commercial boiler - ok to turn it on and off each day?

[zeppy]

relating to a non-condensing hot water cast iron commercial boiler (16 sections) - 2-3 million btus

is it ok to turn it on and off during the transitional period into the colder season? (ie. warm some days and much colder other days)

i have heard it really shouldnt be turned on and off once its been turned on for season (ie. once its on you should leave it on to avoid stress on boiler and also to reduce condensation corrosion issues)

thanks for any feed back

[ga1279]

That pretty much is a valid point as you keep expanding and contracting those sections. That's why you have hot water reset. As I don't know your hydronic layout. Is it a 2,3 or 4 pipe system, do the hot water lines go all around the building, are your coils stacked. Is it a constant volume with reheat, yadda, yadda,yadda. I don't need any of these questions answered as I'm just making a point-GEO

[supertek65]

the boiler usually comes on when it is 55 or so outside???

so it will come off and on quite a bit in the spring and fall anyhow????

the night set back would usually be on the 3 way valve or the loop??

the outdoor reset ratio would be on the boiler temp itself!

[zeppy]

i might add that the hot water boiler is not subject to water treatment so there are other corrosion issues which are not good.

is it healthy for this large boiler to be turned on and off each day (will it impact long term health of boiler)? it sounds like a good way to save on short term operating costs but is it causing unnecessary thermal strain and condensation corrosion?

thanks

[supertek65]

it is clearly not a good idea to short cycle the boiler.
especially if it is not modulating!

just set back the boilre temp

instead of 180 degrees
at night lower it to 110 deg or so on

do you have the honeywell building control book/ manual??
it explains normal boiler controls?




i might add that the hot water boiler is not subject to water treatment so there are other corrosion issues which are not good.

is it healthy for this large boiler to be turned on and off each day (will it impact long term health of boiler)? it sounds like a good way to save on short term operating costs but is it causing unnecessary thermal strain and condensation corrosion?

thanks

[zeppy]

its a conventional boiler and water temp setting should not be below 140-150 due to condensation corrosion issues.

short cycling is a non-issue

should the boiler by started up for 6-8 hrs and then turned off when not needed. or simply left on till colder weather arrives all the time? what is recommended to avoid thermal stress and condensation corrosion issues?

thanks

[chillerguy81]

Is the boiler in question being controlled by an ems or is someone manually turning it on and off for the season? If it is controlled by an ems is there a modulating bypass valve for the boiler to control loop temp?

[zeppy]

hi Chillerguy

the boiler is being turned off manually.

there is a boiler loop and facility loop with a outside air temp controlled mixing valve in between. in this situation, there should not be a need to turn off boiler as the NG costs are reduced since mixing valve is barely open when warmer; there is a 15 hp motor thats on continuously and thats most of the savings when boiler is manually turned off for a half day or day.

thanks for any info on how turning on and off boiler could be harmful

[chillerguy81]

hi Chillerguy

the boiler is being turned off manually.

there is a boiler loop and facility loop with a outside air temp controlled mixing valve in between. in this situation, there should not be a need to turn off boiler as the NG costs are reduced since mixing valve is barely open when warmer; there is a 15 hp motor thats on continuously and thats most of the savings when boiler is manually turned off for a half day or day.

thanks for any info on how turning on and off boiler could be harmful

Since you do have a mixing valve being controlled by oda temp, I would recommend keeping the boiler on and using the outdoor reset to control the load. If you don't have much load the boiler isn't really going to fire much anyway other than to keep it's self warm. This will be a lot less stressful to the boiler. I'm assuming that the 15 hp motor is the hot water loop pump, which I can see they're energy concern about running 24/7 if not needed. I have seen applications were a smaller pump is installed on the boiler loop just to maintain a minimum flow through the boiler so it can run and keep itself warm.

[zeppy]

thanks so much

its for a church facility

so hard to make changes - a lower HP motor for pump would make sense (maybe 7.5 hp)

i could close the mixing valve and keep it closed and turn off pump and just leave the circulating pump on for boiler but then it takes a long time to gradually open the mixing valve to balance things out before setting mix valve controller on automatic (have to be careful to avoid thermal shock)

would like to get a new motor with a variable speed drive but its pricey but would be payback

again thanks - it confirms what i believed to be the case

since its the only hot water boiler it would be nice to take care of it and a few hundred dollars savings due to turning off boiler to save doesnt make sense when annual fuel bill is 35k and cost of boiler is $110k to replace. I fear its been corroded (10 yrs old and no water treatment) and unfortunately it is tough to detect.

[flange]

we did one of these setups last year. pulled out the cast iron, and installed mod cons, and outdoor reset. based upon first year fuel savings, payback will be roughly four years, and building comfort is better.

at any rate condensation may not be an issue if venting is done right. flue gases will condense in the stack right? many boilers of this size use a tee fitting off the back, with a drain out the bottom. this collects condensate instead of dumping back on the boiler, which is what kills sectionals. biggest concern i have is thermal shock, and dirt in sections, causing overheating and stress cracks from it.

[supertek65]

we have had problems with the payback being too good and paying back too fast!!!!!!!!!!!1

i know that sounds stupid!!!!!!!!!!!!!

but here the power company gives incentives and rebates calculated by payback!!1

i assume flange has similar programs back there?

i guess if they get there money back in two years? they do not get to participate in these programs????????????????????




we did one of these setups last year. pulled out the cast iron, and installed mod cons, and outdoor reset. based upon first year fuel savings, payback will be roughly four years, and building comfort is better.

at any rate condensation may not be an issue if venting is done right. flue gases will condense in the stack right? many boilers of this size use a tee fitting off the back, with a drain out the bottom. this collects condensate instead of dumping back on the boiler, which is what kills sectionals. biggest concern i have is thermal shock, and dirt in sections, causing overheating and stress cracks from it.

[supertek65]

condensation is usually a problem where there is LONG horizontal runs!!!!!!!!!!!

[zeppy]

there is a T connection for the flue gas and maybe corrosion is not a major concern

how much is the thermal stress a concern? is it a cumulative issue? i guess its also a problem due to lack of chemical treatment of water.

is it tough on a large commercial boiler to heat the water temp from 95 degrees to 150 degrees several times each week (ie. turning on and off the boiler) until it gets very cold outside. also, the 95 quickly goes lower when boiler is turned back on due to all the water in faciility that will flow back into boiler at 70 degrees.

thanks

[flange]

that aspect to me is a bigger threat. when bringing a boiler up from cold, it should be done slowly. on larger boilers you will see a low fire hold for a period of time to prevent cold slugs from entering while in high fire. on a church, this could occur on a chilly morning when they fire it up and send all the water from the space back. i am not a fan of what you are doing, but from an energy alone standpoint, i understand.

[genduct]

Do you have a low fire then high fire? If so does the high fire happen after x seconds?
You can't help it not but be a problem under low partial loads so it would seem helpful if the boiler were to stay on low fire so as to not stress the boiler in the spring and fall.
Or perhaps not allow high fire untill the water temp reached say 150 degrees.
had a problem with a steam W McL 98 14 sections sometime back with stress from same description.

[chillerguy81]

that aspect to me is a bigger threat. when bringing a boiler up from cold, it should be done slowly. on larger boilers you will see a low fire hold for a period of time to prevent cold slugs from entering while in high fire. on a church, this could occur on a chilly morning when they fire it up and send all the water from the space back. i am not a fan of what you are doing, but from an energy alone standpoint, i understand.

I agree with Flange 100%. Thermal shock is what I was referring to when I said stressing the boiler. I don't work on a whole lot of sectionals but I've seen quite a few start to leak when they're shutting them on and off. If I owned the boiler I would start it up in the fall and not let it get below 140* - 150* until I shut it down in the spring. As far as water treatment goes is there a shot feeder? Have a chemical company take a sample and add treatment as needed. Hot water doesen't require as much maintenance as steam, which I'm sure you know.

[viessmann]

keep shutting boiler on and off every day or every other day and you should see some savings so you could pay us to install new one when it starts leaking pretty much a guarantee.

[supertek65]

brilliant!!!!!!!!!!!!!!!!!!!!!!!!












keep shutting boiler on and off every day or every other day and you should see some savings so you could pay us to install new one when it starts leaking pretty much a guarantee.

[supertek65]

it is a closed system!!!!!!!!!!!!!!!!!!!

throw some boiler treatment in it

follow instructions and BAMMMMMMMMMMMM!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!! !!!








I agree with Flange 100%. Thermal shock is what I was referring to when I said stressing the boiler. I don't work on a whole lot of sectionals but I've seen quite a few start to leak when they're shutting them on and off. If I owned the boiler I would start it up in the fall and not let it get below 140* - 150* until I shut it down in the spring. As far as water treatment goes is there a shot feeder? Have a chemical company take a sample and add treatment as needed. Hot water doesen't require as much maintenance as steam, which I'm sure you know.

[disquew]

Honeywell makes a great controller now that has an adjustable reset which would solve your dilema. I installed one on a boiler of mine last year and is working flawlessly.