Boiler aquastat and cycling


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Old 12-10-05, 08:11 AM
Turbo
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Boiler aquastat and cycling

Ok so I have a boiler that was set to 180/160. This seemed very wasteful to me (we have an indirect for DHW) so I lowered it to 160/140. The boiler radiates a lot less heat in the room where it's installed now...

So anyway this seems to be working fine, but the way everything is setup it seems very inefficient. For example, boiler is at 130 degrees and we use enough hot water for the indirect circ to engage. Furnace activates and runs to 155 (I have set 25 degree diff on the low). Fine. Circulator runs and boiler temp is down to about 150.

Now, one of the zones in the house calls for heat. Furnace immediately kicks back on after being off for only a minute or 2 ??? It runs for 5 minutes or so, kicks off, and the circ runs for another 5...

Also, sometimes the call for heat is so short that the boiler stays hot. It stays hot until the next call for heat, so the furnace doesn't engage - BUT after 10 minutes the furnace is now cold, and it kicks on for about 20 seconds at which time the call for heat ends!!

This seems extremely wasteful.

So it occurs to me to make the aquastat forget about heat for the house I could set the low and high diff to the same temperature, say 150 degrees. This means that when the temperature drops it will ALWAYS kick on the furnace which will run for 10 minutes or so and should prevent this cycling.

The aquastat (8124A) has a note on it to set the high limit at least 20 degrees above the low limit though. Is setting it to the same temperature going to cause a problem?

Is there a better way to prevent this cycling?

I've seen a pretty dramatic difference in oil usage since lowering the aquastat to 160/140 from 180/160. At 180/160 the furnace would fire for every call for heat and the circulator would stop running at the same time the furnace stopped. While it is better now, it seems that more could be done.

Any input would be appreciated.

Thanks.

- Mike
 
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Old 12-10-05, 06:06 PM
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There is no need to keep the boiler hot for any reason when you don't need heat or hot water. You could turn the low limit down to 100 or as low as it goes. Then set the high at 180 for normal usage and things should work better.

Ken
 
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Old 12-10-05, 06:13 PM
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L8124

Honeywell is pretty emphatic about that 20 spread. You could replace the 8124 with a L8148 which would make the boiler into a "cold start" configuration. I think by snipping one wire on the 8124 it too can be set up for cold start. In a cold start configuration, the boiler would only fire upon a call for heat from somewhere.
 
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Old 12-11-05, 04:34 AM
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Grady is right on both counts. The reason for the 20 degree split is that each part may be inaccurate by as much as 10 degrees and if you set them closer, you can get into a condition where the high limit has the burner turned off and the low limit will not let the circulator run. In a situation like that the two have each other locked out. I don't know why you would want them closer than that anyway. In order to make the aquastat cold-start I think you cut the red wire....or is it the blue wire? Just kidding. I prefer Honeywells 7124 aquastat. It replaces all the others (almost) and you can select it not to maintain water temp at all right on the dial. It has replaceable relays and a replaceable temp sensor and a couple of LEDs. It is not expensive either.

Ken
 
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Old 12-11-05, 08:53 AM
Turbo
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Just to clarify what I'm trying to describe is a way to disable the high-limit (I guess the opposite of a cold start, where it maintains 150 all the time). This causes it to cycle less, only once every 45 minutes. With it set to 140/180 the furnace is running for 3-4 minutes every 20 minutes... I'm wondering now if it fires so often for such a short time because it's vastly oversized or something.

A cold start setup won't work the way it's hooked up - the aquastat never gets a "heating call" from the thermostat on the indirect. When the indirect needs heat it just runs it's own circulator and the aquastat is oblivious. It only heats because it hits the low limit.

- Mike
 
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Old 12-11-05, 12:56 PM
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Mike

I understand what you want to do but trying to hold a fixed temperatrue will cause the burner to short cycle even worse. You could be oversized. If this is an oil fired boiler, maybe you could reduce the firing rate.
 
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Old 12-11-05, 01:56 PM
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That is a sloppy installation of an indirect heater. When you spend a thousand dollars for an upgrade like an indirect, you should get the savings it will create. Not stay trapped in the realm of a boiler that doesn't drop below 150 degrees. You could try a Beckett Heatmanager. Its sole purpose is to lessen the burner run cycles. I think the web site is www.becketthm.com

Ken
 
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Old 12-12-05, 08:49 PM
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Originally Posted by KField
That is a sloppy installation of an indirect heater. When you spend a thousand dollars for an upgrade like an indirect, you should get the savings it will create. Not stay trapped in the realm of a boiler that doesn't drop below 150 degrees. You could try a Beckett Heatmanager. Its sole purpose is to lessen the burner run cycles. I think the web site is www.becketthm.com
That's what I was afraid of... After specifically asking the guy who came to service it if that was really the best way for it to work. All he said it was "a really nice setup"

I did ask them about changing the nozzle size. Am I correct in thinking that a smaller nozzle is what I need to lower the firing rate? I just got it replaced with the same size it was when that was what the guy recommended...

The heat manager sounds good. Now all I need to do is find somebody competant to install it!!!

Thanks.

- Mike
 
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Old 12-13-05, 04:32 AM
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If you look at the specs on the indirect, it will tell you how many btus it takes to provide the output it is rated at. Approximately 100,000btus per hour per gallon of oil input to burner should give you a starting point to compare what you have to what you want. There is no problem with running the boiler below that rating if you don't run out of hot water. For a 40 gallon indirect, you can probably drop a nozzle size or two below the rated one and not impact your hot water delivery too adversly. You may have to experiment but I wouldn't just stay at max rate to be safe. You could probably install the heat manager yourself with a little guidance from this forum. I have a few tips to offer and if you have a fair understanding (which it seems you do) of your aquastat, you could do it in less than an hour. If you could shoot a few photos or give us some model numbers of boiler and indirect, there may even be more bright ideas available.

Ken
 
 

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