Buderus GB142 regularly cycling between OA & OY

dgkula

Hi, my home and hot water is heated by a Bureaus GB142 with AM10. It was installed in 1999 and we bought this house last November and until we lost heat last week, I had no curiosity about how this works

dgkula

update

Just sat in front of the boiler for two and a half hours. It runs fine and bums right along at the target temp of 169 when 3 or 4 zones are calling for heat, but when the zone valves start to close as the demand is met, the supply temp starts to creep up. Since zone 1 is never satisfied all other zone valves close and when only zone 1 is left, the boiler shuts down when it hits max supply temp of 180 with code OY as temp is 11 degrees over target temp calculated by AM10.

Then it either waits until supply temp lowers to 158, 11 degrees below target temp per the operating schematic for OY and then starts and in about a minute is back at 180 and boiler shuts down with OY until 158 again when OA shows and we wait 10 minutes to repeat.

Main loop circulation pump is new, all zones are being heated, just when only zone 1 is on it continually bounces off the max temp. this would probably be the case if any 1 zone was on alone, just all other zones are well insulated and smaller so can have their heating demands met. I tried lowering the load to 80% L80 as a guess to see if that would make it ramp more slowly but that had no effect.

Thanks,

David

dgkula

another update

Just took zone 1 out of the equation. This bouncing off the AM10 calculated top limit happens whenever only one zone is calling for heat. With more than one zone the flow seems sufficient to move the heat away fast enough. Not sure what this means though or how to remedy.

Ray Landry_3

water chemistry

Has the boiler been taken apart and cleaned yearly? This is very important. Also, maintaining the water chemistry (ph) is critical in aluminum block systems. I would recomend finding a condensing boiler expert in your area to check the delta t of the system, ph, and clean the boiler. Where are you located?

dgkula

located in sudbury, ma

Thx.

Zman

Normal?

This sounds like normal boiler operation. When the heating load is less than the lowest modulation of the boiler the boiler will increase the on/off differential to prevent short cycling. Is the house heating ok?

Carl

dgkula

house not heating ok

your reply makes sense - the boiler is shutting down for 10 minutes and throwing the OA code to avoid short cycling. the problem I have is that one zone never reaches thermostat temp, and I would like to understand the condition that is effectively causing the short cycling that the boiler is preventing.



over the course of an hour, with the target supply temp at 163 based on the outside reset, the boiler runs at 163 for a total of 10 minutes. it spikes to 180, shuts down, spikes to 180, shuts down, then enough zones are open and calling for heat that it will run at 163 for 8 minutes before zones start closing and it spikes again and enters the phase where it will do 5 or 6 short cycles spiking to 180 to try to feed the one zone that isn't at temp.



as a result of the spiking and shutdown (short term and 10 minutes depending on OY or OA code), the average water supply temp is around 120 to the zone that won't heat. it is my largest zone.



So I feel like whatever condition is causing the short cycling that the boiler is shutting down to prevent is also keeping me from heating zone 1 to 68, as it never gets to 68.



does that make sense? it seems like with just one zone running, there isn't enough "heat velocity" in the system to move the heat away from the boiler fast enough so it goes into short cycle mode and shuts down. when 3 or 4 zones are calling it runs in a nice long cycle. sorry not up on hydronics concepts or terms.



thanks, David

Zman

Type of heat

Do you have hot water baseboard? How any lineal feet of baseboard is on the zone. How many square feet does it serve? How is your outdoor reset setup?

Carl

dgkula

setup

the house has hot water baseboard. It appears to be a single pipe system. this zone heats 4 rooms and half of the entry area by the front door. each of the five heated spaces has a baseboard radiator that starts with an old style automag controller and then a main on top and a stretch of copper fin/tube below. I do not believe that the automags are wired to anything and so assume that they are in the open position. there is no visible automag controller in the house. the total length of fin/tube is 533 inches and the heated space is approx. 805 sq ft.



I don't understand how the single pipe system with automag is designed to work - presumably there is some component in the automag that  directs some of the main water into the radiator? schematically it looks like a monotube type layout with the automag on the supply side of each radiator and a y-shaped connection on the return side of the radiator with a small bleeder valve. in the baseboard the main is on too and fin/tube on the bottom.



Thanks,

David

dgkula

am10

forgot to mention that the outdoor reset am10 appears to be working properly. it is in a shaded location away frm the boiler exhaust and wired directly into the gb142 boiler. thx, David

Zman

Trying to figure....

Is the boiler behaving normally and you have an issue with distribution to the rooms or is there a problem with the boiler. The am-10 has temperature settings. You need to find out how it is set. A drawing and/or some pictures of the piping would be helpful.

What are the t-stats connected to?

Carl

dgkula

schematic, etc

Hi Carl,



Thanks for your help. Here is a schematic showing the baseboards http://tinypic.com/r/14bm0pd/6



The AM10 is set as follows:

Target Point = 180F (@ 14F)

Base Point = 115F (@ 68F)

Warm Weather Shut Down = 70F



There is a single thermostat in this zone connected to a Taco ZVC406 in the basement next to the boiler. The automag zone valves do not appear to connect to anything, and so I assume are in the open position.

I guess I would be surprised if the boiler was behaving "normally" but its behavior could be a function of the system. At face value it doesn't seem to make a lot of sense that each hour it runs steady for 10 minutes at the target temp and spends 50 minutes in a cycle where it runs for 1 - 2 minutes, spikes to max temp, cools down for 4 - 5 mins, runs 1 - 2 minutes, spikes to max temp, cools down for 4 - 5 mins, runs 1 - 2 minutes, spikes to max temp, shuts down for 10 minutes. Repeat.



That type of behavior doesn't seem very efficient. It would seem to be "normal" to hold a consistent temp for a longer period of time. But maybe the boiler is responding to system conditions - as a guess, I can't see the return water temp on entry to the boiler but I am guessing that this loop is not radiating enough heat and so even when modulated to the lowest setting, the supply temp spikes because the delta T is so small? I don't know if that makes sense?



Thanks,

David

dgkula

Thermostat

The single thermostat for the zone is in the top room. It is drafty: it is over the garage, has two single paned windows with storms, has a bay window with 5 casement windows and a large fireplace with chimney on an external well. We definitely need more radiation. Just not sure if/how that would change the burner behavior.

dgkula

thoughts

So maybe if I try to reduce this down, what I am seeing is that when the GB142 is heating only one zone (and in my case it happens to often be heating only zone 1, my drafty zone which can rarely be properly heated to thermostat temp), it is unable to maintain a supply temp less than target temp +11F and so shuts down either until the temp drops to target temp - 11F or for 10 mins depending on circumstances.



The temp ramps so quickly from say 120F to 161F with only one zone running that it makes me wonder if the heat is traveling completely around the zone 1 loop, raising the temp of the water into the boiler pump quickly OR if for some reason with only one zone running flow at or past the circ pump is restricted so heat is traveling back from the supply outlet back into the boiler pump.



This second situation might explain why the supply temp will go from 120 to 161F in less than one minute wit only zone one running.



When 4 or 5 zones are calling for heat, flow is sufficient to move heat away from the GB142 quickly enough that it does not back up and I get nice long runs at the target temp. It is only as zone valves start closing as temp requests are met that the boiler starts to bounce of the temp limit.



Does this make any sense?

Zman

Hmm

It sounds like you have a monoflow system with zone valves that have been disabled. You have considerable radiators for the square footage even at lower water temps, I am surprised it is not keeping up. I would start by verifying that all the radiators getting hot. It will be easier to tell when the boiler is running hot. You want to be sure you do not have an air lock,bad circulator or closed valve.Can you take a picture of the boiler room piping? I am still unclear how that part of your system is set up.

I do not fully understand the logimax controller on that buderus. I have seen it do what you are describing, it then began running normally. There is a reset sequence in the manual that may help. Is it a GB142-30?

Carl

Charlie from wmass

When was the last

annual cleaning down on the boiler?

dgkula

cleaning

Bought the house in Nov 2011 and it has not had a cleaning under my watch.



The more zones calling for heat the longer it runs at a stable temp at or near the target temp. As soon as zones start shutting down, the temp starts to climb, eventually triggering shutdown and code OY.



On restart, with one or two zones calling for heat, it hits temp limit in less than 1 min and shuts down again with code OY or OA. OA keeps it off for 10 mins.



So is this boiler modulating down when I only have 1 or 2 zones calling? It seems like the heat being put in the water is constant and with 3,4 or 5 zones calling the supply temp stays steady (wish I could see delta T). With one or two zones calling the temp climbs in seconds and triggers shutdown.



I have a Taco 007 circ pump which runs at a constant speed. I am ignorant of the laws of thermodynamics but with several loops open, I would guess that a single heated piece of water would take longer to get back to the boiler than with just one loop open, and maybe with multi loops open that piece of water dissipates approx. the heat that the boiler put in so supply temp remains steady when it next passes through the boiler. Maybe with one loop, since the round trip is faster, less heat dissipates, the boiler inputs constant heat and the total heat climbs. I thought the boiler should modulate down (not sure based on what though, I would guess delta T) to avoid supply temp spiraling up? 

David

Ironman

GB142/??

What size is the GB142.There is a suffix in the model number, like GB142/24 or /30 or /45 or /60.



The boiler may be over-sized and zoning it would only compound the problem. Europeans don't design their systems to be zoned but Americans do. Putting a mod/con on a zoned system is an attempt to merge two design methods that are very opposite and will almost always result in short cycling unless a buffer tank or other means are not incorporated. Most installers don't realize this and think that because the boiler modulates it will work okay. Short cycling is a common result.



I just took a look at your numbers: 44ft. of baseboard. That's about 23k btu's output. 805 sq. ft. space would require about that on the coldest day (design temp). The smallest Gb142 is the 24kw which is 84k input. It will modulate down to 28k btu's which would mean that the boiler, even on its lowest fire, is over-sized anytime it's above 0* outside. Add zoning to that and the problem is multiplied.



You need to minimize or eliminate the zoning and/or install a buffer tank.

dgkula

it is a /60

I tried to turn down load to L80 but that did not seem to have an effect. Not really sure what that would so but was hoping it might help.

dgkula

what would be the remedies if it is oversized?

I tried lowering target point on the am10 to 150 thinking that if I can just match the heat loss with heat again from the boiler I will always have 4 or more zones calling and it will not short cycle. That is giving me longer runs but inevitably a zone or two close nd it starts short cycling again.

Any thoughts regarding how to fix?

Ironman

60KW

Or about 206k btu's.



Turing it down to "L80" means the firing rate is limited to 80%. It won't do any good because the logic in the UBA control is already limiting the firing to its lowest point to try to match the load, but because the load is far below that, the boiler cycles off for the 10 min. minimum.

Ironman

Fix?

Read my last two posts carefully. Your boiler is way over-sized.

dgkula

now i get it _ thanks!

The 804 sq Ft is only 1 zone of 5 that I was referring to that never got to thermostat temp but I agree the boiler is oversized given how the system is currently zoned. That explains the short cycling. Sad thing is that the prior owner had a monotee-like system off a single loop with valves on each radiator controlled by a themostat for each collection of radiators. I imagine the prior boiler was run continuously at low temp with the radiator valves to pull heat off the main loop to heat the radiators as needed. When the contractor installed the GB142 they killed the continuous loop and broke it into zones, disconnecting the individual radiator valves, and instead put in loop based zone valves, it seems effectively creating this problem. The wires are all still there bundled with electrical tape. I could, and probably should, reverse the procedure and return to a single continuous main loop with the individual radiators put in/out of the loop locally. Then there would be enough flow in the system to let this boiler run. It does run fine when all six zones are calling ... I bet the contractor replaced a prior boiler and matched the btu but changed the system without realizing the problem.



Given that this contractor created this problem on 2009, in your opinion should they be held accountable for fixing it? I bought the house a year ago and didn't pay for the boiler but now am either looking at living with a short cycling boiler that will likely have a reduced lifespan and one zone that won't heat fully owing to the short cycling, or I will have to pay out of pocket to fix it. Shouldn't the contractor pitch in to fix his mistake?



Thanks, David

Charlie from wmass

wait a second

You have two issues not one. The zone not heating is caused by not enough heat going from the boiler to the zone to be heated. The over sized boiler is a separate issue that is even worse because the heat is not getting out to heat the zone. This could be a flow issue for that zone, or not enough heat emitters for the zone.

Ironman

Contractor Liability

In my opinion he should. But I'm not an attorney and I'm not familiar with your state's laws.



I would recommend that you contact "Charlie from WMass" Garrity if he's anywhere near you and let him look at it and make recommendations.