Minimum Heating Hot Water Temperature

[Jamieo]

Hello All,
I have a question regarding the minimum temperature of hot water entering a heating coil. If I want 95 F leaving air temperature from the coil, what would be the minimum entering water temp that I can have and still get the leaving air temperature I need? I have a situation where I have 120 F water entering the coil. I have determined the GPM's by knowing the load and assuming a delta T of 20 degrees. Typically heating hot water is in the 160 to 180 degree range so I have never though about this before, however with such a low temp entering water this has made me concerned that maybe I will not get the temperature rise in the air that I need. All help is much appreciated! Thanks!

 

[walkes]

You can likely get a coil to do that but it would likely be a multiple row coil. The air pressure drop across the coil would probably be greater. It would all be in the coil selection. Coil selection software is available online from heatcraft.

http://www.heatcraft.com/resources/software.asp

 

[TBP]

You're not working with much delta-T, so you'll need a relatively large (as in VERY large) surface area of coil to "make it happen". The coil manufacturer will have selection tables or application software.

 

[ChrisConley]

The problem is approach. The closer your desired air temperature is to the water temperature the more coil you need.

 

[cme]

The "old" days of HVAC design used 20 deg differentials.

A 2 row coil and at least 40 deg differential is usually the norm these days.

You save much pump energy flowing less water and the pipe sizes/pumps/etc are smaller which benefits 1st cost.

I woul advocate primary /secondary pumping when using a boiler system tho. 20 deg differential across the boiler and the secondary for the max differential you can get away with.

That's my way to skin the cat.

 

[TBP]

But if he's only got 120*F entering water temp, he can't take a 40* delta-T and still get anything even approaching 75*F air out of this coil, let alone 95*F, right? I'd suspect that he'd need 160 inlet temp, dropping to 120 to make this go. Either that, or a coil the size of a boxcar

 

[CarolinaPE]

Most any air handler builder program will allow you to enter such information and pick a coil. More coil rows and fins along with greater air pressure drop will surely result. McQuay, Carrier, York, etc all have programs to do this, better yet give the inputs to the product rep and make him do the work for you. The reps usually are very happy to install their selection software on your computer. Our chw coils tend to be 6 row in FL and heating 1 or 2 row. Based on wanting a 55 LAT and 45 EWT a 120 EWT and 95 LAT may be doable with a deeper coil. What is your entering air temp? I did a quick calc (with 60 degree EAT) on the carrier program and it did pick a fairly deep coil for heating apps.

 

[kpntreal]

Of course your fan speed figures in as well.