# Thread: How to Use Outdoor Temperature Reset and Maintain Boiler Minimum Return Water Temp?

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Originally Posted by beenthere
That boiler is allowed a max GPM of 90 GPM, and a min of 40 GPM.

Its a low temp rise boiler.
I confirmed the information in the O&I manual. I think I see what you were getting at because the data table also shows ∆T at that flow rate (at the max firing rate?):

Max GPM = 90 with ∆T = 12 F
Min GPM = 40 with ∆T = 26 F

So I need to know the pump flow rate to determine the ∆T at max firing rate.

I checked the pump nameplate. The flow information isn't given but the model number is. It's a Grundfos model "UPS 50-80 280" pump. This pump has three speed settings and it is currently set up to operate on speed 3.

Using the pump curves (see attached) and assuming that the system curve falls along the original design flow point of Flow=50 gpm Head=18 ft, it looks the pump should be pushing about 57 gpm.

Data-Booklet-Large-UP-UPS_pg45.pdf

Then interpolating the ∆T values across the boiler for the max and min flow rates I get:

At Q = 57 gpm, ∆T = 21 F

Damn, I should have been able to figure it out without any help!

Anyhow it also looks like the system flow rate is nicely within the operating range of the boiler (90-40 gpm). And Grundfos is a good pump make, as far as I know. It's made in Germany, which is usually a good thing.

But what is a "low temp rise" boiler?

2. 12°F rise is a low temp rise. Would be a problem on some boilers.

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Originally Posted by beenthere
12°F rise is a low temp rise. Would be a problem on some boilers.
Strange. I would have thought high temp rise would be more of an issue due to thermal stresses.

How does low temp rise cause a problem for boilers?

4. Originally Posted by nielkfj
Strange. I would have thought high temp rise would be more of an issue due to thermal stresses.

How does low temp rise cause a problem for boilers?
Causes hot spots in the heat exchanger.

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Originally Posted by beenthere
Causes hot spots in the heat exchanger.
Stange. I would think that with a low temperature drop across the exchanger there would be a high flow rate through the exchanger. With the high flow rate I would think there would be better heat transfer and hence fewer hot spots. Anyway I'll take your word for it.

I have another question.

Assume the following:

Indoor temperature set point = 70 F

Condition 1:
Outdoor temperature = -30F
Temperature differential across boiler = ∆T = 20 F

Condition 2:
Outdoor temperature = 70 F
Temperature differential across boiler = ∆T = 0 F (i.e. no heating required. I believe this would be the case, assuming perfectly insulated pipes with no wasted heat loss.)

Now consider Condition 3:
Outdoor temperature = 20 F
Temperature differential across boiler = ∆T = ???

So, the question is, what is the expected ∆T across the boiler when the outdoor temperature is 20 F?

6. Boiler water temp at different outdoor Temps, varies with the type of heat and BTU output of the heat emitters it has. Copper baseboard might need to be 150 or 165. Cast iron rads might need to be 110 or 125. Fan coils may need to be 140 or 160. All of this also varies with the BTU loss of the structure/home.

7. Saying boiler temp, is actually a misnomer. Its actually the water temp of the heat emitter that is important.

And in a condo, there will be one unit that requires the leaving water temp to be slightly high than what the other units require. Due to what direction it faces, and how long the piping is the water travels through to reach its heat emitter.

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Originally Posted by beenthere
Saying boiler temp, is actually a misnomer. Its actually the water temp of the heat emitter that is important.

And in a condo, there will be one unit that requires the leaving water temp to be slightly high than what the other units require. Due to what direction it faces, and how long the piping is the water travels through to reach its heat emitter.
Actually, I'm not considering the boiler water temp, just the boiler water ∆T - the difference in temperature between supply and return temperatures (which is the same as the ∆T across the boilers). The supply temperature could be 180 F for example. I'm assuming that whatever the supply temperature is, the heat emitters can maintain the indoor temperature setpoint.

So I'm just considering the total heat loss from the building at the different ambient temperatures. I can't remember all the details about conduction, convection and radiation, but won't the heat loss from the building to the outdoors be basically proportional to the differential temperature between indoors and outdoors?

Assuming this is correct, since the differential temperature between indoors and outdoors for Condition 3 is 1/2 that for Condition 1, the heat loss for Condition 3 is 1/2 that for Condition 1.

We can also say that heat loss from the building indoors to the outdoors is equal to the heat loss from the hot water heating system to the building indoors. So the heat loss from the hot water heating system to the building indoors for Condition 3 is 1/2 that for Condition 1.

Then since the hot water heating system loses only 1/2 the heat in Condition 3 as compared to Condition 1, ∆T for the hot water heating system in Condition 3 will be only 1/2 that of Condition 1, which is 1/2*(20 F) = 10 F.

Does this make sense, or is there an error to my reasoning here somewhere?

9. The delta across the boiler will be the same no matter what the building or how much heat is lost from the water before it returns to the boiler.

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Originally Posted by beenthere
The delta across the boiler will be the same no matter what the building or how much heat is lost from the water before it returns to the boiler.
OK, I see what you are saying - a boiler will typically only have a fixed 'firing rate'. In other words the rate of heat input to the water is fixed. (Our boiler is a two-stage with a high flame and low flame setting, which allows two different heat input rates. But your point is still correct.) My statement "... the difference in temperature between supply and return temperatures (which is the same as the ∆T across the boilers)" is incorrect.

However, considering the ∆T for the hot water heating system from supply to return only, the ∆T for Condition 3 should be 10 F, correct?

11. The return to supply delta should remain the same under all conditions.

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Originally Posted by beenthere
The return to supply delta should remain the same under all conditions.
I don't understand this.

The way I see it is if the outdoor temperature is colder there will be more heat loss from the building, so the return to supply delta will be higher.

In other words, if the outdoor temperature is colder, there will be a higher rate of heat loss from the building, which will result in the zone valves for each apartment opening for longer periods of time, which will send more lower temperature water to the return line, resulting in a lower return temperature arriving at the boiler.

Am I missing something here?

13. Yes.

The building losses are not the relevant factor.

What generates the ∆T across the boiler is the heat losses of the building's heating units. As the the air temperatures inside the building will remain fairly constant - the temperature drop across the heat emitters will remain constant.

The control modifies the ∆T of boiler water to outdoor ambient.

PHM
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Originally Posted by nielkfj
I don't understand this.

The way I see it is if the outdoor temperature is colder there will be more heat loss from the building, so the return to supply delta will be higher.

In other words, if the outdoor temperature is colder, there will be a higher rate of heat loss from the building, which will result in the zone valves for each apartment opening for longer periods of time, which will send more lower temperature water to the return line, resulting in a lower return temperature arriving at the boiler.

Am I missing something here?

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