HELP! Sizing Hot Water Unit Heater

willie t

Can anyone help me with the following?

I have a customer who wants to use 80 degree water to put heat into a room at a hockey rink. He has 80 degree water left over from the ice making process machines. He wants to use a hanging type unit heater. (Radiant not an option.)
He knows he needs about 50,000 btu's to heat the space and he calls and asks if it can be done. I go to my hot water unit heater guys and they say use a chart that derates the heater output based on lower temp input water. No problem so far.
I go to the chart which says use a fator of .143. I divide 50,000 btu's ( what I need to heat the space) by the factor (.143) and I get approx 350,000 btus. This is the input of the heater needed to give me 50,000 btu output using 80 degree supply water. Sounds about right.

BUT:
This is based on 60 degree entering air temp. So I figure we may have less than 60 degree entering air so I go to the chart again and use 40 degree entering air.
You would think that a lower air temperature entering would require more BTU's, at least that's what I'm thinking.
I go back to the chart and the factor for 80 degree water with a 40 degree entering temperature is .293!!!!!

So if I do my math right ( 50,000 btu / .293 = 170,648 btu's) I only need 170,000 btu's ?????

This does not make sense to me!

I called two major unit heater manufactures ( the reps agreeded with me that it did not make sense but they could not explain it) and they could not explain why I needed less BTU's with a lower entering air temp.

Can anybody explain this so it makes sense?

Forgot to mention that we only want to keep the room at the rink around 50-55 degrees.

Any thoughts would be appreciated!

Brad White

It is all Delta-T

Personally, I would target that 80-degree water toward pre-heating domestic hot water, where you have a more constant delta-T to work with. A double-wall plate exchanger if code requires, can get you within a few degrees of fluid equilibrium.

To stretch the factors in order to illustrate your apparent conundrum:

Narrow things a bit. Say you wanted to maintain your space (or assumed entering air temperature) at, say, 75 degrees with your 80-degree water, that would be a 5 degree approach which means a lot of surface area indeed. Try that selection and see a unit heater Torvil and Dean could dance in.

Stretch this to the other extreme for an example: Say you wanted to maintain the space (assumed entering air temperature) at 35 degrees. That is an approach of 45 degrees, that much more heat output and that much less surface area.

It is not that you need fewer BTU's but that you need a unit that can generate fewer at normal (180F or 200F rating basis) temperatures.

At the end of the day, you are NOT going to have 40 degree entering air temperature entering the unit. Rather, it would be about space temperature, so select it for 50 to 55 degrees entering air and be done with it.

Be sure to pipe it correctly such that the warmest entering water sees the warmest leaving air ("counterflow"). This will extract the most BTU's from your water flow.

Another few suggestions:

1) If the space is high, use a down-blast propeller type heater if you can. It will act as a destratification fan in the bargain.

2) Beware of drafts. Moving air has that issue on bare skin if any.

3) Install a variable speed control for the fan, appropriate to the motor type. You will find yourself balancing the right combination of air volume and outlet temperature. Sure, you can move more air and extract more BTU's but your leaving air temperature may be too cool for comfort. (More air warmer than ambient but still at a lower temperature.)

With the speed control, you dial the speed down to get a discharge temperature you can live with.

Maximize the water flow and know that you will cool it down to within a few degrees of room temperature; it will never get below room temperature of course.

When you get the return water as close to the room temperature as you can while maintaining a comfortable discharge air temperature, you have achieved Thermal Nirvana.

Say "Ohm" somebody.