Questionable Engineering

Rob Krawic

A new school has been built in my area with a new control scheme. I will try and represent this as best as possible:
Boilers are to shut off above 50 degrees outside air temperature. Back on below 50 degrees. These are Weil McLain BL1128's. The controls utilize a heat wheel for energy savings which relates to the 50 degree (lower than usual) boiler start - stop temperature. I have not seen a engineered control scheme that actually powers off the boilers like this. Usually the jobs that we have we regulate the pumps and the secondary heating loop has a outdoor air reset schedule controlling the output temperature. Also at this new school, the primary heating pump is turned on by a call for heat regardless if the boilers are "on". My question is: Will these boilers have any negative effects due to thermal shock or oxidation corrosion? Is this a viable control scheme? Will this realize the energy savings? I have always believed that it is better for the boiler to keep it functioning to drive out the oxygen and prevent great temperature swings. The engineer says I don't know what I'm talking about. HELP...

JimGPE

Heat wheel?

That's usually a device to exchange heat between two air streams. How does that relate to the boilers?

I've done jobs where an outdoor stat shuts down the boiler plant. At some point it only makes sense to shut it down, and if you are going to do it automatically, outdoor air temperature is a pretty good way. The engineer believes that 50F outside air temp is the building's balance point - neither cooling nor heating is required, as the internal heat gain exactly offsets the skin loss to the cold outside.

Question in my mind is once the plant is ON how is it controlled? Surely not full on or full off....

Outdoor temperature reset? Primary secondary loops with a mixing valve controlled by a reset temp in the secondary loop?

Dan Law

Engineering questionable

We see the ambient lockout scheme commonly, along with system pump lockout as well - works well for us here in Michigan. I've heard a lot of misuse of the term "Primary Pump". Some folks refer to the system pumps as primary, some refer to the boiler pumps as primary. According to B&G, the pump(s)and the loop that circulate the heating source (boiler or heat exchangers) are the primary. This may or may not be the system pump as well. In a primary / secondary arrangement, the system pumps are the secondary, and the individual AHU pumps are "tertiary".
Re: thermal shock - how so? Assuming the boilers are down for at least several hours on ambient lock out, they should reach nearly ambient conditions. When the system restarts, it's brings back what? 70 - 60F? to a boiler that's at say 100F? Not much shock potential as I see it.
Re: the 50F setpoint as it relates to the heat wheel, I would guess the engineer came up with 50F as follows: When the heat wheel is brining in a minimum of 50F air, his heat wheel has sufficent capacity to heat that air to a satifactory discharge, without any supplimentary heat from the boilers.
All this having been said, when I'm the control contractor on such a job, I make sure all these setpoints are easily adjustable, just in case the real world is a bit different than the drafting table (what a concept!) You know if it doesn't work, it won't be the enginner they point the finger at. It'll be the mechanical and the control contractor. Have you ever come across engineers using "diversity factor" in their calcs? Another brainstorm from Neptune's outer orbit!

Dan Law

Engineering questionable

We see the ambient lockout scheme commonly, along with system pump lockout as well - works well for us here in Michigan. I've heard a lot of misuse of the term "Primary Pump". Some folks refer to the system pumps as primary, some refer to the boiler pumps as primary. According to B&G, the pump(s)and the loop that circulate the heating source (boiler or heat exchangers) are the primary. This may or may not be the system pump as well. In a primary / secondary arrangement, the boiler pumps are primary, the system pumps are the secondary, and the individual AHU pumps are "tertiary".
Re: thermal shock - how so? Assuming the boilers are down for at least several hours on ambient lock out, they should reach nearly ambient conditions. When the system restarts, it's brings back what? 70 - 60F? to a boiler that's at say 100F? Not much shock potential as I see it.
Re: the 50F setpoint as it relates to the heat wheel, I would guess the engineer came up with 50F as follows: When the heat wheel is brining in a minimum of 50F air, he feels his heat wheel has sufficent capacity to heat that air to a satifactory discharge, without any supplimentary heat from the boilers.
All this having been said, when I'm the control contractor on such a job, I make sure all these setpoints are easily adjustable, just in case the real world is a bit different than the drafting table (what a concept!) You know if it doesn't work, it won't be the enginner they point the finger at. It'll be the mechanical and the control contractor. Have you ever come across engineers using "diversity factor" in their calcs? Another brainstorm from Neptune's outer orbit!

DanHolohan

Mark J Strawcutter

primary vs secondary designation

I wrestled with the primary vs secondary designation and came to the (perhaps incorrect) conclusion that the primary loop is the one with the expansion tank.

You can have a primary loop running around a building with boiler secondary loops providing heat and emitter secondary loops consuming it.

Another configuration is a primary system (emitter) loop with multiple staged boiler secondary loops.

In each case the expansion tank is on the primary.

Or have I totally missed the mark?

Mark

Terry

SOUNDS GOOD

This is the way I see it.
Primary pumps are on same circuit that the "point of no pressure change" is.

Any other secondary loops, whether source or terminal in nature, I consider "secondary".

Good Point Mark.

Regards,
Terry

[Deleted User]

Is it

a primary/secondary pumped system? Does it have a 3 or 4 way mixing valve? How are boiler temperatures controlled? What turns the system pump on and off? Are the boilers staged or both run together? If the boilers maintain a constant temperature (when o/s temp is below 50 degrees); is there a pumped blending loop(s)?

Variables. Variables.