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# Actual Average Water Temp in Hot Water Boiler System

Member Posts: 80

If I have a 20 degree differential on my aquastat and I have copper baseboard heat. I am assuming that halfway through the baseboard run water temp drops 10 degrees, 20 degrees for the whole run. If I set my aqua stat at 200 degrees is my average temp 190 degrees because of the aquastat setting or is it 180 degrees because of the run temp drop and the aquastat drop.

• Member Posts: 23,905

Eh? It's not quite that simple. The average temperature in the piping loop will be the average, as you state, between the temperature at the beginning of the loop and the return at the end of the loop.

However, the aquastat will cause the boiler to run in a range from its cutin temperature and its cutout temperature (assuming that the boiler does cycle). This has nothing to do with the average water temperature in the loop, but does give you a clue to the average water temperature at the aquatat over time — not distance.

Br. Jamie, osb
Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
• Member Posts: 80

Thanks Jamie

• Member Posts: 405

The difference between science and engineering is that in science you try to find exact answers, and in engineering you try to find close-enough answers.

Imagine a boiler that cycles on at 180F and off at 200F, with a burner that produces a constant flow of heat when it's on and radiation that provides a constant load. The temperature will move up in a straight line when the burner is on, and down in a straight line when the burner is off, it will look like a sawtooth. The average of that will be the average of the minimum and maximum, or 190F.

Now the scientific answer will be to say that the load isn't really constant, the radiators will produce less heat when the water is at 180F than when it's at 200F, and come up with an integral equation that takes that into account. The engineering answer is to say the load is close enough to constant that the straight line approximation works. You're not going to be designing systems so that one degree is going to be the difference between it working and not working.

Similarly, the average water temperature in the radiators can be approximated by the average of the send temperature and return temperature. So if your delta is 20F, with an aquastat setting of 200F your average water temperature is going to be 180F and will range between 200F an 160F.

When looking at manufacturer's specification of heat output vs temperature, read carefully what temperature they are using. Some will say "aquastat setting," which means they have already made the correction for you.

• Member Posts: 249

I built a monitoring system that actually checks the supply and return temperatures for each zone once a second, computes the average water temperature and then figures out the heat output for that zone using interpolation, based on how many feet of baseboard are in each zone and the heat output datasheets that the manufacturer provides for the baseboards. My supply/return delta is typically only 8-10 degrees F (measured peak-to-peak after the aquastat cuts off) in practice. This system is pretty overkill, but assuming that your boiler is way oversized and bouncing off the aquastat all season, just using the midpoint of the aquastat minus half of the delta-T as the average temperature probably gets you within 5-10%.

• Member Posts: 80

Thanks for your answer. My aquastat is only set for 180. My true delta t is only 10 degrees. I have some very old very high output half inch baseboard and I was trying to do some calculations I found specs for my 1954 triad baseboard. I have a total of 100 ft. I was trying to run calcs for total btu output at different temps.

I did a manual J before new boiler install it is correct size. All boilers are oversized until you get to a design temperature day.

• Member Posts: 22,669

The delta T through the heat emitters is always changing based on the load of the space it sees. Indoor gains, open and closing doors, wind, solar gain all can be moving the load.

Even if a system is designed around a 20∆, it may never run at that exact delta.

All systems try to get to thermal equilibrium. if you were to watch the system, measuring supply and return temperature, when they stop moving you have reached thermal equilibrium.

Another way to look at is ∆T is and indication of the heat energy being transfered. If you know or can measure the flow rate, this hydronic formula gives you the btu being transfered.

500 f (∆T)

500 X 6 gpm X (180- 165) 45,000 BTU/hr

Bob "hot rod" Rohr
trainer for Caleffi NA
Living the hydronic dream
• Member Posts: 80
edited August 6

• Member Posts: 80
edited August 6

I guess my question was do you totally ignore the aquastat differential when you are designing a baseboard run? The boiler will theoretically be banging off the aquastat until you get to a design degree day.

My aquastat is set to 180, my differential is 20 degrees, my delta T in each run is 10 degrees.

What is the water temp I use to pick my baseboard?

• Member Posts: 23,905

Pretty much, unless you have a modulating boiler.

Br. Jamie, osb
Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
• Member Posts: 22,669

Basically the boiler aquastat protects the boiler from running at unsafe conditions, with a high limt safety as a second protection usually.

The actual ∆ the system runs at depends on flow and loads.

Bob "hot rod" Rohr
trainer for Caleffi NA
Living the hydronic dream
• Member Posts: 5,037

why are you running that high?

My system needs 180 supply when it single digits and the winds blowing.
most of the time it’s 140 or 150*

• Member Posts: 80

Where are you located? How old is your house? Are you using cast iron radiators , cast iron baseboard, or copper fin & tube? How is your insulation and windows? What is your house construction? Do you have more heating element than you need.? What kind of boiler do you have and how old is it? How big is your house? How many floors do you have? Are you using a condensing boiler?

Without knowing all the above you are comparing apples and oranges