Cold Start Can we save energy this way

exqheat

Different advisers speak to benefits an harm from cold start. Long term low temperature operations during cold start flu gas (Natural Gas) exhaust condensation in Boiler can cause slow but sure boiler damage. My problem is the Spring and Fall operations can cause unwanted boiler shock from cold return water ( 75F) will cause boiler shock when the boiler goes on. Any thoughts?

hot_rod

If you have extended cold run times, just add a return protection device.  

Keeping the boiler hot all the time is the other option. If all the jacket loss is inside the structure, maybe that is an option?  Combustion air grills to the outside tend to be the sieve for that jacket loss from an always hot boiler, as well as any loss up the flue

cycle efficiencies from a short cycling boiler can drag efficiencies down into the 70% range

some of the arguments for mod con boilers even if they run out if condensing mode. The “mod” part really lessens cycling under low load conditions

exqheat

The temperature rise from cold start to 140 supply temp is 15 minutes, and keeps rising. With 60% circulation delta is 20F. A loop is not that needed, as I see it. The discussion has to change from myth to facts as energy conservation becomes more important. it seems more efficient boiler operation will allow boilers to be a real solution, VS heat pumps in multi family buildings. No question protection loops should be built in at new construction. Retros need a realistic analysis.

hot_rod

There is no definitive number for the amount of time to reach return temperature above the dew point of the fuel, it varies a bit from different fuels and conditions. You can calculate that number based on your exact conditions


Call it 140f for an oil fired boiler, the rule of thumb out there is within 10 minutes of burner on you should reach that minimum temperature to assure the boiler is not condensing, and be considered an acceptable minimum run time.

If in shoulder seasons the boiler runs 5 minute on cycles, and the boiler is “wet” not reaching dewpoint, both the boiler and flue piping may be taking a hit

No doubt many boilers run this mode in low load conditions, not ideal, preventable, just the way it is 

exqheat

My application is a Weil McClain LGB-6 600,000 btu cast iron gas boiler in a 10 family apt building. When resting on a warm day in the shoulder season Feb. 23, the return pipe was 75 degrees. It took 15 minutes for the supply pipe to rise to 140 F. I'm sure there are many situations like this for older boilers out there. So at what readings are the condensate risks increased. Perhaps the Manufacturers should have a chart for mechanics to use, to determine the necessary for a add on protection loop. With this definition, definitive guidance would be available mechanics would have firm grounds to recommend this "add on" when converting systems to cold start. Energy savings could be trade off for the addition.

hot_rod

I suppose it would depend on how long it runs after reaching that 140 return. Long enough to dry out the combustion chamber and warm the entire flue above condensation temperatures?

How often does it run under those conditions?

About all you will get from the boiler manufacturers is “protect the boiler from extended condensing temperatures”. Maybe some random drawings and suggestions for protection options

Its application specific and should be the designer’s responsibility to know how and when to protect the boiler from excessive cold runs.

EBEBRATT-Ed

Read any boiler manual you want for any CI boiler and it will state in so many words. "consult the mfg for jobs with extensive piping and long warm up times" The actual wording may be different but that is what they mean.

hot_rod

EBEBRATT-Ed said:

Read any boiler manual you want for any CI boiler and it will state in so many words. "consult the mfg for jobs with extensive piping and long warm up times" The actual wording may be different but that is what they mean.

But their manuals don’t show appropiate return protection options, what would a consult with them accomplish?

I feel the installer should have the knowledge to understand how and why to pipe a boiler?

exqheat

I'm going to get some new data, to better describe the conditions. Later

exqheat

I observed the system on 2-22 with outdoor temps at 35 degrees. The boiler system was circulating a caclulated157F water with a 3 degree differential between supply and return temperatures. I will have to wait for a warm day to measure the recovery of return temperature to 140 minimum. See you then.

Steamhead

@exqheat , does this boiler have weather-responsive controls like outdoor reset, Hydrostat etc, a stack damper, or other energy-saving features?

exqheat

Exquisite Heat

EBEBRATT-Ed

@hot_rod

I agree, the installers "should" have the knowledge and ability to know when return water temp protection is needed. But most don't have a clue.

All steam boiler come with a specific piping diagram and (some) installers don't follow them or even read them. We see the horror here every day. No different with water.

The HW CI boiler mfgs are just pointing out (covering TA) that unusual systems may require deviations from standard practices.

exqheat

Cold Start

When to run a hydronic boiler with maintaining system temperature at all times. OR Cold start when thermostat calls for heat. Who Knows?

Given a multifamily building 8 family Weil McLain 650,000 btu LGB Boiler. No boiler protection loop. One circulator for one loop of baseboard radiation.

The Threats:

On cold start, the boiler begins to circulate, the gas fires and within 15 minutes the exit temperature is 140F and the return temp. is 68F. The boiler continues to fire until the thermostat is satisfied, or the high limit is met at 160F.

What is the danger of flue gas condensation causing acidic damage to the fire side of the boiler sections?

When maintaining a constant range of temperatures from 140 to 160 degrees, when activated the circulator returns 68F water creating a delta of 80F. Is this not a cause for boiler shock fragmentation of the iron sections?

Is there research on this anywhere?

The common answer is to add a boiler protection loop, between the supply and return, to raise the return temperature to 140 F. The boiler in question has been operating for many years with no protection.

In the interest of optimizing energy efficiency and emission reduction, it would seem the risk of boiler shock damage accumulation is reduced in the cold start mode.

The question is, what is the risk of flue gas condensation in a boiler that is firing through out the recovey to high limit at 160F. from 68F?

mattmia2

Shock isn't really a concern but low return water temp is, the return temp should be above 140 realtively quickly. The cold return water temp will cause a CI boiler to condense. Was this a gravity system originally? Whoever installed it should have installed a termostatic bypass or somethinkg similar. With a system with that much mass warm start won't help you much.

There is astrong possibilty that the sustained low water return temps will shorten the lif of the boiler and the vent and could possibly cause a condition where the vent doesn't draft properly and cause CO poisoning.

hot_rod

The damage from extended cold run conditions, boiler in condensing mode, can also damage the flue piping. Is it a masonery chimney or metal B vent. Look for indications of corrosion on the pipe or damage in the brick or tiles.

The vent cap on a B vent system is sometimes an indication. Rust, water streaking marks.

F11E5515-F83B-4638-9203-E288A583192E.jpeg

exqheat

Is there any definitive research in this area.? Have you witnessed a condensation failure of boiler sections in a building. With fire times above 7 minute, wouldn't the chimney be warm enough to pass the exhaust before condensation occurs. How long does the mnimum fire time have to be. With that limit the fire time can be extended by increasing the differential. On cold start, fire times usually are extended. This would reduce condensation risk.

How does exhaust gas condense while a fire is present? Once the fire stops the continued draft would take the flue gases away.

mattmia2

Teh problem isn't ime, it is the temp of the heat exchanger. The heat exchanger never gets hot enough for the moisture in the porducts of combustion to evaporate because the very cold return water temp keeps it cold. Usually this plugs and sometimes corrodes through the boiler. You can see examples of boilers that have plugged and corroded through if you look through posts here.

The colder flue temps from the cold return water also may prevent the vent from drawing properly which can be deadly. This isn't just a equipment life thing, it is also a life safety issue.

The plugged heat exchanger can also cause products of combustion to spill out or in some rare cases start a fire.

exqheat

Just how cold can a boiler fire wall be with 68F water coming and mixing with higher temperature residual boiler water, as apposed to the temperature of the flame? With the boiler water raising the temperature of the return water, the boiler fire is heating the exchanger walls. The exit temp raises quickly (15 min) to 140. The flame is still on to reach exit or 160. By the time the water reaches limit the flue is quite hot and the heat exchanger is heating the moving water. Again is there any reseach showing the limiting parameters in this area, or is this discussion all conjecture.

mattmia2

You can find pictures of rotted out and plugged boilers on here from this.

hot_rod

https://forum.heatinghelp.com/discussion/comment/1745008#Comment_1745008

I think you would need to test a specific boiler at specific points on the heat exchanger, or watch it warm with an IR camera. High mass cast boiler would look different compared to a low mass copper fin boiler, for example.

The actual temperature depends on the dew point of the fuel and the ambient conditions surrounding the boiler, to some degree.

The industry "number" seems to be within 10 minutes you want to see return temperature above that dew point

Another generally suggestion is the boiler run 20 minutes above that return temperature to assure the entire HX and flue piping is warmed and dried.

possibly the IBR or Brookhaven Labs has looked at this?

Derheatmeister

1. Some cast iron boiler manufacturers such as Buderus have a high silicon content which will help with your concern about the thermal shock. Not sure about the Weil McLain ?

2. How old is this 10 plex/8 plex boiler setup and do you see signs of corrosion on the cast sections and the flue?

3. Is the anual fuel consumption in Therms high ?

4. It sounds like your equipment is short cycling during the shoulder season due to Mirco loads ?

5. One Single stage 650 000 BTU/HR Boiler ? Sounds not like a very desirable boiler for this situation

6. At what altitude?

7. Regular or High output Baseboards ?

As others have mentioned it seems that you could possibly benefit from a repipe/Control upgrade

Not sure…But in your postings you mentioned a 8 plex and a10 plex ?…You also mentioned a 600 000 btu/hr and 650 000 btu/hr Boiler ?…. Regardless in order for you to save energy and prolong the life of your equipment you should consult with an expert which will evaluate your situation and make suggestions based on it.

Some things that you may benefit from:

Tekmar (Floating action) 3 or 4 mixing valve with sensors that will protect your equipment from condensing/ Thermal shock and Reduce your anual fuel consuption !

Tekmar Thermostats that will use PID which will Group calls for heat vs.a stop go situation which will again Prolonging the life of your equipment and reduce your anual fuel consuption !

OD reset…Proper system fluid..Auto delta Tee/ Delta P circ ..Buffer tank ? Latent heat storage tanks

Or Change the equipment to Larger heat emitters,Panel rads/infloor heat/ radiant walls with a Condensing boiler..Maybe some Solar for the DHW with Latent heat storage tanks..Phase changing storage..Window and insulation upgrades ?

BTW Thermal shock,Flue gas condensation, high fuel comsuption and premature boiler failures are a real thing when the equipment is not installed or maintained correctly.

Hope this helps.

exqheat

Again is there any reseach showing the limiting parameters in this area, or is this discussion all conjecture

exqheat

exqheat

captainco

exqheat- Is there a study or is this just conjecture? What a great question, From the answers it is something people heard at a bar. There are no studies. I am pretty sure a new ruling came out that said manufacturers of residential boilers were not allowed to put controls on them to maintain temperature when there is no call for heat. The exception would be if they had indirect HW coils.
The only way you are going to shock a boiler is to dry fire it and then add water or heat it up to full temperature before turning on the pump and even then the water would have to be quite cold.

Condensation has nothing to do with the return water temperature. That is an old fairy tale. Condensation is caused by venting problems, combustion air problems, underfiring problems and improper control settings and sometimes mechanical failures. I don't care how cold the water is, a burner with a 2500 to 2800 degree flame is not going to condense ud the above things are correct!

Since the early 80's I have shown that turning the burner off when not needed will save money and cause not harm whatsoever to the boiler if the other things are corrected. This has been done on heating and process equipment. I am sure I could write many pages on this subject based on experience not hearsay.

exqheat

I appreciate your observations. I thought there might be help in finding research. I called a manufacturer, and the tech folks told me that it would be difficult to detect condensation in an active fire box. I agree with your cold start comments. I needed to install a protection loop for a green house system. The volume of water in 2 inch pipes for acres of greenhouse caused shock and broken sections. After the protection loop, the boiler ran without failure for Years until the nursery closed.
With my control strategy I can set temperatures of operation for any cold start, minimum temp maintenance, and Dom Hot water use. The question is what are the limitations for condensation damage in boiler and flue. As for flue the stack temperature and minimum fire times could be established. These requirements can be met with the extension of fire, by means of a properly adjusted differential setting. The larger the differential the longer the fire time. In a cold start mode the start up target high limit combined with proper differential could be established in each boiler. Properly set, this could minimize fuel use, and extend boiler life. I believe this study will help to reduce emissions in millions of boilers.

captainco

Maintaining a flue temperature above 250 degrees, above the drafthood should prevent condensation issues with proper draft. This should only take a few minutes to reach. The actual temperature should be above 300 degrees when fired correctly and that varies with the water temperature.

hot_rod

captainco said:

exqheat- Is there a study or is this just conjecture? What a great question, From the answers it is something people heard at a bar. There are no studies. I am pretty sure a new ruling came out that said manufacturers of residential boilers were not allowed to put controls on them to maintain temperature when there is no call for heat. The exception would be if they had indirect HW coils. The only way you are going to shock a boiler is to dry fire it and then add water or heat it up to full temperature before turning on the pump and even then the water would have to be quite cold. Condensation has nothing to do with the return water temperature. That is an old fairy tale. Condensation is caused by venting problems, combustion air problems, underfiring problems and improper control settings and sometimes mechanical failures. I don't care how cold the water is, a burner with a 2500 to 2800 degree flame is not going to condense ud the above things are correct! Since the early 80's I have shown that turning the burner off when not needed will save money and cause not harm whatsoever to the boiler if the other things are corrected. This has been done on heating and process equipment. I am sure I could write many pages on this subject based on experience not hearsay.

Take a gravity conversion system for example, lots of water volume in the piping and radiators. Lots of thermal mass in the steel and cast iron.
You fire the boiler on when everything is at say 68 degrees. It takes an amount of time before the boiler warms that mass up above the dewpoint, so the surfaces of the cast iron boiler sections will sweat or condense. The distribution system drives the boilers operating condition.
Boiler manufactures caution against run the boiler at that condition for extended periods of time.
The show piping and pumping bypass options. Specific thermostatic valves are built for boiler return protection. Tekmar, most all boiler controls offer return temperature protection sensors 

Captianco, I’m not seeing how a perfectly tuned burner overcomes this thermal inertia

Id like to think my car and truck run efficiently with a multitude of sensors, yet the tailpipes drip condensation until the pipe from the engine to the tail tip warms up. Isn’t the concept the same.

A water heater fired up with 40 degree water will drip and hiss also until the center flue warms

what am I missing?

exqheat

The fire in the fire box is above dewpoint in very short order. As the boiler warms the return water will be mixing with warmer water. There is no replacement for data. With spring coming I will be able to record data from both the return sensor, supply sensor and stack temps and times. My question is how do all those boilers without a protection loop last so long. Cannot be much going on. My original question remains. How low can we go? To save energy and minimize emissions from existing installation that will be in operation wasting fuel. We need to establish a definitive knowledge base. The savings could be close to 20%, provided we have a protocol for setting the necessary conservation limits.

exqheat

"Maintaining a flue temperature above 250 degrees, above the draft hood should prevent condensation issues with proper draft. This should only take a few minutes to reach. The actual temperature should be above 300 degrees when fired correctly and that varies with the water temperature."

Really? How does the stack temperature become affected by a 60F difference in water temperature. Needs closer observation.

hot_rod

Here is a look at what could be going on inside a typical boiler. Flame temperature wall temperature on fire side and temperature outside the HX wall.
If the boiler starts out at 68F inside the combustion area it needs time to get above the fuels dew point. That time period could be minutes or hours on high mass, undersized boiler applications.
It's quite possible even on fin tube systems, see attached.
We know condensation happens and we encourage it in condensing type, mod con, boilers.

ALL boilers are condensing boilers until that HX wall gets above the dew pioint.
Old thick casting boilers could deal better with there corrosion caused by wet cast iron, for years. Modern cast boilers are not as robust.

Why do some last years, decades maybe, and others fail in 5 years? I suspect it has to do how often, how long the boiler runs in condensing mode.

https://www.achrnews.com/articles/135513-protecting-boilers-against-flue-gas-condensation


The intent of this drawing is to show how micro bubbles form when the fire hits the HX. The importance of micro bubble air seps.

captainco

Not sure how those pictures represent the real world. First the flame temperature showing is 1000 degrees low. I think that would change the heat exchanger temperature but I think this is just a computer simulation.

What temperature does it take for hydrogen to mix with oxygen to make water? It takes a spark or flame which would be above condensation temperature. Is the moisture we see from initial start-up condensate from flue gas or condensation from the air? One is acid and one isn't. The condensate is above the flame which is over 2000 degrees so I am wondering how it would fall below dew point.

Colder water does absorb more heat so 65 degree water will lower the initial flue temperature and is more efficient. If the combustion is set up correctly the flue temperature will be above dew point. Wait a minute. According to industry standards we are not allowed to set up combustion correctly. That would require us to control venting and combustion air under all conditions. That would require us to verify the equipment is actually producing its rated btus.

Going back to the original question - turn if off unless needed!! Doing anything else is just a bandage covering up problems we don't want to accept and wasting energy.

hot_rod

When a mod  con runs low temperatures, below @ 130f we get what has been described as “waters of combustion”. That fluid has a low ph, 4- 5 possibly.

A mod con running 40 SWT  in a snowmelt will “pee” a lot of condensate, slowing to a drip at @ 130. Can the burner be adjusted to stop that condensation? 

If blending the return up with supply temperature to prevent boilers and flies from corroding is that a bad practice?

Yes the example has hypothetical temperatures to show air coming out of solution at the warm metal surfaces. On many mod cons you can read the flue gas temperature and watch it change from start up, so that number does exist at some point.

Solid_Fuel_Man

My experience is only with oil and low operation temps. 

Commercial building I worked in had 3 tenants. Each had 4000 square feet of radiant slab. Pex was 12"OC. Single zone all with that same NewYorker pinner with a becket fired at .85gph if I remember correctly. A single 0011 and that's is. Boiler would run the ENTIRE call for heat and was piped directly to the slab. Boiler SWT never got over 110F RWT never got over 90, after a long call. These shops were all set to around 60 degrees. 

All 3 of these boilers lasted and had over 500F stack temps after being cleaned and tuned. I hate pinners...did I mention that? 

Last I worked there all 3 boilers were still operational and 22 years old. For the life of me, I cannot see why any of this even worked. Must be due to poor heat transfer of a single pass pin Boiler. 

A fee years ago I did replace one when the owner of the building removed the chimney for space reasons. I put in a fire tube mod/con direct piped. 

Oil boiler sat right by the leak free manifolds, I didn't want to touch them so I just piped over to them. 

Derheatmeister

Captainco..With all due Respect..
As a NCI certified Carbon Monoxide and Combustion specialist #16-258-18 i can ensure you that even with the proper combustion adjustment as per your,manufacturers instructions and industry standards a cast iron/Fin tube boiler (NOT A MODCON) that does not have proper return temperatur protection and is subject to prolonged cold return temperatures due improper piping techniques WILL CONDENSE and cause serious issues.. We had to "Red tag" boilers that had seriously clogged HX due to this and were spilling large amounts of Carbon Monoxide into the Mechanical room ...Expecially on Snowmelt systems in subject becomes a reality and i think that many here will agree...

exqheat

"OLD DAYS
Several decades ago, sustained flue gas condensation was seldom a problem in hydronic systems. They were designed around heat emitters that forced the system to operate at relatively high water temperatures, typically in the range of 180°-200°.

During a cold-start condition, a small amount of intermittent flue gas condensation would form within the boiler. However, the operating conditions quickly increased the water temperature entering the boiler above the dewpoint and any initial condensate quickly evaporated. This situation occurs in every boiler during a cold start and is generally not a problem." J siggy.

exqheat

So... If the controller fires the boiler until the return temperature is above 130F, condensation should not be a problem? Or if the normal delta 20F, you could set a minimum high limit start up temperature at 150. In this case condensate would be evaporate as the return temps 130. Try to remember that there is a whole lot of heat added by the fire. If the thermostat is not satisfied at 60% circulation and extends to more than 60% the temperature for high limit can be adjusted upwards. ( Out of Condensate danger range.) At 60% circulation there is a good chance that the delta temp could in fact go down. Or to be safe, you can set the initial cold start high limit to 160F. With a 20F differential the return temp would be 140F before the fire stops, eliminating condensation risk. If the BTU delivery is now too much at 60% circulation rate at 160F. The control can ratchet back down proportionately, until the high limit is 150F. At this high rate of circulation there is a good chance the return water is going to be quite warm, and the delta could be reduced considerably. This is of course a result of close observation of the system operation at installation and check back later. This would minimize fuel use, and satisfy the DOE adjustment requirement that supply temperatures be adjusted to meet heat load at minimum fuel use.

hot_rod

As long as the boiler output matches the heat emitter delivery rate.

All systems strive for and reach thermal equilibrium, the settings on the boiler only act as safety limits.

If the boiler is set at 160, but the heat emitters can transfer more than the boiler can produce the system may never get to 160.

So the amount you can force the boiler to operate at specific condition, depends on the system design and match up. The ODR controls either on the boiler control, or aftermarket do a good job of managing SWT temperatures.

Read more about this concept in Idronics 32. See if this link works.

https://idronics.caleffi.com/hashtag/interactive-ed

captainco

derheatmeister - Being certified I assume the boilers you are talking about had their drafthoods removed or blocked and barometrics added. Then the mechanical room was pressurized or combustion air was controlled in one direction? Then the gas pressure was adjusted until the actual input of the boilers were as they were listed. Input btus is not cubic feet of gas. Natural gas is 90% sensible heat so non condensing equipment needs an additional 10% cubic feet. Because we don't have perfect combustion the actual amount of btus available is another 10% below that.

Backing up, you said the piping was screwed up, so is that the return water temperatures fault? What return temperatures do pool heaters see? What temperature do water heaters see?

NCI Certification teaches that flue condensation is caused by venting, combustion air problems, underfiring or other mechanical defects, not return water temperature. That is because for over 40 years that has been verified in the field.

Have I seen what you are talking about? Yes!! But not after we fix it.

Derheatmeister

"captainco"
These systems that we encounter were installed by others that did not take your NCI classes.
Thank you for all of your lectures/Classes and making this industry better !

No Drafthood blocking, No european style barometics such as i did dating back to the 80ties and no fan in a can installs !
Yes i am aware of the derated gas Xcel energy supplies us with here in the high country.

As you have maybe seen: Frozen homes or homes that are coming out of a deep setback will also Condense for a Very long time and with no Boiler Return protection these systems will just Corrode and Clogg up HX in no time !

Our Snowmelts and Solar systems are protected by Glycol to approx.-30F !

"exqheat"never mentioned if his system is protected by Glycol which can bring back some seriously cold temperatures to the boiler by Renters that left the Double hung stat open.

Melting snow for liability at approx. 9000+ feet can be very challeging and a inpropper installed Fin type boiler can see return temperatures well below freezing temperatures.

The correct thing to do with these severly clogged up dangerous system was to change them out for a ModCon type boilers what can withstand/Like these conditions and reduce the customers AFC at the same time..

I Never understud why these ModCon Fluid based boilers are not part of your Class ?

Please also keep in mind that our incoming Domestic water temperatur generally can go down to 38 F sometimes even close to ice cubes,But running water does not Freeze...When you have 16 people taking showers at the same time this can condense you water heater ...which even with a blocked drafthood will make a waterheater condense for a very long time !

Frozen homes or homes that are coming out of a deep setback will also Condense for a Very long time and with no Boiler Return protection these systems will just Corrode and Clogg up the HX in no time !

As you said:
"NCI Certification teaches that flue condensation is caused by venting, combustion air problems, underfiring or other mechanical defects, >>>>not return water temperature<<<<. That is because for over 40 years that has been verified in the field.

Have I seen what you are talking about? Yes!! But not after we fix it."


As far as my memory serves me the NCI classes also addresses fan speed adjustments on Furnaces (Furnaces that blow air thru a Duct)
Improper fan speeds can have a impact on the Combustion..So can low return temperatures on boilers.
So why not address return temp protection on boilers..Guess this is covered by "Mechanical defects" ?

BTW..Buderus's claim to fame for the TUV was to dry fire a boiler and then fill it with cold water without Thermal shocking it...

Hope these discussions Are not as some have been saying "Conjecture"...

Again thank you for making this industrie better.