Delta-T (sorry) again

josephny

So I've been reading previous threads on delta-T and holy cow this is complicated.

I have an IBC SFC-199 LP boiler feeding 6 zones (each with its own Grundfus pump).

If you had to dumb it down a lot, could you make a recommendation for the very basic things to check or measure to see if the system is performing reasonably well?

Despite the complexity (that leads to the derogation of its usage), I see 20* delta-t thrown around as a nice starting point.

Would I just measure the diff in temp between the supply and return lines in the basement feeding each zone? If I have ~20* diff can I reasonably conclude that that zone is performing reasonably well?

Thanks.

pecmsg

Basically yes.

dko

You can check the supply and return temps from the IBC unit.

Hold the wrench for 2 seconds, let go.
Hold it again for 2 seconds until you see the red leds on the left (by default will show flame current, and shows code "A" on the right).
Press - / + to scroll the options.
Code 1 will be your supply temp
Code 2 will be your return temp

josephny

dko said:

You can check the supply and return temps from the IBC unit.

Hold the wrench for 2 seconds, let go.
Hold it again for 2 seconds until you see the red leds on the left (by default will show flame current, and shows code "A" on the right).
Press - / + to scroll the options.
Code 1 will be your supply temp
Code 2 will be your return temp

That's a cool feature.

Will this work with the IBC primary-secondary loop piping manifold in place?

Would you say that knowing the delta-t at the boiler is useful, but not as useful as knowing the delta-t for each loop?

EdTheHeaterMan

The difference in temperature between two different locations will determine 1/2 of the information you need to determine the output (or input) of heat energy between the measuring points. The other number you will need is the gallon per minute flow rate.

With the GPM and the Flow Rate you can determine the amount of energy being exchanged from one location to the other location.

The reason that 20°F is used in Hydronics as a benchmark, is because the number adds up to a nice round number for using a rule of thumb on smaller projects (like residential heating systems). On large industrial jobs, the residential rules of thumb go out the window because the larger the project the smaller the margin of error must be, so engineers with degrees from prestigious schools actually figure out the equivalent lengths of every 24" fitting and valve and length of pipe to determine how a system might work. But to take that accuracy to every residential boiler install would take so much extra time that no one would be able to afford a new heater based on the cost of designing alone. …I digress.

That rule of thumb goes like this:
The definition of a British Thermal Unit is the amount of heat needed to change the temperature of one pound of water one degree fahrenheit.
So let's look at what one pound of water might be in gallons.
One gallon of water weigh about 8.33 pounds
Now let's look at how we measure water flow in heating systems.
Pumps are rated in gallons per minutes
Heating equipment is rated in BTU per hour.
Now let's look at how hours and minutes relate to each other
One hour and be broken down to 60 minutes.

With this important information we can now calculate how much heat a gallon of water can move in a minute if one place is 20 degrees different than the other place.

Pounds of water X minutes in an hour X temperature difference = BTU per hour
OR
8.33 X 60 X 20 = 9996
OR
About 10,000 BTUh per one GPM

This is the main reason that 20 ∆T is used in Hydronics as a rule.
But some systems like Radiant floor heating may actually work better with only a 10°∆T with double GPM flow rate. It all depends on the system you are working on

I hope this explanation makes this topic “clear as mud”

josephny

EdTheHeaterMan said:

The difference in temperature between two different locations will determine 1/2 of the information you need to determine the output (or input) of heat energy between the measuring points. The other number you will need is the gallon per minute flow rate.

With the GPM and the Flow Rate you can determine the amount of energy being exchanged from one location to the other location.

The reason that 20°F is used in Hydronics as a benchmark, is because the number adds up to a nice round number for using a rule of thumb on smaller projects (like residential heating systems). On large industrial jobs, the residential rules of thumb go out the window because the larger the project the smaller the margin of error must be, so engineers with degrees from prestigious schools actually figure out the equivalent lengths of every 24" fitting and valve and length of pipe to determine how a system might work. But to take that accuracy to every residential boiler install would take so much extra time that no one would be able to afford a new heater based on the cost of designing alone. …I digress.

That rule of thumb goes like this:
The definition of a British Thermal Unit is the amount of heat needed to change the temperature of one pound of water one degree fahrenheit.
So let's look at what one pound of water might be in gallons.
One gallon of water weigh about 8.33 pounds
Now let's look at how we measure water flow in heating systems.
Pumps are rated in gallons per minutes
Heating equipment is rated in BTU per hour.
Now let's look at how hours and minutes relate to each other
One hour and be broken down to 60 minutes.

With this important information we can now calculate how much heat a gallon of water can move in a minute if one place is 20 degrees different than the other place.

Pounds of water X minutes in an hour X temperature difference = BTU per hour
OR
8.33 X 60 X 20 = 9996
OR
About 10,000 BTUh per one GPM

This is the main reason that 20 ∆T is used in Hydronics as a rule.
But some systems like Radiant floor heating may actually work better with only a 10°∆T with double GPM flow rate. It all depends on the system you are working on

I hope this explanation makes this topic “clear as mud”

It's a great explanation and very helpful. I had read other explanations for why 20*delta was used, and they certainly connected it to GPM and delta-T, but many other explanations omit the actual math that explains the connection between the variables. Thank you.

Now I am pondering how to reconcile this with the idea that the GPM is of very little importance in the BTU rating of baseboard heaters. (Rather, the temperature of the water is the more influential variable.) Looking at tables references in instructional videos, for example, the difference in output between 1 and 4 gpm is a pretty small percentage.

Using your math of moving 1gpm with a delta-t of 20 results in 10,000 btuh.

Moving 4 gpm with the same 20* delta-t should result in 40,000 btuh. This is not the type of result I would expect.

So, I clearly am misunderstanding or using inappropriate assumptions. Perhaps the "GPM is of very little importance" statement applies to a different calculation (radiation emitted from a radiator)?

Hot_water_fan

Moving 4 gpms with the same 20* delta-t should result in 40,000 btuh. This is not the type of result I would expect.

This is the maximum potential. For the same length of baseboard, 4x the GPM won’t 4x the output, it’ll increase output by about 5%. What will happen is the 4gpm scenario will have a lower delta T.

Widening the delta T has a few benefits: lowering return temps and lowering pumping energy. 

A tighter delta T, via higher GPM, means higher velocity flow, which you might need for turbulent flow. It also makes a floor more consistent in temp. In a scenario where you need high average water temps, you might not be able to reach that if the delta T is very large. (IE you need 190F AWT, you have a ceiling to how hot the supply water can actually be so a higher GPM makes it easier). 

josephny

OKay, starting to grasp.

I need to start thinking about inputs vs. outputs.

If I understand correctly, inputs (variable I can directly control) include: velocity (gpm) and supply water temp).

Output variable include: Delta-T

The other measurements (turbulence level, evenness of heat across locations, efficiency, etc.) are all outputs (perhaps secondary outputs).

So, given my desire to just have a basic (i.e., simple) approach to optimizing this (yes, oxymoronic, I know), is it safe to say that I should just focus on choosing LOW, MEDIUM, or HIGH speed on the zone pump and setting the supply water temperature and keep my fingers crossed that the system works well?

Right now, the pumps are set at high speed and the water temp is at 165.

mattmia2

Is there someplace that you're too warm or cold?

Jamie Hall

"So, I clearly am misunderstanding or using inappropriate assumptions. Perhaps the "GPM is of very little importance" statement applies to a different calculation (radiation emitted from a radiator)?"

This is true. The heat output of a radiator is somewhat independent of the flow rate in the radiator, It is a function only of the average temperature of the radiator. The reason flow rate gets considered is that the average temperature is just that -- the average of the input temperature and the output temperature -- and the output temperature is affected by flow rate; a higher flow rate will give a higher output temperature for a given input temperature.

The actual mathematics to do a full analysis gets rather messy, so it is not usually done for residential size systems; it is simpler to assume a delta T and thus an average temperature. The only place that varying flow rates can make a significant difference in a residential system is when there are a number of radiators in different rooms connected in series. There increasing the flow rate and thus decreasing the delta T results in the downstream radiators getting hotter water, and thus having a higher average temperature -- and greater output as a result.

EdTheHeaterMan

@Jamie Hall that last comment was insightful and I agree and I think it is awesome, because you nailed it and made it more real to the non engineer folks that come here to learn more about this stuff. But I could only pick one icon to add.

Maybe others can click on the other two for me!

EBEBRATT-Ed

A 20 degree delta isn't picked because it is a round #.

If the return is too cold condensation can result.

Also a big delta like 40 or 50 could stress the rear section from too much TD and crack the boiler.

josephny

Jamie Hall said:

This is true. The heat output of a radiator is somewhat independent of the flow rate in the radiator, It is a function only of the average temperature of the radiator. The reason flow rate gets considered is that the average temperature is just that -- the average of the input temperature and the output temperature -- and the output temperature is affected by flow rate; a higher flow rate will give a higher output temperature for a given input temperature.

So the temperature drop across the emitter is inversely related to the velocity?

And with a lower temp drop, we have a higher output temp.

And with a higher output temp, we have a higher average temp across the length of the emitter.

And with a higher ave temp across emitter, we have a higher emitted BTUH output.

But, the increase in emitted BTUH is only minimally correlated with the velocity because other factors play a more influential role.

Correct?

Jamie Hall

Reasonably close anyway... and within reasonable velocity ranges. Keeping in mind that the output temperature can never equal the input temperature, not can the output temperature drop below the space temperature... and the BTUh output of the emitter is almost linearly related to temperature, but not quite. nor is it constant across the length of the emitter.... but as a first approximation, you've got the idea.

hot_rod

The flow velocity is the limiting factor, and to some extent the amount of pumping power required.
4- 5 fps is accepted in the industry for copper tube and pex. In larger size pipe, over 2" it is more of a pressure drop calculation as opposed to velocity number.

The hotter average temperature of , the higher the out put is a simple way to explain it.
Increasing average temperature or increasing flow will increase output. Flow increase in not linear like temperature increase, for adding output.