Outdoor Air Reset

John Ketterman

> Without outdoor reset, the <BR>
> boiler will make long cycles up to (or <BR>
> approaching) maximum temperature, followed by <BR>
> long off periods. With outdoor reset, the boiler <BR>
> will make a larger number of shorter cycles up to <BR>
> the reset temperature. Since the btu demand <BR>
> remains constant, the total firing time in both <BR>
> cases will be approximately the same.<BR>

Not really.

When the thermostat calls for heat, the circulator runs all the time, but the boiler goes on and off as needed to hold the water temperature steady within a range (deltaT). The temperature droops to 170, the boiler comes on and kicks it back to 180, over and over. The steady-state cycle time does not depend on the water temperature; it depends on the aquastat deltaT setting. It takes the same energy and thus the same firing time to heat water back up from 170F to 180F (w/o reset), as it does to heat it from say 90F to 100F (w/ reset). These steady state cycles are the most commonly occurring cycles.

When the heat call is satisfied, the stat tells the boiler and circ to shut down. IF the off time is long, and during this time both 180F water and 100F water cool to 70F, then indeed the boiler cycle at the start of the next heat call will be shorter with reset than without reset. But it is still much longer than the steady-state cycle time, so it doesn't matter.

What you appear be thinking is that in both cases, the boiler heats cold water to the target temperature, then it immediately goes off and the water is then allowed to get completely cold. But that's not how it works except in some poorly-designed (oversized) "bang bang" or "blast furnace" heating situations. It's certainly not how it works when there is reset and so there are very long heat calls (up to 24 hrs/day).

Finally, remember that 2-min boiler cycles are bad, but there is no major difference between a 20-min cycle and a 200-min cycle.

Nathan Wagner

Outdoor Air Reset & Traditional Boilers

This is a question from an engineer who knows just enough to be dangerous. Please explain to me the energy advantage, if any, of adding outdoor air reset to a traditional on/off boiler.

My thinking is as follows. In an on/off system, the system temperature will always fluctuate around the actual steady-state demand temperature. By installing outdoor air reset and reducing maximum system temperature, the system will respond more slowly to demands for heat, and the magnitude of the temperature swings will decrease. This will make the system more comfortable, but the average btu demand will remain unchanged.

Without outdoor reset, the boiler will make long cycles up to (or approaching) maximum temperature, followed by long off periods. With outdoor reset, the boiler will make a larger number of shorter cycles up to the reset temperature. Since the btu demand remains constant, the total firing time in both cases will be approximately the same.

Where, then, is the energy advantage here in outdoor air reset? The more frequent firings in the outdoor reset case may actually cost you efficiency.

Outdoor air reset makes efficiency sense on a condensing boiler, where consensing heat is only recaptured below about 140 deg F - and on a modulating boiler, where it can help in determining steady state system demand. But on an on/off traditional boiler, does it help at all? If so, why?

Thanks,

Nathan

Brad White_9

This from an engineer who actually is dangerous

You bring up some oft discussed points and not all apply at all times (like other aspects of physics, we both know).

"It depends".

(Let's get it out of the way and stipulate first that the boiler is properly selected to the heat loss, not grossly over-sized and hence limited cycling in coldest weather.

The ideal setup is to separate the radiation loop from the boiler loop. This allows the boiler to be kept at a safe temperature although reset to the extent that it can be. At the same time, the radiation loop can be reset from full boiler temperature down to near room temperature, the Full Monty so to speak.

Let us also stipulate that this is constant circulation, maybe with TRV's as a treat.

As you said, the comfort is much improved.

The boiler water volume is maintained at a temperature somewhat higher than the radiation loop requires in order to maintain authority of the mixing device (Valve, injection pump, whatever).

But this volume, being disconnected from the radiation, is rather small now. It is easier to maintain this at temperature than the entire house volume.

We should note that the cycling/firing rate will have a minimum on-time and will drift above and below setpoint (differential) by nature of the beast. Our injection means will compensate for these spikes and give our radiation loop a very steady temperature keyed to outdoor temperature -the goal.

Indeed, as you said, in milder weather/lower demand times, the cycling will increase. But it does this anyway.

But because the secondary (radiation) loop sips water from the boiler, the draw time between cycles increases in milder weather. This lessens cycling to a degree.

In the end, the input to the space is far more closly matched to the heat loss from the space. And therein lies the efficiency.

A mod-con is the best of all worlds for efficiency we agree.

Doing de-coupled reset as I described is second best, sort of like kissing a second cousin compared to a Mod-Con.

If you do reset only on the boiler itself with a low-limit, you are cheating yourself of the full benefits. Like kissing a tree.

Weezbo

Huh?

What?

that sure seems like a unique viewpoint. mostly i would like to suggest that comfort is an important aspect of our school of work....both physical and financial.