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Old 01-30-11, 03:56 PM   #1
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Default Boiler Short Cycling

I have a question about my boiler. It's an 80yr old fuel oil powered boiler. We live in a 110 yr old house that has cast iron radiators. Every room contains a stand up radiator. We have the temperature set for 68 degrees when we get up in the morning, 62 during the day , 68 at dinner time and then finally 62 over night again. I've noticed that when the thermostat temp changes from 62->68 the boiler burns full blast until it comes up to the set water temp. Then say every 10 minutes it'll come on full blast for 1 minute and then shut down again. Anyone know why it's doing this? My guess is the thermostat is calling for more heat and causing it but the boiler can't go above the set water temperature ( aquastat i think it's called? ) Is there a way to stop this? My guess is that it's inefficient. If I can't fix it it's not a huge worry, I plan on installing an air source heat pump with my tax return this year

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Old 01-31-11, 12:36 AM   #2
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I hate short cycle burns.. I got a little box to install on top of the Aquastat, and it helped a lot.
I very rarely get a short cycle anymore.

Residential Boiler Controls | Beckett HeatManager

It's the gadget on top.


Nowadays, this boiler is my back-up system and I've started using
the free-flow valve and the timer to keep hot water circulating on these really cold nights.
The circulator pumps don't come on at all..
We have baseboard and convection does the job.

The timer is kinda crude, but it works okay for my needs..
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Old 01-31-11, 02:09 AM   #3
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It appears that boilers are prone to short cycling After moving into my wife's house I noticed that the gas boiler was cycling on and off 2-3 times per minute! The water temperature was set to minimum and it appears that the heat radiation from the flame just after start-up was enough to trigger the aquastat. Setting the water temp a bit above minimum solved the problem. Adding a thermostat in the house also helped.

Cholcombe, just to make sure I understand your setup: You have an aquastat that governs the water temperature and a thermostat somewhere in the house that turns the boiler on/off depending on room temperature? This is how it should be and the best thing to do is to set your aquastat higher. This will make the radiators hotter, but the thermostat will make sure the house doesn't overheat. Also, maybe you can increase the tolerance on the thermostat, say from 1F to 2F?

Three more things to try (if possible):
  • Make sure you don't have air pockets trapped in the radiators, or elsewhere in the system (parts of the system, like one end of a radiator, would be colder than others),
  • Clean out the inline water filters,
  • Speed up/slow down the water circulation pump.

New boilers and furnaces reduce the flame size as the return temperature approaches the set limit, but I understand that your old burner doesn't have that capability. I would advise you to buy a new boiler, maybe slightly smaller, since that would be oh-so-much-more efficient, or ask a technicial if your boiler can be retrofitted to be more efficient, but since it is to be replaced shortly then there's no sense in spending $$$.
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Old 01-31-11, 09:13 AM   #4
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Modern aquastats have two temperature settings--a high and a low--which lets the boiler reach its maximum temperature and then coast down to the low setting, thus preventing rapid cycling. I have an old boiler like you which only has the high setting, so it runs until the house gets up to temp and then cycles as the thermostat calls for heat.
Piowslaw's idea of adjusting the tolerance on the thermostat is a good one and would have a similar effect to having a "low" setting on your aquastat.

You guys have had some cold temperatures lately, so your house is losing heat more quickly than normal. Maybe you should get the snow plows to push snow around your house to turn it into an igloo . But seriously, if you can stand it you can turn down the thermostat. A lower delta T between inside and outside will mean less heat loss and fewer cycles. Barring that, it comes back to insulation and weatherization. Your money will be much better spent there than trying to mess with an 80 year old boiler. A higher R value will pay for itself no matter the heating system you use.
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Old 01-31-11, 09:42 AM   #5
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Thanks for the ideas guys! I'll try to adjust the delta on my thermostat if possible. I'm sure that will help. I also looked into the heat manager that XRinger recommended. I really like that idea. It looks like an arduino, temp sensor and a relay. I've built a project very similar to that before. I think my aquastat is one of the super old kinds that only has a high temperature limit. I might look into replacing that with a newer one if possible also. This has produced a lot of good ideas!
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Old 01-31-11, 10:55 AM   #6
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I was looking at replacing the old AquaStat with a new model..
One of these maybe "L7224A,C; L7248A,C,L Oil Electronic Aquastat Controllers"
but the price was high and the install looks difficult. (Since I'm not a boilerman).

I think the Heat Manager will extend the life of my old 1980 system.. It's not so hard on it.
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Old 01-31-11, 11:26 AM   #7
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Yeah I really l like that idea. I'm going to ask my oil company how much it will be to install it. If it's too expensive I'm fairly sure I can do it myself. How hard could wiring up some stuff to a relay be right?
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Old 01-31-11, 11:59 AM   #8
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If you are talking about the heat manager, you should be able to download the install manual and check it out.
I'm a tech type and didn't have any problems installing. It's pretty easy to un-install also.

My problem with the Aquastat is installing the probe.. I'm not sure I want to do it..
Unless I have to..

There are some brand new (non-Beckett) versions of the Heat Manager. Some have a digital display. They might be an improved version.
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Old 01-31-11, 12:59 PM   #9
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hmm... ok. Well I'm a linux administrator. I'm not afraid of getting my hands dirty with electronics. I should check out the manual. From their learning module online it looked like just splicing a few wires and attaching the temp prob to a pipe close to the boiler. They also insulated around it with pipe insulation and zip tied it. Probably to keep the outside air from throwing the readings off.
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Old 01-31-11, 06:00 PM   #10
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HI cholcombe

The short cycling is usually caused by the heat anticiapitor inside the thermostat. It adds a small bit of heat to keep the thermostat from overshooting ,but it cools quickly ~< 1 to 2 minutes,causing a call for heat(short cycling). They are adjustable, a millamp setting ~.1 to 1.5 milliamps is usually found. A reading is taken of the thermostat circuit and that is used as a baseline/first setting. They also usually have the words longer/shorter marked , moving it one or two marks at a time should stop the short cycling.(toward longer) Old thermostats also can do this do to a accumulation of dust on the bulb or because the bimetal strip has gone through so many on off cycles it becomes "sprung".

Also I'm assuming this is a mercury bulb thermometer,it also has to be level if you had it off the wall for remodeling.

It's possible it's a aquastat by my money would be on the T-stat.

Digital based thermostats use algorithms to anticapate heat , and the better models can actually "Learn" not to short cycle or over shoot.


Last edited by wdrzal; 01-31-11 at 06:16 PM..
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