Oversizing

Boston

So, next queston after many of you helped out on the boiler question.

They have 2 125,000 air handlers in a house that shows a total heat loss of 90,000(excluding garage numbers)

I will see the contractor tomorrow and see what he claculates the heat loss to be, but I assume it is right around 100k.

Is this a big problem to have excess capacity in a air handler, or could it be a good thing, unlike a boiler?

Thanks for the education.

Steamhead (in transit)

Depends on the type of boiler you're using

The oversized AHU coils mean that you don't have to get the water as hot to heat the house. If you're using a condensing boiler this is great, as these units love low water temperatures. With a cast-iron boiler, the low water temperatures could cause problems with flue gas condensation unless a bypass or a primary-secondary piping setup (P-S is the better of the two choices) is used.

Whichever boiler type you decide on, size it to the heat loss, NOT the AHU capacities.

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[Deleted User]

As with ANY hydronic question, the correct answer is...

It depends :-) But if you've ever been here before and stayed for very long, you ALREADY knew that;-)

Technology today has changed. A few years ago, I'd tell you it would be problematic. Similar to what we call driving a tack with a sledge hammer, except for when you are at design condition, which is typically the SMALLER percentage of the time.

Kind of like trying to control the speed of your Corvette with the acclerator pedal GLUED to the floor, and you are controlling the speed by oscillating the ignition key on and off as fast as you can. Pretty herky jerky ride eh...

Todays controls have the ability to vary not only the flow of water going to the individual FCU, (non electric TRV) but also have the ability to modulate fan speed (Variable speed fan motors) based on deviation between set point and actual room temperature.

Automatic cruise control for your heating system if you will. This cruise control wil extend backwards all the way back to the mod/con heat source as well. If it "sees" a small load, it idles along, putting in only what's necessary to maintain human comfort.

If the load increases (open doors, windows, weather changes whatever) it will see this increase in load and adjust its output accordingly.

Pretty nifty eh... So, as I previously stated, it depends. It depends on how YOU set up the system.

Herkey jerkey whiplash style, or smooth, efficient yet powerful cruise control.

ME

Boston

ok, got it

So, I have these super sized air handlers - so if I tie in a nod/con like the ultra(I had a whole other educaton on that) we can modulate water temp to fit the load. I can actually see why installing too small of an air handler could make a mod/con struggle.

So, we have some large air handlers, I will get the mod/con and pray the installing contractor can figure out how to put everything together.....

Thanks again.

Actual load is around 100k. I see the ultra 155 hasd anet of 123. Are those numbers close enough to my heat load to make sense?


Can't thank you guys enough. By the way - it is a LOT OF WORK to learn this stuff. But if I had not said something I would be having a 250k btu burner(net) being installed instaed of a 123k.

I'd love to hear predictions of differential in fuel bills between these 2 scenarios. I am guessing 40-50%.

Rodney Summers

Considering your garage

is taking up 14000 of the 100000 btu heat loss, the Ultra 105 might be the best choice when considering REAL eff. to heat loss calcs. Considering theres a 10 to 20% fudge factor built in to heat loss software the 105 appears to be the best suited for your calcs.

Never heard what your domestic load is expected to be, you'll be using an indirect?

Boston

domestic

bob:


5 people in house, i did not calc dhw, i thought i understood NOT to add that in.

and you are correct that 14k of the total is garage, that we would not be heating to 60 on the coldest days of the year for sure.

So I don't need to worry about the 81k net on the ultra 105. 81 is a long way form 100 if its cold out.

Rodney Summers

Chilton

I agree ,9 times out of 10 the domestic hot water needs of a boiler should be secondary to the heat loss needed from the space heating, however there are those times when the domestic HW load is greater than the heat loss of the house and those are the times when domestic water needs should be looked at carefully.

If your domestic load includes large capacity tubs (75 gallons) and the expectation of filling the tub and running the washer at the same time, than the capacity of the indirect and or the size of the boiler need to accomodate this.


I didn't remember reading what your domestic needs were. So thought I'd check and see.

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