What is wrong with this aquastat?


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Old 12-06-08, 11:20 AM
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What is wrong with this aquastat?

I have two long running problems with my aquastat. For a bit of a primer, I live in a mid-size 2 bedroom apartment on a 4th floor. The apt is heated with one air handler/heat exchanger, whose heat is supplied by water from a boiler all the way down in the basement. The aquastat is attached to the incoming hot water pipe in the attic adjacent to the air handler.

My main gripe is that the air handler power cycles continually, and stays on for an average of 5 seconds or so. Occasionally, it will stay on for a max of a few minutes in much colder weather. But, generally with temps around freezing, I'm seeing 5-10 second ON times. Is this crazy or what?!

Now, for the two possible underlying causes for this. The first observation is the fact that the aquastat, for whatever reason, is resetting itself. Every few weeks, I find that the aquastat's setting rises from 140 to about 180. I then reset the temp back down to 140 in order to maximize efficiency. However, it seems to climb back up after a period of time to the point where the power cycling becomes even more pronounced, i.e. shorter durations.

The second observation is that there is no "differential" adjuster, so far that I can see at least (perhaps the aquastat needs to be opened?).

Last year, my landlord's HVAC man stopped by to "tighten up" the aquastat to the pipe, which did in fact slow the upward migration of the temp setting. He set it to 140, and stopped by the following week to check it. Sure enough, it was still at 140 a week later, so for whatever reason, he doesn't really buy my story of the aquastat changing its temperature. Furthermore, he explains that all this power cycling is perfectly normal, and that even if the temp were to rise to 180, nothing, or VERY little, is lost.

Questions: A.) Is this temp movement of the aquastat normal, and is a setting of 180 not any less efficient than 120-140?

B.) Is this annoying continuous power cycling (ON times of 5 seconds) normal??

Thankyou!!
 
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Old 12-06-08, 04:08 PM
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Where is the thermostat located in your apartment in relation to the hot air vent? Tell me there isn't hot air blowing at the thermostat.

After the blower kicks ON for 5-10 seconds, how long is it OFF afterward ? In other words, that 5 second blast of heat is enough to satisfy the heat call?

It sounds to me as though there isn't enough flow in the system to keep the coil above the setpoint of the aquastat. Your t'stat calls for heat, boiler fires, water starts circulating... gets up to 140, blower kicks on and immediately cools the water below 140, and the cycle repeats until the thermostat eventually satisfies. First thing I would look for is an airbound coil in the air handler. You get _some_ flow, but not enough to keep the coil above the blower setpoint. Has anyone tried bleeding the air out of it?

The aquastat on the air handler coil only controls the blower, doesn't do anything with the firing of the boiler. It is supposed to disable the blower if the water temp in the coil falls below it's setpoint. This prevents it from blowing cold air. 140 is probably the right setting. If it rises to 180 all by itself, there are ghosts in the attic. If it's set at 180, the blower won't come on until the water is at 180 ...

Disclaimer: As a tenant, you shouldn't be touching anything on the heating system. It's the landlords equipment, and his responsibility.
 
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Old 12-06-08, 05:26 PM
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Originally Posted by NJ Trooper
Where is the thermostat located in your apartment in relation to the hot air vent? Tell me there isn't hot air blowing at the thermostat.
First, thanks for the reply! The thermostat is located sufficiently far from one of the vents to be affected by any hot air.

Originally Posted by NJ Trooper
After the blower kicks ON for 5-10 seconds, how long is it OFF afterward ? In other words, that 5 second blast of heat is enough to satisfy the heat call?
Well, last night for instance, the blower remained ON for about 5 seconds then turned back on 10-15 seconds after it shut off. It did this repeatedly for about 15-20 minutes, after which, the blower remained off for a longer duration.

Originally Posted by NJ Trooper
It sounds to me as though there isn't enough flow in the system to keep the coil above the setpoint of the aquastat. Your t'stat calls for heat, boiler fires, water starts circulating... gets up to 140, blower kicks on and immediately cools the water below 140, and the cycle repeats until the thermostat eventually satisfies. First thing I would look for is an airbound coil in the air handler. You get _some_ flow, but not enough to keep the coil above the blower setpoint. Has anyone tried bleeding the air out of it?
I'm not sure where I would be able to find an airbound coil. It almost sounds as if the cover would need to removed to do that? The air has not been bled to my knowledge. Is this something one needs to do on a regular basis? Does this lead to increased efficiency? While up in the attic, I do hear the sound of trickling water through the incoming and outgoing pipes, so I can assume there is air in there as well.

Originally Posted by NJ Trooper
The aquastat on the air handler coil only controls the blower, doesn't do anything with the firing of the boiler. It is supposed to disable the blower if the water temp in the coil falls below it's setpoint. This prevents it from blowing cold air. 140 is probably the right setting. If it rises to 180 all by itself, there are ghosts in the attic. If it's set at 180, the blower won't come on until the water is at 180.
Only in the last hour did my landlord's HVAC man stop by again to "reassure" me everything was working as it should. He told me that in order to lengthen the power cycles, I should lower the aquastat to 110, which I have just done now. It seems to have reduced the power cycling somewhat, but it is still present. He also said that he's never heard of the aquastat resetting itself to higher temps and gave the suggestion to tape the dial in place to prevent that from happening. So, we'll see what happens.

I also brought up the fact that I've seen the suggestion in various places on the net to increase the "differential" point in order to reduce the frequent power cycles in order to lengthen the ON duration. He confirmed that my aquastat does NOT have a differential and is simply an on/off switch. My instinct tells me that such an aquastat with a differential would be beneficial in my case and potentially an energy saver in terms of electricity usage by lowering the amount of air handler power cycles (not necessarily to save on heating costs) - on at 140 and off at 125 or so. He said that such aquastats are only useful for situations where the aquastat itself controls the firing of the boiler, and NOT the thermostat, as is the case in my apartment. However, this contradicts things I've seen on the net where such dual control aquastats (temp + differential) were setup to indeed control only the air handler.

In any case, the lowering of the temp may partially solve the annoying constant on and offs. But, at such low temps, I will most likely have to deal with relatively cool air blowing and potentially longer, more energy-consuming ON durations, right?
 
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Old 12-06-08, 05:43 PM
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If you can hear water 'trickling' in and out on the pipes, then I think the pipes are full of air ... if none of the techs have considered this, then they don't know WTH they are doing, IMHO.

Those 'coils' that I spoke of, I mean the 'fan coil' ... your air handler has a big ole 'car radiator' in it that the fan blows air across. That 'radiator' is what I was calling the 'coil' ... There's air trapped in that thing, $$$-->donuts.

There IS a differential on that aquastat, probably a fixed 10°. The contacts on the control MAKE ON RISE to setpoint, and BREAK ON FALL to setpoint MINUS fixed differential... that's what's typically used anyway.

The tech IS correct about there being no need for an adjustable differential on that control. It's not an 'operating' control. It's just a switch that turns the blower on when the water temp is above setpoint. And off again when it cools.

The way it's SUPPOSED to work is this...

T'stat calls for heat.

Boiler fires and starts circulating hot water to air handler coil.

Blower aquastat senses temp above setpoint, and switches on blower.

Blower CONTINUES TO RUN FOR THE DURATION OF THE HEAT CALL, because the boiler is pumping sufficient hot water to the air handler to maintain that water ABOVE the setpoint of the aquastat.

Thermostat satisfies, boiler shuts down, air handler cools, blower shuts down.

System goes to sleep until next heat call.

What's NOT happening in your system is that there isn't enough hot water flowing through your air handler coil in order to maintain the temperature above 140°. With a plug of air stuck in the coil, enough water can't flow to keep it hot once the blower runs.

Tell them to find the bleeders and get the air out and that blower will stop cycling.
 
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Old 12-06-08, 05:58 PM
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Ok thanks. I'll get them to bleed it. That makes a lot sense. However, what doesn't compute, is that if there really is a built-in set non-adjustable 10 degree differential, how could it be that the temp of the incoming hot water pipe could drop in temp by a full 10 degrees in that short period of time (5-10 seconds), even if there is air trapped in the coil? It seems to me that the aquastat is faulty if that is happening. If it is indeed possible for the water pipe temp to drop that quickly, then perhaps the problem lies solely in the trapped air.

Listening to the air cycling as I type this, the blower is stays on for about 20-30 seconds now with off times of about a minute or so. This is more acceptable, but still seems a bit too frequent. Maybe it's the nature of the beast.

I'll report back as to what effect the trapped air removal has.
 
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Old 12-06-08, 06:09 PM
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No, it's not the nature of the beast. That blower should turn on and remain on for the duration of the heat call. Then shut off until the next one.

I would expect a lower setting of 110 to reduce the cycling somewhat, but if it cycles at all, it ain't right... there should be enough hot water pumping through that coil to keep it hot with the blower running. If it ain't trapped air, then the circulator pump isn't up to the task, or the boiler isn't big enough ... but I'm betting on the air.

Think about how much air is moving past that fan coil... sure it's gonna cool off fast... especially with just a piddle of heated water flowing through it.

Bottom line here is that the problem is most likely NOT with the aquastat 'causing' the cycling, that's just a SYMPTOM ... the REAL cause is elsewhere.
 
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Old 12-06-08, 06:25 PM
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OK, great. You've been a big help and I've learned a lot..
I just want to make sure I was clear that the aquastat is on the *incoming* hot water pipe and placed outside the handler about 3 feet or so before the water hits the coils. I understand what you're saying about the coil itself and how it can cool off very rapidly, especially with trapped air, but it still doesn't make sense how the incoming hot water pipe would cool off that rapidly and shut off the blower that quickly. After all, the hot water is being continually replaced with additional hot water until the thermostat is satisified. Do you think that the water that is cooled off in the coils is rapidly backtracking into the incoming pipe? Sorry if I'm sounding a bit dense, but just want to arm myself with a little knowledge. Thanks..
 
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Old 12-06-08, 07:46 PM
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Ohhh, now I see why yer skeptical! And ackshully, I am too at this point. I guess I was assuming again ... that the 'stat was right on the air handler.

Ya know, the thing to do I guess would be verify the temperature of the water at that point. Maybe it _is_ a problem with the a'stat! How long can you hold your hand on the pipe? Have you ever felt the pipe when the handler is cycling?

But, I would still insist that someone at least check if the thing is airbound.

What floor are you on?
 
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Old 12-07-08, 09:20 AM
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OK, gotcha. I'll get it checked for air. I live on the top floor (4th floor) of a building with an attic, which is where the air handler and aquastat sit. So, you stand by what you said earlier about how all a'stats without an adjustable differential have a fixed differential of about 10 degrees, before I make a fool of myself when the HVAC guy returns? Clearly, this a'stat is overly finicky, and if there is a differential, it's not working.

He's already said I could get a new a'stat, "if it makes me happy". Clearly, he sees nothing wrong with all this crazy power cycling. Theoretically, there should be one power cycle of the air handler for every thermostat call, but I do realize there might be a couple or even a few cycles. But for the ratio to be somewhere in the neighborhood of 10-20 cycles per t'stat call seems ridiculous.
 
 

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