Hitting the stop

nicholas bonham-carter

Minus 3 here in Omaha, with a 15 mph wind, and the temperature is dropping.
My vaporstat is cutting out at 10 ounces, which is unusual, so cycling on and off.
If I had my street clothes on I would go down to the basement and play around with the gas shutoff to see if I could reduce the gas somewhat to see if I can get the burn down to the point where it might just continue steaming without hitting the limit. This would show me whether there was any value in getting a two-stage gas valve, or the optional Peerless Mod-U-Pack gas train mod. Either of these are a bit pricey, so I want to get some idea how effective it would be.—NBC

JUGHNE

Nicholas, same temp here, 2 was about the high for the day.
For my steamer experiment, I had to remove/cap 2 of the 10 burners in a church steamer as they had cracks in the ends of the burner tubes.
So that leaves this with a 15% pick up factor.
The building sets at 55 overnight and tomorrow will start up from a 9 degree setback to get to 64 for mass lasting 45 minutes and then setback to 60 overnight.
If it does not hit the comfort range of 64 I am sure someone will notice and I may hear about it.
The piping is all insulated with 1" FG.
Today (2 degrees) it was cycling on the Tstat. Just a few ounces would heat the place.
If 15% pick up works in this case it sure should work with a minimal set back of the 3 degrees one might use for residential.

mikeg2015

If you can slow venting and balance it, I’d argue you don’t need pickup at all. Maybe just estimate actual piping losses. Then size using DOE of the EDR plus header, jacket and riser loss.

I’m close to pulling there trigger on converting to vacuum and size for building heat loss at design plus 10%. Just a little Less than my radiation at 240 per sqft. Probably use 200.

I’ll have to throw some ideas out there in another post.

nicholas bonham-carter

I certainly wish I had 2-pipes to play with in a vacuum, but unfortunately have only 1-pipe.—NBC

acwagner

I think adding a duty cycle timer to modulate the burner is a good alternative to the options you listed. I've been running this season with one and I like the results. Generally cheaper and easier to add, especially if you're the owner/operator.

nicholas bonham-carter

I don’t think the duty cycle timer would help in this situation. As the temperature keeps dropping into sub zero temps, the thermostat is constantly calling for heat, and the pressure can rise during these extended burns to the cutout point.
If I had a 2 stage gas valve, I could have it cut down to 80% when the pressure hit the stop.
Modern digital thermostats appear to have a sort of timer in their control, with their cycles per hour.setting.—NBC

acwagner

I think it would work. It's just another way to reduce burner output to match radiator demand.

One method, as you suggest, is to down-fire or add a two stage burner to reduce the burner output to 80% and have the burner run 100% of the time.

Alternatively with the timer, you keep the burner running at 100% capacity but limit it to run for 80% of an hour (for example 48 minutes of burn followed by 12 minutes of idle and repeat as necessary or any combination of times depending on your goal). Basically, you are virtually downsizing your boiler. In this case, the thermostat still governs, so if the thermostat is satisfied after 20 minutes then it turns off the boiler.

My thermostat doesn't have cycle per hour setting (you can adjust the temperature swing instead), so I don't know how the timer would work with a CPH thermostat. I suspect it wouldn't matter in your situation since it sounds like this is only a problem when the temperature is low enough that the call for heat is almost continuous. At that point you're at one CPH anyways. Just my guess, though.

The main disadvantage is it takes some trial and error to the get the on/off timing right depending on your objective, so you have to be willing to spend the time to do that. But, if you're the owner and don't mind spending the time I think it's a cost effective alternative to adjusting the burner rate.

Jamie Hall

@nicholas bonham-carter -- you've been on this wall long enough to know better. Controlling your boiler with the vapourstat is going to be the simplest, most reliable way to modulate its output to the load in very cold weather. If you could go two stage, that would be better (still using vapourstats to control the staging). A timer solution doesn't do anything the vapourstat cycling can't do, and adds a level of complexity and operator intervention (unless it is controlled by an outdoor reset with wind and solar sensors) which you just don't need.

Remember: KISS

nicholas bonham-carter

That might be useful in moderate winter temperatures, but in the case I spoke off, with the temperature dropping, with lots of wind, and the pickup factor rearing its ugly head-the pressure rises, while heat is still needed, (almost exceeding the output of the radiation).
Reducing the output of the flame 20% might keep the boiler on for extended periods, With no interruption of burn, and no air evacuation time/fuel wasting.—NBC

Jamie Hall

What I found in this latest cold snap with Cedric (when it did have to modulate a bit) is that the pressure dropped fast enough that the cycle off phase was controlled by the burner's post purge/pre purge timing. The on phase by the vapourstat. Since the firebox and flue were still at steam temperature (pressure never actually go to zero) flame stabilization was very quick -- hard to time without continuous recording instruments, but at a guess on the order of 10 to 15 seconds into a 10 minute burn phase. I can live with that... the timings correspond well with Cedric's known "pickup" (how I hate that term) oversize of 11% (fired at 2.75 gallons per hour, 86% efficiency by the combustion test, 1229 square feet installed EDR).

PMJ

A vaporstat may be simple but it will not control a boiler anything close to the lowest pressures that these systems will run at and still heat just fine. That is because the maximum pressures that can easily be run are too low to actually have a cutin and cutout with any distance between them. I don't ever get over 2 inches water at the header even at -20F. Why? Because more pressure isn't ever necessary and more pressure is certainly not more efficient. My vaporstat is a safety device.

Jamie Hall

Oh dear. Well, I knew that someone would chime in... I beg to differ, but you all know that.

PMJ

Differ away. It is just an open discussion of ideas isn't it?

If a vaporstat is being used in control at all it is at pressures higher than I ever wish to be. Set to it's lowest setting mine will never trip. It is not a useful control instrument for me at all.

nicholas bonham-carter

Lucky you, as your boiler must be perfectly sized to the load.
The next size down in the Peerless 211A range is 80% less big, so too small.
These cold snaps are not frequent enough to convince me to rework the gas train to down fire yet.
If I would use setbacks, then I would probably need the changes.—NBC

PMJ

nicholas bonham-carter said:

Lucky you, as your boiler must be perfectly sized to the load.
The next size down in the Peerless 211A range is 80% less big, so too small.
These cold snaps are not frequent enough to convince me to rework the gas train to down fire yet.
If I would use setbacks, then I would probably need the changes.—NBC

NBC,

Not perfectly sized at all. Way too big. I don't need net total burn of even 40% to heat at -20F. The conventional wisdom is that this can only result in pressure and short cycling against pressure. Not so. I simply pace the boiler through even burns separated by even waits three times an hour. The result: no pressure and much more even heat. It is a remarkably simple solution that receives much resistance here.

acwagner

I agree with @PMJ . Here's the timer I got:

http://www.galco.com/buy/Macromatic/TR-6512U

It's reasonably priced, and it doesn't take any permanent alterations to hook it up into the system that can't be reversed. It's also easy to adjust and has a huge range for timer settings. It might be worth a try just as an experiment. If you don't like the results, you can always remove it.

By the way, I'm in a similar situation to you NBC--my system demand fell in between the outputs of two boiler models, and I chose to go with the larger model. I contemplated downfiring or adding a two stage burner, but decided to add the timer as an experiment. I like the results and wouldn't consider altering the burner output at this point. Once you set a timer sequence that meets your goals, you don't have to change it.