Short cycling because I reduced heatloss through new windows and insulation any ideas to solve probl

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Andersonmill

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Hello,
I am new to Forum. I have a Buderus G200-4 with a Riello gas burner. (Not a variable or two stage output) I have a Honeywell Aquastat with no lo limit. High limit set at 180 degrees. No outdoor restet or computer. System is 8 years old. 179,000 btu net output. 200,00 max. Burner diaphragm set to 200,000 btu. 50 gallon indirect water heater. Forced hotwater. Both cast iron and baseboard radiators.

I did a new heat loss after the remodel. We are now at 80,000. We were at 150,000 before remodel.Antique house built 1850. We live in Boston
I am trying to find the right balance of efficiency and keeping my house at a steady temperature. Should I just change the diaphram in the burner to match the new heat loss calculation??
Many thanks:)
 

Dana

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How short are the burner on-times, and how many burns per hour? How many zones, and how much/type of radiation is there on the smallest zone (smallest in terms of baseboard length, not floor area.)

At "only" 2x oversizing you should be close to hitting the efficiency numbers, but not if it's 10 burns/hr at 1-2 minutes per burn during the shoulder seasons. If that's what is happening you may be able to tame the beast with retrofit heat purging boiler controls (eg Intellicon 3250 ), which suppresses the burner during the begnning of a call for heat until the boiler temp hits the programmed low-limit, and "learns" the system, cutting the burner before the call for heat, anticipating that end based on recent burn history. Cutting the burner early and finishing the call for heat by purging the boiler's heat into the zone lowers the boiler's standby temp, which also reduces idling losses. These economizers maximally exercise the thermal mass in the system, reducing the number of burns, as well as the total burn time. (There are other economizers with similar functions from other vendors as well, the Intellicon is one of the simplest.) The net result is that the average water temp rises in falls with heat load, but the water temp varies by quite a bit during a call for heat.

Outdoor reset without heat purge control raises & lowers the output temperature, which may increase the numbers of burns, since it then operates in a narrower temperature band between high limit & low.

Derating the burner on a cast iron boiler isn't something to be done lightly, since the heat exchangers are designed to work well only within a range of BTU intput. A 20% cut is often do-able, but a 50% cut is likely to result in lower efficiency or even lower the lifespan of the boiler.
 

Andersonmill

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Hello Dana,
Thank you for taking the time for a well done reply.
I have two zones. The small zone is about 40' of base board about 20,000 btu's. The large Zone is 7 large (20 sections at 4' tall)cast iron radiators. Each zone is driven by a taco pump. Some of the older pipes are large old steam pipes being used for the forced water(3").
 

Andersonmill

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Dana,
This Intellicon. Does it not do the same thing that is happening now. Once my burner hits the high limit ( i have no lo limit), The burner shuts but the circ pumps keep pushing water until the aquastat drops 10 degrees then turns on again.
Also, wont a longer period of burner off and circ on reduce the water temp even more causing a cold shock. Also, the water has to be heated up from a lower temperature?
 

Dana

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The Intellicon allows you to program a low limit rather than operating within a narrow 10F band the way it does with your single-aquastat control. If you set the high limit to 200F and program the Intellicon to purge until the boiler temp hits 130F, you now have a potential hysteresis band of 70F instead of 10F. Read the installation & operation manual in that link- it's a pretty straighforward, do-able as a DIY by folks with some electrician skills, but you have to understand the control schematic for your boiler (which may or may not be available online- check your boiler's manual.)

What model Honeywell is on the boiler?

A boiler low-limit of 130F safe for a gas-burner, but you'd want to hold the line at 140F for an oil burner, maybe 135F for propane, due to the different exhaust condensation temperatures of the different fuels.
 

Andersonmill

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Dana,
I will check which aquastat I have. When I designed and installed the system, I remember using a hi temp shut off only.No need for a lo limit.
Today I set the differential on my aquastat to a 25 degree differential so I would obtain the purge results you are telling me about. I now have a 180 degree high limit ( which is reached in 10 minutes) then the circ pumps run for another 15 minutes. At that time there is no longer a call for heat but sometimes not.

I always thought that ideally, the few heat up and cool downs the better for the boiler life.
 

Dana

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Dana,
I will check which aquastat I have. When I designed and installed the system, I remember using a hi temp shut off only.No need for a lo limit.
Today I set the differential on my aquastat to a 25 degree differential so I would obtain the purge results you are telling me about. I now have a 180 degree high limit ( which is reached in 10 minutes) then the circ pumps run for another 15 minutes. At that time there is no longer a call for heat but sometimes not.

I always thought that ideally, the few heat up and cool downs the better for the boiler life.

The longevity of the boiler is not adversely affected by allowing it to cool to just above the temperature at which condensing occurs on the fire side of the heat exchangers. If allowed to cool to 10s of degrees below the condensing point on every burn cycle there will be corrosion issues. It's OK to cold fire the boiler occasionally, but during continuous operation it needs to stay out of the condensing range.

Cycling it with a temperature differential of even 50F from low-limit to hi-limit does not mechanically stress it much. But having it cycle on/off a gazillion times wears out the ignition system & gas valve components, and wastes fuel during those ignition cycles.

It typically takes at least 10 seconds from the time the gas valve opens to the time the thing approaches it 's steady state combustion efficiency, during which time it's efficiency is pretty abysmal. If the burn cycle is only 100 seconds long, it becomes a fairy substantial hit in average efficiency, but if every burn cycle is 10 minutes, it's negligible. If you're getting 10 minute burns out of it, you should have an as-operated AFUE fairly close to it's steady-state combustion efficiency. As a rule of thumb (not super-accurate), a short-cycling boiler will end up averaging 10-15% below it's steady-state combustion efficiency.

The economizers do it one better by allowing even bigger temperature differentials, and by cutting the burner in anticipation of the end of a call for heat, so the boiler is never parked near the high-limit between burn cycles. The standby loss when the boiler is at 150F is substantially lower than if it were left at 180F at the end of a call for heat. And during the shoulder seasons when the water temperature requirements are even lower, the average temp at the boiler will be somewhat lower, since it may never has to reach 180F to satisfy the thermostat during light loads, and will purge even closer to the programmed low-limit at the end of the cycles.
 
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