hydro air coil reset control

clammy

Looking at a job and i am stuck on picking a control and or a piping straigity 3 hydro air units and a max coil temp of 160 on all units what approach is best injection pump to control water temp tp all 3 coils and have them all on the same reset temp or seperate control for each .Each zone has different heat looses will 1 reset for all be the simplest and most cost effective .My true goal is to modulate water temp seperatly at each unit without effecting my gpm flow as to achieve the proper air temps to pervent short cycling and overheating of zones also these unit have a minuim ventalation air flow setting when unit is not running for heating or cooling i would like to adjust water temp to air flow am i just going over board here ? Or should i pipe her in and out with a by pass and some globe valve to adjust water temp manually

David_5

Questions

Is this conventional or High velocity? Why not just use 1 reset for all 3 units? Are the 3 units the same size? If so you can also adjust the blower speed. I have 2 hydro air in my home and reset but I don't think most people would like 110 degree supply air temps.

David

jwade55_3

I am doing one now.

Very similar to this. I designed it to reset the loop temp via a taco I-Valve. We then have one variable speed setpoint circulator per air handler, it's speed being varied based on ahu discharge temp. As long as the ahu is constant speed, the changing loop temp will not affect the discharge temp.

Now for your ventilation, same thing applies, the circulator will see the increased discharge air temp, and slow down to maintain.

J

don_42

Help

me to understand if you will! If the temp of the water is
being lower but the fan is staying the same how can you
not have a decrease in the temp coming out of the duct?

Bill Nye

David

Who said you have to reset to 110° ? How about 130°?

I think the 180° blast is very uncomfortable, in very cold weather you might like it or at least live with it. In mild weather or shoulder seasons like spring and fall "warm air heat" is a lot nicer than "hot air".


Clammy I am not sure what you are trying to do but if you reset the water to the zone that is the biggest or requires the most btu's I think the other zones would be allright. If you needed less water temp at a smaller zone you use a thermostatic mixing valve maybe?

jwade55_3

You would, if you allow the water temp to drop below a certain point. The key here being that you set up your reset curve so that this cannot happen, also the fact that you are using a variable speed pump, set to maintain a discharge air temp. The thing we are really trying to acheive is allowing the air temp to stay the same, but lowering the btu output of the coil as the load changes, Right?

i.e. If we have an ahu moving 1000 cfm across a coil and we want to maintain a 120 deg discharge with a return air temp of say 65 deg.

BTU/h=1.08 cfm * DeltaT
59,400 btu/h= 1.08 * 1000cfm* 55 deg delta T

So using a default valve of a 20 deg delta T on the flow side we come up with.

59,400= 500 * gpm * 20
5.94 gpm = 59,400/500 * 20

Lets say 6 gpm for ease of figures.

The other piece of info we need is the coil capacity chart, which takes into account the size of surface area, tube passes etc. I'll guess here, say 180 deg water at 6 gpm with 1000 cfm will give us a 55 deg rise based on coil chart.

Now if we lower the water temp, without changing the flow rate, our temp rise will decrease, because the coil will loose capacity, but if the load in the space changes dramaticaly, the temperature rise requirement may change as well, so we vary the flow to maintain the discharge air temp.

Clear as mud?

J

don_42

Thanks

J,got it.Too bad they have not found away to modulate the
fan speed along with the water temps.

David_5

I didn't say it

I said most people wouldn't like 110 supply air. It dosen't bother me though. 180 water temp will not produce 180 air temp. The type of system installed also would help to determine answers.

David

Bill Nye

With no

body language or voice inflection......... I didn't mean it either, I mean, I didn't mean YOU. But I know what your saying

I am amazed at how many hydro air systems I find with boiler high limit at 200° or greater. Hydro air with a tankless coil boiler, go figure