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Furnace cycles on when heat is turned off???

Hi -- I'm hoping someone can answer a question that I haven;t been able to find an answer to. I have an older oil-fired furnace,with a tankless hot water system. The furnace will "cycle on" every so often, and run for anywhere from 5-10 minutes. The thing is, it does this year-round, even when the heat is off, and no hot water is being used (i.e., no sink faucet turned to hot, no showers running). And because I don't have a hot water tank, it's not like it needs to run to keep the temp up for that..... The only time I can get it to not run at all is if I turn off the emergency boiler switch to cut the power. Obviously, an old furnace + cost of oil = me not wanting the furnace running all the time. Any ideas would be appreciated - thank you!

Re: Furnace cycles on when heat is turned off???


This is a very common complaint for homeowners who own a hot water boiler---if you have radiators or baseboard for heat you have an oil-fired BOILER with what's known as a tankless domestic hot water coil (DHW); the beauty part of an oil-fired boiler is that it can produce both hot water for heating the house radiators/baseboard, and at the same time it can produce a small amount of clean, DHW for showers, dishwashing, cleaning, etc.; the DHW is produced inside the boiler by installing a TANKLESS COIL inside the boiler which is a separate coiled 3/4" or 1/2" copper tube about 10' in length that holds approx. one or two gallons of DHW & is kept hot by being surrounded inside the boiler by the hot boiler water; the DOWNSIDE of the TANKLESS COIL is that once the 1 gallon or so of DHW is used up by whomever gets to use the shower first, the hot water in the little coil is replaced by cold water & the boiler must come on to heat up the gallon of cold water every time this happens.

This is the traditional way that has always been used by boilers going back at least 50 years for combining hot water space heating with the need for DHW, and is still used today in some HW boilers as well as more modern "combi" boilers; this also hails back to bygone days when Americans used much less DHW than they do today; most families (especially if there are over 2 or 3 people in the family) insist on unlimited DHW for daily showers, an automatic dishwasher, clothes washer, jacuzzi, etc.---these items require considerably more gallons of DHW than a tankless coil could ever deliver.

Consult the site below to determine how many gallons of hot water you need during the day---especially during the PEAK HW HOURLY USAGE--that hour during the day (usually the morning rush hour) when the most HW gallons are used--this is the time when most families have the problem of running out of HW---quite often a separate HW tank sitting next to the boiler is the only way to solve this (see ALTERNATIVES below).

The tankless HW coil you have now is really designed these days for perhaps a family of ONE, or perhaps an elderly family of TWO.

On the other hand, you have to realize that the FIRING RATE of most boilers is on the order of burning 1 gallon of oil per hour, or even 3/4 gallon per hour---therefore, if you can hear the boiler come on for 10 minutes to heat the 1 or 2 gallons of DHW in the tankless coil, you're only burning approx 1 PINT of fuel oil; so it wouldn't get to burn 1 GALLON of oil until it went on & off 4-8 times during the day (depending on climate, boiler efficiency, etc.) in mild weather; during the winter, when the boiler water is hot anyway (because the house radiators are being heated) the use of oil for DHW is even less; on the other hand, older boilers tend to have approx 15 gal. of water that has to be heated up each time, as opposed to newer boilers.

ALTERNATIVES: If you have more than 1 or 2 people in your family and you're constantly running out of DHW for showers, cleaning, dishwashing, etc., consider having a heating contractor come over the house to quote you a price on an INDIRECT HOT WATER HEATER; this is a well-insulated, separate 30 gal or 40 gal heat exchanger tank that sits next to the boiler as a separate zone; this is in effect a large heat exchanger like the little coil you have in the boiler now, but it is much more fuel-efficient because it is highly insulated----you'll never run out of hot water with an INDIRECT HOT WATER HEATER (IHWH), and since they obtain their heat from the boiler water, there is no separate burner or chimney flue needed---the DOWNSIDE is that they don't come cheap---the unit itself costs between $700 to $1500, plus installation costs (approx $300); you would have to decide if the present control inside the boiler that controls the tankless coil (the Aquastat) should also be changed so that it doesn't fire every time the boiler water gets below a certain temperature; this would add another approx $300 to the update for a new Aquastat.

Click onto the Pex Supply site below to learn more about indirect HW heaters, then click onto "indirect water heater" (in red letters) to view the various mfgrs of these units; then click onto each unit to get an approximate suggested price for each unit; Triangle Tube (Google "Triangle Tube Indirect HW heater") is considered top of the line, but is pricey; the Amtrol unit costs approx $700 plus installation, & I've had good luck with them; ANY indirect HW heater has no moving parts to wear out & usually only has to be drained of a gallon or 2 of dirty water each year as the only service need----they last for decades just sitting there next to the boiler, quietly doing their job.

A less-expensive alternative is to call your boiler service technician to have them de-rate the oil burner nozzle if it is now larger than 3/4 gallons per hour---the oil burner nozzle (a $3 item) is replaced with a smaller orifice nozzle that uses less oil per hour-----this would save you some $$$, especially during warm weather, but is only really a stop-gap measure if you have a family of more than 1 or 2.

I DO NOT recommend you go out & buy a separate gas-fired or electric-fired hot water heater as an attempt to resolve this problem---many people who own an oil-fired boiler go out & do this and they end up spending more than ever on heating bills, plus the installation of an additional flue (chimney) for the new unit.


Re: Furnace cycles on when heat is turned off???

Pelton, thank you for that explanation. Unfortunately, I'm all-too familiar with the downside of a tankless system, as this is my third house with it (I'm not a fan). A follow-up question: if there has been no hot water use, and the thermostat has been off, why would the furnace cycle on? Presumably there's been no change in the water level.... (please forgive my ignorance if I'm not "getting" this)

Re: Furnace cycles on when heat is turned off???

While there is no traditional tank, there is the pipe that runs through the heater which has to be kept up to temperature. Your older furnace isn't as 'smart' as modern tankless HWH's To not have this happen, you'd have to electrically engineer your older system to duplicate a tankless HWH while operating as a home heater.

For the really OCD;

1- Time how much the heater cycles to keep itself warm
2- Calculate how much oil is burned. You can pretty much ignore the shortened life of the heater as apparently this beast will not die.
3- Compare the annual expense of running the old heater compared to buying a new one.
4- See that you're way ahead of the game keeping the old one (see Pelton's well written answer)
5- Learn to smile every time the heater cycles on, as it has become the sound of you saving money and not that of buying a new expensive system.

Re: Furnace cycles on when heat is turned off???

That's funny-- thank you! (Though my smile is a bit smaller when it goes through a tank of oil in about five weeks-- with the heat set at 60.... ;-) )

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