Cold start on a gas boiler - Ron jr.

Unknown · July 2002

This Peerless gas boiler with 2 zones for heat , and the indirect Megastor, yesterday . When running the indirect it seemed like it took the boiler quite alot of time to get up to its set temp at 180 degrees . Its been a long time since I set up a system like this , so I kept an eye on the temp rise . Seemed like the boiler hovered in the 110 - 120 degree range much longer than I would have thought .

Does anyone recommend maintaining a minimum temp in the boiler to meet a heavy demand on the indirect ?

J.C.A. · July 2002

Ron Jr.

Having NEVER been a big fan of cold starting , I will still stick by my guns and say to keep it at a temp a few degrees lower than the domestic "wanted"temp. The service associated with "cold starting " oil fired equipment has proven to cost more than the fuel savings were . With an indirect , you will get a few runs a day, but with the super insulated tanks , will they be long enough to keep the moisture out of the equipment and chimnies ? Give and get , Dans' big post asking for thoughts should be the forum here ! Having recently shifted to the other side , I haven't seen the long term implications of cold starting Gas fired equipment. I know what I saw with oil , and man was it ugly !

kevin · July 2002

cold starting

If the water heater is sized out properly you won't have a problem. Unless there is a very long period of time between runs(in the summer) there will be some residual heat left over(especiall w/ the cast iron boiler)

I am curious as to any problems others have had with cold starting with indirects. I have seen some problems when boilers were used for heat only and left to sit the summer in a damp basement....Kevin c.

hr · July 2002

Cold boiler, cold tank

Will take a while to warm especially if you are pumping the with a seperate pump and flowing gpms equal to what the boiler can produce. Make sure you are not bleeding into one of the heating zones at the same time, maybe a flow check not seating.

Make sure your boiler gauge is accurate also. I have seen some cheep-os that are both lethargic and off as much as 20 degrees! I like to see the boiler get above dewpoint in 5 minutes or less. Try throttling the flow a bit thru the indirect. Not too much or you will shortcycle the boiler.

Remember, depending on the indirect aquastat differintal, the boiler should fire as soon as the tank cools 15 or 20. So the boiler should fire long before the tank cools to incoming water temperature.

I wouldn't idle the boiler unless they have a huge dhw useage. Maybe the kids are running a neighborhood carwash

hot rod

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Bill NTSG · July 2002

Reverse Acting ....

...Aquastat . Ron I have seen indirects on steam boilers that have actually cooled the water in the indirect before the boiler started to warm up. Picture a cold start boiler, zone valve to indirect, [taco 90 sec to open] indirect at low ebb of cycle, just about to "wake up" from overnite off cycle and a Huge demand. People getting ready for work and school all at once. First, I would not do the zone valve. But I have used a 6006B or a 4006B when I first started using cold start Burnham V-73's in the early 90's. They had a tapping in the back and I bushed it and installed a well for the aquastat. I wired the indirect to call the burner and the pump. I ran the power to the pump through the reverse acting aquastat . It held the pump off until the boiler reached 120 or 130° or whatever I decided. This worked much faster than warming the cold boiler with the cool indirect water.

Unknown · July 2002

Thanks guys for all the replies

Im gonna recommend that on the next install we at least have a triple aquastat set at a minimum for the boiler . What Bill Nye said makes sense - say the indirect , after dropping its temp over a long time , calls for heat . With cold start , those first few precious moments will be wasted actually cooling down the tank with colder boiler water . Not the best of circumstances when theres a rush to take showers in the morning. And like Chris said - we all know what happens to the smoke side on cold start oil boilers.

Hot Rod - we have had a run on those cheap gauges . As a rule we always check to see if the temp on the gauge matches the temp set on the aquastat. This one was dead -on .

Being nowhere near an expert on atmospheric gas boilers, does having them cold start affect them like it does to most oil boilers?

Harv · July 2002

how Long is to long

What size was the boilers output. Remember that the delt t across the indirects heat exchanger is huge when filled with cold water. At 110 degree boiler water and 50 in the indirect you have a 60 degree delta T. That will suck up a lot of BTU's. When in real use the tank will rarely be filled with 40 gallons of 50 degree water. Do the math. 40 gallons of 50 degree water will take about 16,000 but's to raise the temp 50 degrees. 40 gallons X 8 lb's per gallon = 320 lb's of water x 50 degree rise = 16000 btu's. If the boiler has 80,000 btu input and a 70,000 output ( always go by output) it will take about .23 hours to reach 100 degrees in tank. That's about 14 minutes.

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Unknown · July 2002

Cold Start vs. Temp. Maintainance

Ron,

Depending on the standby losses of both the Boiler and Indirect Heater, the differential of the tank aquastat and the ambient temperature of the basement, this decision may be a difficult one. In most cases though, a boiler sitting at or near a limit may have to wait to turn the burner on and this waiting period will mean time and precious hot water for showers. Ideally, the boiler aquastat may have a 15°F diffential, but reality dictates that this may vary in some cases.

Most gas boilers now incorporate a motorized vent damper which will help keep standby losses to a minimum, provided the basement is reasonably warm. Standby losses of most indirects are also at a mimimum, say a degree or two per hour. If you have a boiler fire in the morning for the Indirect, it will shut off at or near it's high limit of 180°F and the Indirect will now have about 140°F water. Over a period of 6 hours or so, the tank may cool down as much as 12°F and the boiler as much as 30°F (based on 5°F per hour standby loss for gas boiler), this will still have the boiler hotter than the Indirect and when the Indirect does fire. The boiler will fire immediately instead of waiting for the boiler differential to drop.

I have seen oil boilers that can drop as much as 10°F per hour based on draft conditions. In most cases though, this is not the case with gas boilers due to the vent damper. This information, by the way, is not based on theory, but is based on fact from data compiled from tests and actual jobsites. Therefore, my vote will have to be for Cold Start controls Ron. Our Indirect Heaters come eqiopped with an aquastat with a 5°F diffential, thus providing precious hot water quicker and keeping the boiler warmer throughout the day by cycling a time or two! Hope this helps!

Glenn Stanton

Manager of Training

Burnham Corp.

Unknown · July 2002

Thanks Glenn

Its good to have info from tests done in the field. Im gonna pass the info on to my supervisors . The job was installed 2 days ago without a callback so far on cold start. Hey werent we supposed to get together one of these days for a beer? Maybe you might make it to the roundtable seminar . Im trying to get my boss to pay the tab for me - but if that fails , Im still going . This ones looks too good to pass up. Take care.

Unknown · July 2002

Harv

I didnt get the ratings on the equipment - its always figured out for me in advance ( I hope ) . I would say it took about 10 minutes to get the tank to 125 degrees at the aquastat . And that was with a startup temp of whatever the streetwater was . I was just wondering how much time might be cut off heating the indirect if the boiler was started at say , 120 degrees instead of room temp. I was never good at math though . Thanks Harv.

Unknown · July 2002

Sorry about that Ron!

I was down in your area last month for a few days. I was conducting evening seminars and that didn't leave me a whole lot of play time! I didn't think that you would be able to break away during the day, so I did other things instead. I am unsure at this time as to my attendance in Massachusetts. I am now concentrating all of my efforts with the New England States, NY and NJ and working with our salespeople and individual companies and organizations in addition to the all day seminars we offer. We have brought an additional person, Ron Beck, on board to handle my previous resposibilities in the Mid-Atlantic and Western Regions. I am also working out of my home in Rhode Island which is pretty much central to the region. All of this has kept me very busy and I am enjoying it tremendously. I suspect that you will see me there, but no commitmtnt at this point in time!

Glenn

Jeff Krawic · July 2002

Cold start?

I am in agreement with Glen. I like cold start on certain applications such as with indirect. And incidenally, the Megastor unit is well insulated with only 1/2 degree loss per hour at 65 degree ambient. If you are seriously concerned about making the hot water demand with cold start there are 2 other things you may consider.
1.) Set the Megastor at 140 degrees storage and use a mixing valve on the supply side to the house.
2.) Use a "Zone priority control" for the domestic zone.
Hope this helps. The only disadvantage I can think of on the boiler side of an atmospheric gas boiler with cold start is it MAY condense the gasses at start-up and you will see indications of this on the top of the burners when it first starts. You should be well OK though. Cold start saves alot of bucks in stand-by losses throughout the year. And with the low heat loss of the Megastor, it should be a decent payoff to the customer and make you look like you saved them alot in annual fuel costs. Good luck. Oh and a quick PS for the next installation, you should always use a "heat trap" on the DHW supply outlet, even if there is no mixing valve. It prevents hot water migration by convection from leaving the tank keeping stand-by losses even lower.

Jacob Myron · July 2002

Cold Start?

All answere above are good but the real issue is it take BTU input to heat teh water.

Assume the tank is a 30 gallon storage heater and the the water temperature is 50 degrees you need to input 30 gallons times 8.33 times 90 degrees=22,491 BTUs to get the tank up to 140 degrees.

Another factor to remember is to make 1 gallon per minute of domestic hot water you need to input 50,000 BTU per hour.

Many homes that have boilers used for water heating rarely require more more than 100,000 BTUH for heating. Often when a boiler is installed people tend to forget that domestic water heating has a value that needs to be added to the boilers size.

Typically a thumb rule can be used to add BTUs to the boiler for domestic hot water (thumb rules are just that and applicable in all instances.)

I used to 25,000 BTUH to the boiler for up to a family of four and 5,000 BTUH for each additional person in the house up to eight people for domestic hot water production.

If the the heating load is shed when the boiler makes hot water you will be O.K. If heat and hot water are made at the same time you could have a problem.

Cold starts are bad when the tank gets cold while in use.

A cold start should never happen unless service or change outs require the boiler to be shut down.

Jake

Bill NTSG · July 2002

Cold start

The thing is I look at it from an oil burner service tech stand point. At start up you make a little smoke. When the boiler water temp is held below 130° you are condensing . Soot and water make acid and other nasty stuff. I have to clean the gook out of the heat exchanger. The faster the flue gas gets above the dew point the happier I am. A warm start boiler is a head start in this race as far as I am concerned. I am no engineer or rocket scientist I just know what I see. With gas it is not a problem maybe . Just in todays society of MacDonalds drive thru I want it now mentality a head start on the hot water game has got to help.

Cold start on a gas boiler - Ron jr.

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