Temperature Drop

Mark Hunt

be mindful of the effect that the lower return water temps will have on the heating appliance.

Boiler protection may be required.

Mark H

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MW_2

Help with Delta T

Can anyone help me understand why most hydronic systems are designed around a 20 degree temperature drop? Given that a higher delta T can allow us to use smaller piping (and circulators) because of the reduced flow requirements, how do I design systems with more than the standard 20 degree delta T?

Mark Hunt

I recommend

This book.

Good stuff in it.

Mark H

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Mike T., Swampeast MO

The best explanation I've heard for the 20° delta-t design standard is that it makes math simple. 10,000 btu = 1 gpm. Plus, the 10° drop in average temperature corresponds conveniently with the ±10° differential of many plain aquastats.

There's utterly no reason that most systems cannot be designed around significantly higher delta-t assumption at design conditions. Significantly higher delta-t with floor heat can be a problem as the temperature difference across the floor can become unacceptably large--there are however ways around this little problem if you think hard enough...

To use a different delta-t:

1) As always begin with a room-by-room heat loss!

2) Size the emitters to achieve this design loss at whatever average emitter temperature you deem acceptable given the nature of the boiler, the available space for the emitter and the available budget. Be aware that if you have sized the emitters based on a Manual-J heat loss that this sizing will likely be VERY generous when it comes to maintaining heat loss--a "cushion" is there for unusually cold weather, the ability to raise space temperature quite rapidly in nearly any conditions, and a good measure of CYA... You can use only a portion of Manual-J losses for sizing your emitters based on your experience.

3) Add all the outputs of emitters on this circuit. Compute the design-condition flow based on whatever delta-t and this output ability. The formula is extremely simple.

gpm = btuh / (500 * delta-t)

So, say you need 45,000 btuh for this circuit at design and want to use a 30° delta-t.

500 * 30 = 15,000

45,000 / 15,000 = 3 gpm

4) Compute your maximum head loss in the circuit using 3 gpm available flow and choose a circulator that best achieves this rate and your computed head loss.

If you haven't already compensated for the general and substantial overage of heat loss calculations and you find one circulator a bit small and another a bit large, choose the one that's a bit small.

5) When it comes time to set the aquastat or set the heating curve DO NOT FORGET that higher delta-t means lower average temperature! Say you sized for 150° emitters at design conditions with a 30° delta-t. You would need a supply temperature of 165° to allow 150° emitters. If you still haven't made any adjustments for the general and substantial overstatement of heat loss compared to heat maintenance realize that unless very rapid recovery from deep setback is needed a substantially lower supply temperature or curve will almost certainly suffice.

If this is a one-pipe loop or diverter-t system do be aware that higher delta-t means that elements further down the loop have to deal with increasingly lower temperatures requiring increased emitter size!!! Neglect this little detail and you'll have balance problems that will likely be "solved" by installing a bigger pump...

MW_2

Thanks, I'll give it a shot.

Mike T., Swampeast MO

A few other things to remember.

If you're zoning with pumps and have rather small zones, your biggest difficulty may well be finding circulators that are small enough!

If you're zoning with on-off valves there's always that "little" problem of changing flow rates and head losses depending on which zones are calling for heat at the time.

You can do away with BOTH of these problems by using thermostatic radiator valves (TRVs). You have ONE circulator and ONE circuit in most cases. The only major additional step in calculating is adding in the pressure drop that the TRV body itself adds to an individual emitter at the design flow rate it requires. Done this way you'll find out why the Europeans really don't give a hoot about delta-t. While they still BEGIN with a design assumption what they really care about is delta-p (change in pressure). The goal is to produce a system that can be directly connected to a mod-con by operating within its acceptable range of flow and using the smallest piping and valves to keep the pressure drop within an acceptable range given the ability of the circulator and head loss through the boiler.

MW_2

I'm a big fan of TRV's as well. I don't think we use them to their full potential in this country.

Mike T., Swampeast MO

Thanks Mark. Sometimes I'm so much in mod-con mode that I forget that some boilers can't deal with low return temps...

Just as the increased delta-t adds to the design supply it reduces design return by the same amount!

Mike T., Swampeast MO

From what I've seen we barely touch the potential of TRVs in this country! Meanwhile China, of all places, is embracing them for the energy-saving and comfort-making potential and they're installing millions of square feet of evacuated tube solar collectors per year to boot.

At least we still have our Hummers!!!! Yee hah! Is SERIOUS four-wheel drive much different than SERIOUS circulators??? Both are MUCH more than is needed for the typical job of merely cruising along with the flow...