Expectations for boiler cycle length and temp rise

jfleming

We've been fixing a steam system for the past few years -- fixed header and boiler piping, main venting, insulation, new vents. One pipe counterflow, if it matters.

We notice that, with a set back of 59 degrees at night, that it can take a few hours to get up to 65 in the morning. We have maid-o-mist vents and are currently testing with moving from #4 orifices to #'s 5 and 6 to see if the issue is venting air from the rad fast enough (mains are vented)

Also, while temp builds over the course of the morning, the pressuretrol seems to be correctly kicking in and cutting the boiler in and out (main 1.5, subtractive diff of 1.25). However, the cycle is pretty short - 2min on before cutout, 2min off before cut-in. Boiler might be oversized, but no calculation has been done 

I'm missing external context - does a 6 degree setback reasonably take a few hours to overcome? And, if the boiler is at pressure, is a 2 on /2 off cycle too fast?

clammy

As a comparison I have a older home single pipe and for a 2 degree temp rise from a cold start ,after the system has steam to the main vents it usually takes about 45 minutes to 1 hour for the stat to be satisfied . I think there a lot of widget room in there especially w a counter flow system being there’s only so fast you can vent everything through the radiator . If all else is kosher I would look to possible increase the radiator vents from 4,5,&6 to 6 ,c and d . I find in older system the owner best bet is limit set backs to only 2 or 3 degrees ,there seems to always be issue w either slow recovery ,start up cycling on low water due to heating mains until piping is hot enough to prevent excessive formation of condensate . On slow reacting system if I have personally pulled the boilers return plugs and washed out the boiler of it mud and debris and upon re firing found the system slow response had a lot to due w mud build up in the boiler directly effecting the boiler abililty to produce dry steam and to transfer all the burner heat to produce that steam . I ve done this on quite a few boilers who,s Performance was poor but piping and everything looked better then most just going on a hunch I drained the boiler from a 1 inch ball valve directly on my boiler spare return tapping ( 21/2 x1 1/4 reducing ell) seeing the real condition of the water ,drained wanded from bottom and top filled and a fast skim ,afterwards the mains went from 45 minutes to about 5 minutes all due to poor water quality . As clean as a sight glass is it truely isn’t the indicator of the internal cleanliness of the boiler . Often discounted is the internal cleanness of the boiler .
I find out of all the types of steam system out there that counter flows are often the toughest being you truely need dry steam for any type of decent performance . I ve found that an over head feed off a drop header w the header take off being tied into an upward facing tee piped into the run and the Start of the fitting continues and drops into your wet return . I ve usually always used a vapor stat instead of a pressuertroll . If the boilers is short cycling I would probably recheck the edr and if oversized possible add a 2 stage gas valve to get a longer cycle and less on and off cycling . I never discount a dirty boiler ,you never know until you pull the boiler plugs it seems to be the thing nobody does but if installed without provision to due so ya might as well wait till it kicks the bucket .
Peace and good luck clammy

Mad Dog_2

Additionally, High Mass Systems (Steam, Radiant in a slab) should not be set back so far. You burn alot of energy gunning it to try to get back up again.  Mad Dog 🐕 

Jamie Hall

Your timing, as the guys above have said, is about right for a 6 degree setback -- which is about 3 times the maximum setback you should be running, if you run any at all. As to the boiler cycling on pressure, it's just telling you that it's oversize. Probably at least 50 percent oversize,, in fact, if not more.

jesmed1

Mad Dog_2 said:

Additionally, High Mass Systems (Steam, Radiant in a slab) should not be set back so far. You burn alot of energy gunning it to try to get back up again.  Mad Dog 🐕 

I know the OP is talking steam here vs. hydronic, but I've stopped using setbacks in our 4-unit condo building with old cast iron radiators and 2 oil fired hot water boilers. I don't think it saves much energy, if any, and it disrupts the heat balance between our lower-floor and upper-floor units when the boilers run longer in the morning to recover. So for our high-mass hydronic system, I've found that maintaining a steady temperature throughout the heating season works best.

ethicalpaul

hey @clammy something you said caught my eye and I wondered if you could clarify?

especially w a counter flow system being there’s only so fast you can vent everything through the radiator

Are you saying there are no main vents in a counter flow system? That strikes me as strange--can't you put a main vent on a counter-flow main? Wouldn't you put it just before or just after the last radiator runout on that main?

Daveinscranton

I have a good friend who is a stone mason by trade.  He also does general contracting work when he has to, which is often.  His specialty is fixing mistakes.  Costly ones usually, after others have tried and failed.

We talk a lot in his slow season.  He is of the opinion that big temp setbacks are not good for building envelopes.  Materials have different rates of expansion and contraction with temperature, that over years, become problematic.

AdmiralYoda

Steam doesn't play well with setbacks. As mentioned it is a high-mass system that heats evenly, gently, and slowly.

I used to do the same thing, now I just set it and forget it. The house stays more comfortable, the boiler works less and the gas bill didn't change. I didn't do a deep dive analysis but I think I may have even saved a couple bucks.

I also have a counterflow one-pipe steam and my boiler is about 50% oversized so it cycles on pressure in about 30-45 seconds if I'm running a setback. I've installed a vaporstat that keeps it below about 12oz of pressure and it doesn't cycle on pressure unless it is very cold out and the thermostat is taking longer to be satisfied.

clammy

Paul there’s usually not many who will get into cutting into an old counter flow main to get a vent in between the last two take offs unless the system had extremely long mains which isn’t really common for most counter flows I ve come across . Plus unless split into more then serval mains there usually sized larger then a single parallel main for the Btu which can be difficult and costly for the sake of a main vent .Dry steam ,insulating mains and near boiler piping ,clean boiler and water and fast radiator vents , low pressure and also zippo for the setback .
Peace and good luck clammy

109A_5

Hello @jfleming,
Today was about 40 degrees Fahrenheit outside. I setback, usually about 8 to 9 degrees, for about 9 hours when I am away at work. When I return I most often I raise it up 4 degrees, then another 4 or 5 degrees a bit later when I pass by the T87F thermostat. Today for you I did 8 degrees in one shot. It took 58 Minutes and the pressure never went over the normal 1.25 WC or 0.72 Ounces as much as I looked at it multiple times through out the hour. My boiler is undersized at 300 Sq. Ft. and the EDR is 347.

I have not studied this at all as far as fuel savings. However I believe the hour of fuel usage to recover from the setback would be less fuel than the normal cycling of the 9 hours if I just left it at 68 degrees.

On the other hand when it gets to the single digits Fahrenheit outside, I set back less or I don't bother.

Single pipe steam with 9 radiators, most all of the pipes in the basement are insulated.