Simple Control Strategy

scott markle_2

I have a friend who's cheap and not too concerned with comfort. He has a mono-flow system with baseboard.

I'v been thinking about ways to inexpensively improve the fuel economy from his oil boiler. Since the system is a modest length mono-flow loop it seems to me that this ensures a certain degree of boiler protection from cool return temperatures.

I'm thinking of turning his high limit down to 160 and setting the circulator on a aquastat that would keep it running when ever system water is above around 85 degrees. The room thermostat would operate the burner and energize the circulator (if it was off). When the thermostat was satisfied the circulator would continue to pump until system temps dropped below 85.

Seems like this would keep more heat in the conditioned space and less going up the flue, it also seems like it would help ensure longer steady state burns.

Maybe use a PID thermostat to help mitigate the sine wave overshoots?

Any Thoughts?

Brad White_198

Poor Man's ODR

Constant circulation with T-stat to burner is time honored and often done with atmospheric gas boilers. One enhancement which one can argue is essential, is the use of TRVs on all radiators.

The constant circulation alone increases comfort in my opinion; the TRVs even out any spikes and essentially re-size the radiators to the room heat loss. Without the TRVs but with constant circulation, you risk over-heating as you can imagine.

There is no true return water protection, but the old Centra controls as one example, had boiler minimum settings down to 68F. There is, somewhere out there, a paper by Joachim (Joe) Fiedrich, formerly of Stadler, (now Viega), advocating this approach.

I suppose if you wanted return water protection, a Danfoss ESBE thermic return valve is a good choice.

scott markle_2

Thanks for the input Brad. I was pretty sure I had heard of this approach before.

When your willing to forgo comfort for efficiency this opens things up a bit.

Do you think this will work ok with a cast iron boiler? I see some obvious potential for room temp overshoot. But if thermostats are set to 62 what's the big deal if we get up to 66 occasionally.

What about PID thermostats, don't these help with overshoot by looking at rate of change and preemptively breaking on anticipation of set-point.

Brad White_194

Hi Scott

That control strategy is probably is older than you think. Gravity hot water has the same effect, shorter burner cycles (and more tepid water) in mild weather and the inverse in cold weather.

So long as the burner responds to the space temperature, you will tend to have this indirect effect. As for overshoot, I can see that more with higher mass/higher water volume systems but truthfully, the anticipator may ease that a good deal.

Give up comfort for efficiency? I see this system, simple as it is, give more of both. Still, an ESBE valve on the boiler return is added security.

To be clear, many things I did so many years ago were in ignorance of some fundamentals; doing so today, I would add those touches. Maybe because now I can afford to, rather than when I was in my 20's and cash was tight

I like your PID approach! I have not used a PID thermostat (unless the Honeywell Smart Recovery types count as such), except that all knowing PID controls have been integrated into DDC, actuating control valves versus firing an on-off burner. But why not?

As stated, this has been accepted on atmospheric CI boilers but I doubt that wet-base or oil-fired boilers would do well at all. (Supposing the atmospheric castings are exposed to ambient temperature air between firings, the strategy is acceptable is my conjecture.)

RonWHC

Worked for an

old timer, starting in 1962, who called your idea the "Cadillac of Heating." Especially on 1-pipe water systems. Low temp comfort was what we told customers.

Don't do residential anymore. However, keeping the system circ. running is our choice. Some "oops" events got us off the reverse aquastat for pump control. We now use the WWSD feature of whichever ODR Control is in use, or install a separate O/D temp cutoff for pump control.