In floor delta t

smoore

Some manufacturers say to design radiant in floor for 10 degree delta t and some say 20 degree delta t. what is the pros and cons between the two? I just finished a in floor slab on grade system and was playing with the delta t and i thought that with a wider delta t i would dump more btu's into the slab. I noticed that with a smaller delta t my supply water temperature dropped and with a wider delta it raised back up. Why is that or is this an error on my part?

thanks in advance

Mike T., Swampeast MO

"I noticed that with a smaller delta t my supply water temperature dropped and with a wider delta it raised back up."

No error on your part. Think in terms of averages and you'll see why.

Say you need 140° AVERAGE water. With a 10° delta-t this would mean a supply temperature of 145°. With a 20° delta-t it would mean a supply temperature of 150°.

This affects an emission device in a way that sounds "wrong". Under otherwise identical circumstance (air temp, flow rate, supply temperature, emission device, etc.) a device with a lower delta-t will have a higher btu output because the average water temperature is higher. A larger delta-t means a lower average water temperature and lower btu output.

RB_2

excerpt on Delta T's...



"Picture 4 shows the relationship between fluid temperatures and delta t. Let’s assume the water leaving our heater is 180 deg. F (enter from top of chart) and we have chosen to use a 40 deg. F delta t (enter from left side of chart) then we can see the average design temperature in the pipes will be 160 deg.F. Suppose our radiant floor needed 95 deg.F water, we can select any number of combinations of supply and delta t temperatures.

For example, a small radiant zone installed in a thin poured application with tile needs a small delta t; a combination of 100 deg. F supply temperature and a delta t of 10 deg.F would be a good choice.

The delta t decision is based on application.
If we discuss delta t’s in regards to radiant systems we influence the consistency in the surface temperature.

A large delta t is not necessarily a good thing for comfort on low mass applications using low R-value floor coverings such as tile. In such cases, a 40 deg. F delta t could result in uneven surface temperatures and be uncomfortable. On the other hand 30 deg. F delta t using a counterflow pattern in a 4 inch or 6 inch concrete slab heating a garage is quite acceptable.

If we discuss delta t’s in regards to heating plants then we influence the size of distribution piping, circulators, valves and flue gas temperatures. A condensing boiler is capable of handling very low return temperatures whereas the non-condensing boiler needs to keep return temperatures above the flue gas condensation temperature, (typically 140 deg. F)…"..."Example below illustrates possible solutions for a radiant system...the wider the spacings the hotter the average fluid temperature."