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

[nielkfj]

Why would the system delta-T change as a consequence of the outdoor ambient? The system would have no awareness of the outdoor ambient. The system delta is the result of system heat loss to the conditioned space - where the temperatures would be fairly constant, no? So why would the delta ever vary?

This is how I see it:

As the outdoor temperature increases, there is less heat loss from the "conditioned space" (or building) to the outdoors. So as a consequence the conditioned space draws less heat from the heating system in order to maintain the desired temperature. Hence the water returning to the boiler is at a higher temperature - in other words a smaller ∆T.

At the extreme, when the outdoor temperature is the same as the desired temperature of the conditioned space, there is no heat loss from the space, the conditioned space draws no heat from the heating system, and the water returning to the boiler is at the same temperature as the supply water - ∆T = 0.

You don't seem to have a grasp of how the boiler reset operates. The reset control varies the boiler temperature inversely to the outdoor ambient in accordance with the reset ratio. The system delta would only be slightly indirectly altered.

Or am I missing something important? <g>

PHM
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The only education I have about outdoor reset is from reading the Tekmar 261 operating manual last week, so likely there are a great number of things that I don't understand. I don't know much about reset ratios, just a bit I read on the internet.

I do understand that the ODR controller decreases the boiler supply water temperature as the outdoor temperature increases, and vice versa. The method used to calculate the supply water temperature based on the outdoor temperature could be reset ratios which I understand is a straight line change in temperature inversely proportional to the outdoor temperature, as you said. Or it could be calculated according to a formula - a curve instead of a straight line - which is apparently what the Tekmar controller does.

[nielkfj]

The cooler/colder the water is, the more heat it removes from the combusted gas. So the combusted vapors going into the chimney are much cooler. A chimney with 275°F vapors entering it, may work okay for a 10' chimney, but cause condensation in a 30' chimney.

OK, noted. This could be the reason why the technicians have set the minimum supply temperature to 155 - to prevent condensation in the flue.

I think I understand what you mean about the 30' flue. The flue gasses will lose heat as they travel up the flue. The longer the flue, the more heat will be lost. So heavier vapours could start to drop out if the flue gas temperature gets too low.

A boiler efficiency test is best done when the water is nearing the boilers set water temp, which is when the return water is also near its max return temp.

But the efficiency should be higher when the return water temperature is at a lower temperature, no? This is part of what makes outdoor reset more energy efficient, isn't it?

Check the delta between the boilers return and supply water temp. Also see if you can find the GPM going through it.

Ya, if I do start to record the performance of the system, I'll note the supply, return and outdoor temperatures.

I don't think I can get the GPM. The system has a single centrifugal pump, so I would need the curve and ∆P across the pump. There are only pressure gauges on the inlet line to the boiler and the boiler itself.

Are the heating water valves 2 or 3 port valves?

Not exactly sure what a "heating water valve" is. If you mean the zone valves in each apartment, these are all 2 port throughout the building.

[nielkfj]

Rather then concentrating on the boiler and make up air units in this building concentrate on the "Building" itself. Tightening the envelope has a much higher return on investment.

OK, noted.

However, I think the envelope is already pretty tight. The siding was replaced in 2006, and extra insulation was installed at that time. Also new windows were installed at the same time.

One area that we could make more efficient is two "cabinet unit heaters" that sit inside of the two main entrances. These heaters have no thermostat connected to them, they just have a manual rheostat that controls the fan speed, hot water runs through them continuously whether the fan is on or not. People just turn them on and off and adjust the fan speed to whatever they think is appropriate and leave it.

[beenthere]

OK, noted. This could be the reason why the technicians have set the minimum supply temperature to 155 - to prevent condensation in the flue.

I think I understand what you mean about the 30' flue. The flue gasses will lose heat as they travel up the flue. The longer the flue, the more heat will be lost. So heavier vapours could start to drop out if the flue gas temperature gets too low.



But the efficiency should be higher when the return water temperature is at a lower temperature, no? This is part of what makes outdoor reset more energy efficient, isn't it?

It is higher. But when the burner has only been on for 5 minutes, the flame and flue draft still aren't always completely stabilized, and the test result is false.


Ya, if I do start to record the performance of the system, I'll note the supply, return and outdoor temperatures.

I don't think I can get the GPM. The system has a single centrifugal pump, so I would need the curve and ∆P across the pump. There are only pressure gauges on the inlet line to the boiler and the boiler itself.



Not exactly sure what a "heating water valve" is. If you mean the zone valves in each apartment, these are all 2 port throughout the building.

The pump should have a tag on it somewhere listing its rated GPM at X feet of head. Most likely the system was engineered, and the circ was selected to meet the specs of the engineered system. So you can probably just used the pumps listed GPM.

[nielkfj]

It is higher. But when the burner has only been on for 5 minutes, the flame and flue draft still aren't always completely stabilized, and the test result is false.

I think I see what you are saying now. You were explaining that when the return water temp reaches a maximum when the system has stabilized.

I was explaining something different, that if the set water temp is lower, the boiler efficiency will be higher.

The pump should have a tag on it somewhere listing its rated GPM at X feet of head. Most likely the system was engineered, and the circ was selected to meet the specs of the engineered system. So you can probably just used the pumps listed GPM.

OK, noted - will check the pump tag (nameplate).

Actually, I have the original design drawings for the mechanical systems in the building. However, I am a bit sceptical that the information is accurate because they are an "Issued for Building Permit and Tender" revision, not "As-Built". There are also some differences between the drawings and what actually exists in the building now.

In any case the information stated on the drawings for the "Radiation Water Pump" is:
Flow: 50 gpm
Head: 18 ft
Power: 1/8 hp

The existing pump is not original, and I believe has been changed several times over the years. So who knows what specs were used in selecting the existing pump. But if the tag indicates is the close to the same flow rate and head as stated on the drawings, I guess I could assume that the figures are reasonably accurate, or at least in the ball park.

[beenthere]

That boiler is allowed a max GPM of 90 GPM, and a min of 40 GPM.

Its a low temp rise boiler.

[nielkfj]

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?

[beenthere]

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

[nielkfj]

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?

[beenthere]

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.

[nielkfj]

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?

[beenthere]

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.

[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.

[nielkfj]

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?

[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.

[nielkfj]

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?

[beenthere]

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

[nielkfj]

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?

[Poodle Head Mikey]

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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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?

[beenthere]

If the water temp coming back to the boiler when it’s 30 outside is 130 and the leaving boiler water temp is 150.


Then if the water temp returning to the boiler is 110 when it’s 10 outside, the boiler leaving water temp will be 130.

This would be an example of either a single stage boiler. Or of a boiler at the same firing stage at both return temps.