min delta-T design consideration

Dan Miller_2

When people talk about having a minimum 10 degree delta-T between supply and return on a hydronic system, under what conditions is that measurement take? Obviously the delta-T on a cold day will be higher than a mild one.

Also if the system has a header when the boiler supply mixes partially with the return, is the delta-T difference calculated based on the boiler supply temp or the temp in the header?

For example the output from the boiler is 104F and the temperature from the sensor in the header is 99F. The return temp is 90F. Would the delta-T be 14 or 9? It's currently 50F outside. HVAC calcs use a winter DT of -4F.

Brad White_9

Getting the DT's....

I am not familiar with the concept of a "minimum" delta-T, rather it is a design constant for selection of equipment at design conditions (coldest design day for example). During intermediate seasons, if you have a conventional boiler with on-off control, a gravity hot water system or non-modulating control of any type, the delta-T will vary widely.

For example, if you have a 100,000 BTUH output system properly sized for a design day of say, 0 degrees F. (70 F indoors) and it is, say, 40 degrees F outside, you will have a heat loss of about 42,000. Thus your radiators will heat quickly and the space satisfied likely before full delta-T is reached. Ram that output into the room at a rapid rate. By the time the room is heated, you may only see half your delta-T.

A modulating source if properly set up with PID type controls, will meter the BTU's into the space more proportional to the heat loss at any given time. (The rate of temperature rise or drop is taken into account and the system responds accordingly but with restraint so it does not "over-shoot").

Thus you are likely to see your full delta-T. A lower water temperature range of course, but likely the design delta-T just the same.

A modulating source without true PID control is likely to ram the heat in there at first then back off so you may still not see the actual Delta-T until a design day. Lots of variables.

Now, your question as to where Delta-T is measured: It is reflective of any supply and return line pair serving or coming from a common element.

By this I mean any given radiator has a certain delta-T. It may well differ from another radiator but is comparitive to one radiator only at this level. Measuring the supply of one and the return of another is meaningless by comparison.

A boiler has a delta-T of course as would the line set going out to all of the radiators (aggregate of all radiation).

Measuring the supply header after a mixing valve and the boiler outlet temperature is similarly meaningless. Only the supply and return from the radiation is comparable and the boiler supply and boiler return are comparable.

Does that make sense?

Another point- very few systems in reality see the delta-T they are purportedly designed to. This means to me that the heat loss is less than calculated or, as likely, the system is over-pumped. We move an awful lot of water to heat buildings in this country. You can back off your flow quite a bit and still heat beautifully.

Brad

Mike T., Swampeast MO

It would appear that the boiler is operating with a delta-t of 14° and the emission system is operating with a delta-t of 9°. Why the difference? Because some of the return water is blending back into the supply to the emitters with the boiler itself producing warmer water. In other words, more water is flowing through the emitters than through the boiler. The header is doing its job.

Sounds like this is a condensing/modulating boiler. If so, delta-t can vary significantly--both with load on the system and the weather. As you mentioned, the delta-t will generally tend to increase as the weather gets colder