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Warm-start versus cold-start

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Warm-start versus cold-start

By George Lanthier

RECENTLY ON MY WEB SITE DISCUSSION BOARD AT www.FiredragonEnt.com, we started talking about the operating benefits or lack of them regarding a principle called ‘cold-start.’ Now although most of us know that refers to system operating temperatures, I’m going to define the term and its opposites for the sake of the newbies and greenies out there. After that, I’m going to tell you why I don’t like it and throw a curveball on another cold-start problem.

Heat-only unit: This is any unit that just runs for heat, no domestic hot water of any kind, such as a furnace.

Cold-start unit: This is any heating appliance that does not maintain temperature, but does produce domestic hot water. The concept is most successful with an oversized indirect, proper piping and control applications and ideally with a low-mass, low-water content boiler. The circulator and piping must also be properly sized and pumping toward the coil.

Hot-start unit: This is any heating appliance that maintains a high temperature (160°F to 210°F) to produce domestic hot water. The concept is most commonly used with undersized, poorly applied internal tankless heaters.

Warm-start unit: This is any heating appliance that maintains some temperature (100°F to 120°F) and produces domestic hot water. The concept allows the boiler to remain cleaner throughout the operating year and actually accelerates recovery during extended periods of domestic hot water draw. It also reduces maintenance costs for a minimal amount of energy used.

After a recent move, I was asked if I still believed in the concept of warm-start over cold-start while buying my oil at $3.35 a gallon. My answer? ‘Yup, I believe it should be done across the board. My former residence was a horizontal multi-pass, this one is a pin-type. I think they all need it, JMO!” Why did I say that, simple because it makes sense and I’ve seen the effects of both, here’s the rest of my posting:

‘Here’s a shocker though. I don’t believe that 350°F Gross is a hot enough operating temperature anymore. With what I’m seeing on CO lawsuits and malfunctions across the board, 350°F doesn’t seem to be cutting it.At 350°F in the breech you ain’t getting **** at the top of a 35 foot oversized chimney. TRUE, liners would stop all the BS, FACT, but the industry has been too slow to go and the problems are increasing daily. Back in 1981 with the introduction of low-mass, horizontal,multipass boilers the Euros told me that 350°F Gross – 285°F Net was minimum to operate. These were the same people who convinced me of cold-start, another principle I no longer believe in. But, unlike cold-start they were basing the info on standards that are not accepted here, properly relined (retroffited) chimneys.

‘Cold-start was based on inspection of the unit twice a year by government officials with immediate service, if required. Homeowners here get serviced much less frequently and the quality of the service work is deteriorating quickly, too quickly, JMO! We need to seriously look at our outdated policies in regards to venting, chimney design, retrofit and operating temperatures. We have now not only killed people with oilheat, we are making a lot of people sick. TRUE, faulty installations, carlessness and lousy service procedures heaped on top of a general lack of education is mostly responsible, but when will this BS stop?”

Although that answer is a bit long and opened the door to another subject, it was my way of also showing that things aren’t always as they appear and that no matter what, time and technology move on.

I was once a great believer in cold-start. I put it in my home, I taught it like a religion, and I truly thought that it was the way to go.What the heck, an oilburner is 100 percent efficient only in two modes; it burns 100 percent of the fuel going through it when on and doesn’t burn any when it’s off. The problem with cold-start is that it was based on only using it with an indirect, preferably oversized, and with a low-mass, low water content boiler. It also depends on the burner being set for true zero smoke and the boiler being properly cleaned once a year or more.

I’m a lazy tech, but in my opinion that also makes me a good one. I don’t work hard, I work easy.When I saw the debris building up in my boiler, I knew something was wrong. I don’t like to clean my boiler, but when I do, I scrub it right down to the bare metal with a brush and water. Then, it dawned on me. What do most guys like to clean, a hot boiler or a cold one? Which one is easier to clean? That’s also a no-brainer and the answer to the question’is cold-start worth it? Well, if you follow all of the rules, yes, but since I try to go three or four years between tune-ups, it just doesn’t cut it for me.

Then there’s the production of domestic hot water and the weak spot, recovery. We tear the subject of domestic hot water (DHW) apart in our book The Hot Water Handbook, but here’s the skinny facts.

To make and supply adequate DHW you need one of two things or both: recovery and storage. One of the biggest problems you can create is an undersized indirect with an oversized (water quantity) boiler. Are you really buying this? Can’t you believe that if you have a coil that holds a half gallon of water and a boiler that holds 14 gallons that you must turn over the 60 degree water into the hot indirect 28 times before you start recovering temperature? You’re pumping cold water into a warm vessel and that just doesn’t work. The Europeans who came up with this figured the same coil, but with a boiler that holds three gallons of water, that’s a 6 to 1 turnover as opposed to the 28 to 1 that many try to do.With wall-hung, modulating- condensing (mod-con) gas boilers that hold a couple of quarts of water, cold-start will generally work, but they don’t make them for oil and we may never see one in this country in our lifetime for oil. It’s important to note right now how these boilers are piped too.

Then we have to look at temperature.Not just the temperature of the water in the boiler and water heater, but what’s happening during start-up to the flue gases. Again, I think cold-start has ruined lots of jobs. If the boiler is warm throughout the year, not only is the production of DHW faster, but the flue gases are in better shape. With cold-start you have a tough ignition sequence since there is no latent heat to help vaporize the oil and assist with igniting the atomized oil. Most of today’s boilers have no lined combustion chambers, and so you’re trying to fire into this cold black hole.

Venting is tough except maybe in the dead of winter because the chimney cools during cycles and there is little to no thermal draft present on start-up.Tight combustion heads have helped with offcycle losses over older oilburners, but they also do their job and reduce the flow of air through the chimney causing the oilburner to have to reheat the chimney every cycle increasing condensation. In the warmer weather, cold-start boilers can wreak havoc with service again due to draft problems.

Cold-start, high efficiency boilers, in my opinion, also lead to a lot of damage being done to chimneys and masonry chimney liners that is called spalling. Spalling of a chimney allows deposits to build up and block the flue opening and that can lead to carbon monoxide (CO) backing up into the home.In addition, low temperatures in the flue may never properly heat up and then condense on the top of the chimney, causing the condensation of flue gases and the resultant freezing of the condensation.Until last year I knew of these theories, but had no proof.

Due to some very quick action on the part of a fire department who took pictures of a chimney after a CO incident, I now know these theories are absolutely true and have the pictures to prove it. The pictures show quite clearly ice build-up and spalling of the masonry chimney liner. The photos were taken as the occupants were rushed to a hospital and later placed into a hyperbaric chamber.These occupants did not die, but in New Hampshire in 2007 three people died from a similar event.

I have begged this industry to investigate metal liners to no avail for over 20 years. If you don’t want to do them, fine, as they say, see you in court. But if you don’t want to use liners, then make sure the chimney is clear and in good condition and that the chimney base is open as far down as you can go. Keep in mind that a chimney inspection is probably in your service contract and that an NFPA Level Two Inspection is required in every oil heat state either by code or the manufacturer’s instructions with a new install.

Finally, on the subject of flue gases I am beginning to believe that our industry standard of maintaining a minimum of 350°F Gross and 285°F Net stack temperature is not adequate for most chimneys in the United States and Canada. These chimneys are for the most part too tall, too big and don’t warm up quick enough. In addition, the 350°F Gross is normally with a maximum of 10 feet of fluepipe. In many cases flue pipes are longer and just add to the cooling process along with improperly installed and simply not needed draft regulators. I am now recommending that we maintain a minimum of 400°F Gross at the breeching and that we run a slightly higher temperature to prevent problems and enhance operation. If you line the chimney with stainless, fine; 350°F just may do it, but if the chimney is masonry and masonry lined, I would go to 400°F.

In conclusion let me offer another option for those who are determined to do cold-start. The solution, in my mind, is piping. If I could be assured that beginning tomorrow everyone would only pipe boilers with a by-pass pipe or do them in a primary-secondary configuration, I would have to shut up and go away on this subject. The bad news is it just isn’t going to happen. In my opinion that’s why cold-start works with the mod-con gas boilers, they are only supposed to be installed with primary-secondary piping. Interesting, huh?

Many of us do things because we were taught to do them the way the guy who taught us did them. In most cases he was taught by a guy in 1938 who was taught to do it following the anthem ‘but we always did it that way.” Technology has progressed a long way, and yet many in this industry think that our outdated traditions make more sense than following the manufacturer’s recommendations. That won’t hold up in court because it just doesn’t make sense.


  1. Good day, Thanks for the food for thought. Can I bother you to elaborate on your Statement:

    The solution, in my mind, is piping. If I could be assured that beginning tomorrow everyone would only pipe boilers with a by-pass pipe or do them in a primary-secondary configuration.

    I am thinking of installing a cast iron cold start boiler in Halifax Nova Scotia, I currently have a holding/warming tank for DHW and a two story clay lined masonry chimney.

  2. Speaking of doing things how they were done in 1938, why use a chimney at all? Side venting with most modern retention head burners don’t need assisted draft vents or barometric dampers. Stack temperatures can be “near condensing” (200 F) and eliminated with very little water or ice forming before exiting. Care and good planning need to be done in order to avoid undesirable exhaust and the prevailing wiind. Up to 15% of our energy goes towards keeping the stack warm or hot, to the degree you suggest. The tune of the burner is much more stable and hardly has reason to create soot and will hold it’s tune for several years (residential).

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