us conjure up a sticky of design stuff, real vs. design, real stories....

My humble home has a heat loss of 46K BTU...my HX is rated at 56K BTU @ 180 deg. In a normal situation, using gas/oil-fired boilers, you figgure your boiler size to match the load...there is a fixed BTU net output, sizing it to the load prevents short cycling, etc...but wood is another story. Example; my CB 5036 has a capacity of 199 gal. If it is full of 180 deg. water, and my secondary circ. on my house HX loop calls for heat, at 4 gpm, how much capacity do I really have before the water temp cools off? What if the t-stat calls for heat, but your fire gettin' low, water temp only 165..how much capacity do you have then? Take the rated BTU capacity of an OWB with a grain of salt! You can graft the larger HX into your SUPPLY plenum if you have the room, make a larger plenum or duct transitions. If yo don't have room, look into an "H" style, or slanted A.C. coil.

 

[Stihl310]

Alright,
I got a quick question, I calculated head loss per taco's formula's.. I came up with a total head loss of 11 feet... Now my question is do I need to also add in the physical head that I"m going up against as well, since I will be mounting my circ. pump approx 6-7 feet below my wood burner ( in the basement) This would then give me approx 18 ft of head loss. Causing me to have to jump to a larger pump than what I was wanting to use to maintain a gpm of around 9-10 gpm. I'm assuming that this head calculation is taking into consideration (drag) for lack of better words that the pipes cause on the water, thus causing back pressure...

Hopefully someone understands what I'm talking about here...If this is the case then I may be as well to just mount the pump on the furnace, therefore removing the 6-7 foot of head altogether

Thanks!

 

[mtfallsmikey]

If system is pressurized, no. if it is, I would add the vertical lift.

Got a neat new flow/head calculator in the mail from Grundfos over the weekend....they are promoting their new line of VS pumps.

 

[trialanderror]

http://www.heatexchangersonline.com/airtowater.htm

i'm gonig with the 150k unit. even though i only need 75k, i can easily acheive 88k with 140F water so i don't need to fire nearly as often/hard. if needs be more btu, just turn up the fan speed a bit and raise the water temp. always go a bit bigger. never know when you'll need it.

in my case, i'm going to need 1.25"pex. to get high gallon per minute with the HX i want. so that means, i'm already plumbed up if i want to add more hx's somewhere else, i'll have the water capacity.