Short Cycling...


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Old 11-27-14, 05:08 PM
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Short Cycling...

Greetings all,

Let's just say, I wish I knew then what I know now... But, what's done is done.

2300 sq. ft. house in eastern CT, design temp is 7F. Built in 1997, and we've owned for 3 years. Haven't made many sealing, energy improvements, but is fairly decently insulated from what I can tell.

In May of this year we had the original Crown boiler with tankless DHW replaced due to cracks in the fire box and the burner was going (sounded like a jet in my basement).

We ended up having a Buderus G115WS/4, Riello burner, and Triangle Tube Smart 40 indirect put in. There are 3 zones - two heating and the DHW - running off Taco circulators. The Buderus has the Beckett AquaSmart 7600 control with the ODR installed. The DHW runs off the AquaSmart with priority. The AquaSmart is connected to a Taco SR503 for the two heating zones. Both zones are roughly the same size: one the main floor and the other the second floor which is 3 bedrooms and 2 bathrooms. But the second floor common area is open to the main floor via a cathedral ceiling so the second floor doesn't heat as much as the main floor...it all rises up into the second floor from the main floor.

Settings are: Cold start with High Limit of 180, 25 diff, low target for ODR of 140 (so water temp range is 140-180), low outdoor temp is 5F, high is 67 which is the WWSD temp. Both t'stats are set at 67 during the day with 65 setback at night (I can't stand being hot at night...).

I know I'm oversized... I did a heat loss myself, after the install, unfortunately, and got around 50-55k (it's been awhile, I don't remember exactly).

My issue is short cycling. In reviewing the history in the AquaSmart (because I sensed the boiler was only running for a short time), the longest run is about 2:54 and most of the others are in the 2:30 range.

Short of a buffer tank, what are my options? I'm not totally opposed to a buffer, but space and cost are certainly factors. Anything I can do otherwise to help in the interim? Wider diff? I've considered swapping out the SR503 for the 503-Exp and wiring the main zone for post purge - wish there was something that would post purge both zones, though. Would the Logomatic help - I'm cool with swapping the AquaSmart out for that and getting the indoor sensor as well.

Thanks for any help or suggestions, and love the forum. I read it regularly and have learned a lot!

Damian
 
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Old 11-27-14, 08:04 PM
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you can use your smart 40 as a buffer.
Use the boiler to keep it between 140 and 180 (using your odr), then just pull off the outer shell for your heating load.
I do this all the time with micro loads (bathroom floor warming).
I have done a job or two with just too many zones, and I have used the indirect as a buffer of sorts.
Seems to work well enough. I used a 3 way diverting valve to send the return directly back to the boiler if the tank temp dropped too far.

A 60 gallon buffer would do a better job.
You are limited playing with wider differentials, as I would not want to send much over 200 out to a BB heater (and even then I would not be very comfortable with it).
Thats about all I got.
 
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Old 11-28-14, 03:18 AM
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Check to see what size oil nozzle was installed , you may be able to reduce its size which will help the short cycling . I set up a oil boiler using a 4 way mixing valve and outdoor reset, and a timer to control the burner. It worked like this , the target temperature was set by the ODR and the mixing valve opened until the boiler water was at the ODR target . The burner fired and the timer began to count down, time was set for 1/2 hour and burner ran until boiler reached aqua stat setting of 180f or until timer turned burner off . The burner stayed off until the mixing valve was fully opened or there was a call to heat the DHW indirect. The system had 11 zones for in floor heat and the burner was fired with a 0.40 nozzle .
 
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Old 11-28-14, 08:31 AM
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Anything I can do otherwise to help in the interim? Wider diff?
I don't believe a wider diff will help... It's likely that the water temp is not coming up to the limit setpoint during a heating call. If the water doesn't hit limit and the heat call ends before that, the diff doesn't come into play at all.

I suppose you could have a tech attempt to downfire the boiler a bit, but keep in mind that the firing rate is determined by Engineers and that the combustion chamber is sized for a particular firing rate. Often when downfiring the techs try to go too far and you end up with a LESS efficient system.

Keep in mind that ANY boiler, even if correctly sized for the heat load is only going to be correctly sized on a DESIGN DAY outdoor temperature, which only occurs for a few percent of the heating season. At all other times the boiler is oversized.

Post purge MAY help... because by post purging you are pulling residual heat from the boiler into the home and the next heat call will start at a lower boiler temperature which would demand longer run times. The DOWN SIDE to post purging is that the temperature in the home will not be as constant... will probably overshoot thermostat setting. SOME thermostats will learn this and adjust their 'anticipator' circuits to compensate, but then when the thermostat cycle length is adjusted shorter as a result, you are pretty much back where you started from...

The buffer tank is probably your only sure bet, but it must be engineered properly and have the correct controls.

I personally like the idea of a large buffer that one can use as a 'battery', charging up the temperature in the tank fairly high, and pulling the heat load from the tank itself and not from the boiler. This would require a tank with four 'ports', two for supply and return from the boiler, two for supply and return from the heating load. The control on the tank is what would call the boiler to fire as needed to recharge it, and the heating load would only turn on/off the pumps drawing from the tank. This setup would allow FULL OUTDOOR RESET on the heating load side, while a large enough buffer tank would allow long, efficient, burner cycle times.

This won't be a cheap route to take!
 
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Old 11-29-14, 09:33 AM
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Thank you all!

I ordered the SR503-EXP and will set up the main floor zone with post purge to try to get a little longer cycle. I will also look to work with my heating contractor on planning a buffer tank installation and repiping.

Thanks again,
Damian
 
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Old 11-29-14, 10:17 AM
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This drawing is by no means complete... may not even be correct! it's just something I have been fooling with for a while... just to give you some ideas...

The three way valve is a Taco I-series. It shows a zone valve panel but could be implemented nearly the same with a circulator panel.

The buffer is maintained at temperature, and the central heating pulls from the tank.

Again, just for information and ideas:

 
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Old 11-30-14, 09:18 PM
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Damian,
I don't know how the ODR effects the system with the Aquasmart. I do know that since I installed the Aquasmart on my older Peerless I get a lot of short cycling with the same types of numbers. I don't have ODR just what it reads from the aquastat probe.The Aquasmart uses a moving target for the high limit. It tries to keep your system temperature as low as possible at all times. It might say economizing Hi limit of 150F and then you reach that when demand is on. While circulators are running it sees the demand is still there and changes the target to 160. Now your diff just became 15 instead of 25. 2 minutes later it might be 163 etc until it actually reaches your true hi limit.

I got the Aquasmart when I installed my Superstor and eliminated my leaking tankless coil. I wanted it for the LWC, it was a "bonus" that it had the heat manager built in. I will be switching to the Intellidyne HW+ that I already had. I had it on old house and a couple friends have it on theirs. It works like the heat manager but when there is a demand it will fire all the way to 180 then let it drop to 135 slowly based on how fast it senses the temp dropping. Cold days with high demand it won't let it drop as far. Warm days with low demand it won't fire for long periods.

Hindsight I might have tried the Hydrolevel (Hydrostat?) or other aquastat and just gone with the HW+. Maybe options to discuss with your contractor. Also if you keep the Aquasmart you might want to get/keep a "spare" sensor around for the $20. Mine failed like 2 months after install. The cap nut that holds sensor tip cracked causing failure and no boiler. Of course it was right before one of the single digit nights was coming. Luckily I caught it early in the afternoon. I called multiple shops and only one had it in stock. just something to keep in mind as the service person might not have one either (unless he carries spare Aquasmart he could steal from). Beckett was vary cooperative, said they had "some" complaints of that and sent me a new one free of charge. Of course that would not have helped for 3 days until it came in the mail. It now resides with my spare parts and old honeywell aquastat just because I could always get that hooked up in time of other failure.
 
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Old 12-02-14, 11:37 AM
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Nice diagram - certainly helps to visualize the concept. Thank you!

And sequoiasoon, thanks for the feedback. The ODR attempts to match the water temp to the outside temp. So at say 36F outside, it will heat the water to 162F. It then would use the diff (in my case 25 degrees) during a call for heat to know when to fire again. So in my case, if the ODR sets 162F, the circs will run during a CFh until it is either satisfied or the water gets to 137 or below. It would then heat back to 162 or until the CFH ends, whichever is sooner.

So in my case, the time it takes to go from 137 to 162 is around 2:30 or so... Or sometimes it will start to heat from 137 and the CFH will end, and the burner has only run a minute or so.

I guess I could disable the heat manager and ODR, set the high limit to 180F, and the diff to 40 degrees. But then its a matter of which is more efficient - heating the water to 180 repeatedly, with hopefully a longer burn time from 140-180...or leaving it how it is with a shorter, less efficient burn time for the 25 degree diff, but heating to lower temp. Less standby loss the lower the temp... Dunno. It's probably a wash?

We shall see how the new panel with purging the main zone helps.
 
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Old 12-02-14, 02:59 PM
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Damian, I think the point that Sequoia was making is that perhaps the "Heat Mangler" is 'fighting' with the ODR control. Maybe they aren't playing nice together.

Before you disable BOTH, try them one at a time for a period of time and note the behavior.

It then would use the diff (in my case 25 degrees) during a call for heat to know when to fire again.
This may or may not be true... this is where the 'fighting' between the ODR and the HM starts.

If the ODR cuts the burner off before the HM senses it has hit it's setpoint, fights start over control.

What ODR are you using? How is it wired to the control?
 
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Old 12-02-14, 07:52 PM
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I'm using the ODR for the Aquasmart. It disables the Aquasmart heat manager instead using the ODR curve based on the high limit and low water target and low outside air temperature.

http://www.beckettcorp.com/protect/m...duleManual.pdf

I did disable the ODR (removed the module, thereby re-enabling the heat manager) for a few days last week, and it short cycled a bit more. Like sequoia said, if there is a prolonged CFH, the heat manager bumps the high limit up a bit. That little bit (I think it's like 10 deg at a time) takes a short time to reach.

Good thought, though.
 
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Old 12-03-14, 05:48 AM
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If your still getting a short run time with a ~25 degree diff, then the control isn't your issue.
I feel your issue is that the firing rate is to high. Refer to the boiler/burner manual and see if you can down fire it a bit. That will give you a longer run time at steady state.
 
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Old 12-03-14, 06:53 AM
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A perfectly sized boiler to match the design day ( nozzle size) , the burner will run for 24 hrs. without shutting off and just produce enough heat to keep the home at design temperature . Therefore if the burner runs for 12 hrs. on the coldest day the nozzle is twice the size it should be .
 
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Old 12-03-14, 10:10 AM
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the nozzle is twice the size it should be
Just to be clear though, this does NOT mean that you can install a nozzle half the size in your boiler.
 
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Old 12-03-14, 11:01 AM
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What's your heating element? Slant-fin baseboards or hydro-air?
 
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Old 12-05-14, 01:58 PM
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Slant-fin baseboard. And I will discuss with the contractor in the spring about a smaller nozzle.
 
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Old 12-08-14, 06:29 PM
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Another thing you can do which is not an end all but will help, is turn off DHW priority. I used to be a big priority guy but not I usually discourage people from using it if not needed with positive results I might add. Add more load when hot water and heat is calling together. When boilers are over sized which about 85% are by at least 100% it should not have priority. You need to look at this on a jo by job application. So when is it needed? Really large hot water demands, very small boiler compared to the tank requirement.
 
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Old 12-09-14, 07:09 PM
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I may try that. The DHW comes off the Aquasmart rather than the Taco panel... I will have to check if I turn priority off, will it still bypass the ODR temp and still go to the high limit? This time of year isn't a concern as the ODR temp is high enough...but in March/April...

I will take a look. Thanks!
 
 

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