hydronic hot water coils

maz

It seems I get 10 different answers from 10 different professionals,suppliers when sizing boilers for use with hot water coils Coils are almost always over sized sometimes and very often way over sized Do you match boiler net rating to coils even though over sized or boiler to heat loss of rooms and or home .

Brad White_9

I consider coils to be like radiators

within reason. Like any emitter (radiator, coils, radiant floor) you have to design them for the specified or anticipated water temperatures available and the temperature drop. In a conventional hot water system this may be 180 degrees entering water, in condensing systems, maybe 140 degrees (both on a design day). And for 140 entering, I like to take a 30 degree drop to 110 LWT.

This latter point is to allow better condensing potential.

Yes, you match the coil to the load. And if you are "generous" with your coil selection relative to load, make that generosity equal in proportion across all coils within the system. Don't add 25% to one coil unless you do so to all coils. That way you need one temperature for all.

Key things to keep in mind:

1. Circuiting. Full face not serpentine. This way at low flows you will be less likely to have hot or cold spots. Critical if you are sensing downstream temperatures in the duct.

2. Excellent valve authority- the ability of a valve to throttle down flows as linearly as possible relative to desired setpoint.

3. Preference for valve authority to enact temperature variation, not volume variation. By this I mean, my preference is to have constant circulation if it is a main AHU coil. Bleed in minute controllable quantities of hot water to control output. Remember: Water temperature trumps water volume when it comes to linear coil output control.

For smaller local zone coils, the circulator ought not be necessary but good circuiting is. Managing the water supplied to all coils is really the key.


The other point to consider is the leaving air temperature which is supplied to the room. If too low (even if it technically meets the heating load) and the space is too dry, you may percieve a slight wind-chill in the space. So you have to balance the air side to that variable also. VFD controls with ECM motors are tops in my book. Balance the coils to optimum water conditions in and out, then roll the airflow up and down to fine tune what is going on in the space. Then double-check the water side once again.

With good flow management and constant airflow you can achieve excellent results in the space. Amost imperceptable temperature changes.

My $0.02

Brad