Bosch Greenstar Combi 100 short cycling issues?

BoilerNovice7

Hey all.

I have spent hours reviewing this and other forums trying to understand if I have a problem with this newly installed boiler, and while I think I understand the issue and potential solution I am no expert and would like some insight to help point me in the right direction to capitalize on the full efficiency and comfort of this condensing combi boiler.

The boiler is firing for about 1 minute every 2.5 minutes.  

That seems to be way to often and I would like to confirm this with someone more knowledgeable than I and identify the best potential solution.

I am running the wall mounted version of the Greenstar combi 100, the outdoor temperature sensor is installed and I am utilizing the CrC200 to control my single zone.  

We have 1 zone in 3 loops totaling about 900 feet of 1/2” pex in slab.  There is 2.75” heatsheet insulation under slab and there is r15 slab edge insulation.  The building is 790 square feet.  Well insulated using rockwool r23 exterior wall insulation and r38 ceiling insulation and extremely well sealed with only 3 windows and 1 sliding door.

I am not currently running any setbacks, temp set to 72 degrees.  I am seeing between a 15 and 20 degree delta T.

I have supply temp set to 123 degrees but by the time it makes it to the manifold it is usually between 100 and 110 degrees.

piped primary secondary with oversized 4.4 gallon expansion tank and single taco circ pump that is running 24/7.

I did not purchase and install the supply temp sensor as others on this forum have recommended, so that could be an issue but I am not sure. 

I have the unit in eco mode so it is not constantly firing to keep domestic hot water ready.

I am looking for feedback on the setup, help confirming wether the boiler is oversized for the application (which I believe to be the case) and recommendations on how to increase efficiency and decrease cycle times.

hot_rod

That is too short of a cycle. 10 minute run time is one best practice suggestion.
In the control section there are some adjustments to help lessen short cycling.
First lower the output. If you know the heatload of the home set it to that number. If not some trial and error. Reducing the output does not change the DHW capacity, it still will go to full fire. Maybe set it to 50% to start.
Or ideally do a load calc on the house to get a starting point.

It may have a timed anti cycling function also. Or a step fire setting, read through.

https://www.bosch-thermotechnology.us/us/en/residential/service/documents-media/downloads-for-bosch-products/manuals/gas-fired-condensing-boilers-manuals/

kcopp

Why is the system circulator plugged in to an outlet.? Should be into a relay controlled by a thermostat or the boiler if that has that function. What is the outdoor temp? 123F seems a bit on the high side for a slab.

BoilerNovice7

In response to Kcopp: The system circulator is plugged into an outlet because I was told it would work fine that way and I wasn't sure if I would have to purchase an additional relay and didn't really see the benefit aside from just reducing the run time on the circ pump by spending the extra money. The pump is rated for continuous duty so I wasnt too concerned about it. I just looked through the installation instructions again and it looks like the boiler does provide a connection for a single additional circ pump under 2 amps.

Now I'm thinking it would maybe be a good idea to wire that into the boiler instead of just having it plugged in. I just didn't think it would make much of a performance difference. Do you think that will make any kind of performance difference having it shut off based on commands from the boiler?

The outdoor temp was about 40 degrees at the time I was watching it cycle. Now we are closer to 32 at night. I set it to 123 based on the advice of one of our local mechanical companies, but like i said it never sees 120 at the manifold before going down into the slab. Think I would be better suited to lower the supply temp?

BoilerNovice7

In response to hot_rod: Thanks I thought it was too short but had no idea honestly. I was just sitting there watching it cycle thinking that can't be right. I have adjusted the output, however with the CRC200 room controller installed and a set supply temperature it makes you designate upon setup my understanding is that is designed to automatically modulate the boiler based on the heat curve it selects using the indoor air temperature and the outdoor air temperature to dictate modulation and firing of the boiler.

After plugging in the CRC200 room controller the output heat adjustment knob on the boiler does not change the output or fire cycling times. The boiler will fire around 80 degrees and cease firing at 123 degrees. Then as the temperature drops in the primary loop it cycles all over again about 2.5 minutes later.

I have combed through the installation instructions, the operating manual, and the applications guide that Bosch provides. I have also reviewed the installation and operating instructions for the CRC200 bosch room controller too. I'm sure I missed some critical information however and will be reviewing the documentation from front to back again, it is a good suggestion.

But like I said based on my understanding of the unit, it should be auto modulating based on the outdoor and indoor air temps and auto selecting the most effecient heating curve to maintain the selected indoor air temp, and this understanding has been reinforced by the fact that the zone heating output controls don't seem to do anything once the room controller got installed. The instructions in the room controller state that I should leave the zone heating knob at the "maximum" position and allow the CRC200 to control the supply temp upper and lower limit.

I will try and perform a load calc again, but last time I tried doing one it looked like the minimum btu output of the boiler was greater than the buildings heat loss value but I don't recall the exact numbers I came up with. Will go back to that and document my findings.

I have no idea what an anti-cycling function is or step fire setting. I will definitely be diving back into the documentation for the boiler. Thanks for the insight!

kcopp

72F for a radiant floor seems a bit high to me. One of the advantages of radiant is you can run lower temps w/o the loss of comfort.
The boiler should be able to find the sweet spot and modulate down to run at a proper temp and then run a long time at a lower firing rate.
I think you need to spend some time looking at the program settings. I am not familiar w/ that control, and it has been a long time since I used that boiler... and then it was w/ the older control.

BoilerNovice7

Thanks for the insight I have lowered the thermostat to 68 and may even drop it to 65.  72 never seemed warmer or more consistent to me than it has so far since I commissioned this boiler and radiant slab.

I will certainly be looking through the controls more, I suspect you are correct and know there are more advanced configuration options.  I utilized the “auto configuration “ upon setup.  I may reset the controller and take a closer look at all the options available to me in manual configuration of the room controllet.

Hot_water_fan

I will try and perform a load calc again, but last time I tried doing one it looked like the minimum btu output of the boiler was greater than the buildings heat loss value but I don't recall the exact numbers I came up with.

Your building is 800 sqft and well insulated. I wouldn’t be surprised if your heat loss was < 10,000 btu, so yeah you’re massively oversized since the minimum output of the Bosch is 24,000. 

Use the techniques hot rod recommended and it should calm down a bit, but as you can see it’s a huge boiler for the application and the best you can hope for is still a lot of short cycling. You should be able to drop the supply temp way down, 123F is absurd for this structure. 

Big Ed_4

He needs the extra BTU's for making hot water . The boiler can modulate down for heat. I would let the boiler control the heat circulator . I agree your program needs to be adjusted .

BoilerNovice7

Thank you so much for the input. When I did the heat loss take off I came up with about 8900. When the local boiler contractor heard that he made me feel unsure of my results because he asked me if that was for the whole building as if it couldn’t be correct and told me it sounded wrong. I had no idea what I was doing when I bought the unit frankly. I tried to make an educated decision but hindsight is 20/20 haha. Well aside from your suggestions what else could I do to actually use those btus and would it be worth the effort? I have a concrete porch slab I will be installing outside, and an extra supply and return port on my manifold… Or would a buffer tank be the better solution? A properly sized boiler would be better, but this thing is on the wall and it’s keeping the whole building very comfortable so replacing the unit isn’t high on my list of solutions right now unless propane usage gets out of hand.

Hot_water_fan

He needs the extra BTU's for making hot water . The boiler can modulate down for heat. I would let the boiler control the heat circulator . I agree your program needs to be adjusted .

Agreed on the hot water. This is the downside of combis though as it can’t really modulate down to a useful level even if it’s capped at the minimum, so it’s effectively a non-modulating boiler. 

Thank you so much for the input. When I did the heat loss take off I came up with about 8900. When the local boiler contractor heard that he made me feel unsure of my results because he asked me if that was for the whole building as if it couldn’t be correct and told me it sounded wrong. I had no idea what I was doing when I bought the unit frankly. I tried to make an educated decision but hindsight is 20/20 haha. Well aside from your suggestions what else could I do to actually use those btus and would it be worth the effort?  

You’re hemmed in by the Combi aspect of it. It’s got to be that size for DHW because you’re operating a just-in-time system. You could install a buffer tank or just live with it. When it breaks, you can replace with a Combi boiler with a better turndown ratio or move to a DHW storage system (like an indirect tank or standalone hot water tank). It is not surprisingly the contractor was frankly clueless about the heat loss, that’s a major problem in the industry. 

For a small building that well built, you have many more options for heating too - a small heat pump would thrive and provide AC, but it’s such a small load that you could potentially do electric resistance water heating. The trade off is operating cost vs install cost. As the heat load decreases, operating cost is much less relevant. These choices are rate and location dependent. 

Something like this is tiny and doesn’t require a primary/secondary loop. 

https://www.ecomfort.com/Electro-Industries-EMB-S-9/p18277.html

You’re also in the size range where you could run the central heating off a tank type water heater, provided that it’s designed for that. 

EBEBRATT-Ed

15-20 delta is a lot your delta should be 10.

Big Ed_4

Where is the supply temperature sensor ?

Without it the boiler may fish ?

BoilerNovice7

Big Ed_4 said:

Where is the supply temperature sensor ?

Without it the boiler may fish ?

No system supply temperature installed, was not sure if I needed it and don't know what you mean by "fish", can you elaborate?

BoilerNovice7

EBEBRATT-Ed said:

15-20 delta is a lot your delta should be 10.

Any reason why? I've spoken to at least 3 or 4 different pros and have seen 15-20 degree recommendations, I have seen 10 being considered acceptable too though and not sure whats right. Would adjusting the GPM on the loops on the manifold help me adjust my delta t?

BoilerNovice7

Hot_water_fan said:

He needs the extra BTU's for making hot water . The boiler can modulate down for heat. I would let the boiler control the heat circulator . I agree your program needs to be adjusted .

Agreed on the hot water. This is the downside of combis though as it can’t really modulate down to a useful level even if it’s capped at the minimum, so it’s effectively a non-modulating boiler. 

Thank you so much for the input. When I did the heat loss take off I came up with about 8900. When the local boiler contractor heard that he made me feel unsure of my results because he asked me if that was for the whole building as if it couldn’t be correct and told me it sounded wrong. I had no idea what I was doing when I bought the unit frankly. I tried to make an educated decision but hindsight is 20/20 haha.

Well aside from your suggestions what else could I do to actually use those btus and would it be worth the effort?  

You’re hemmed in by the Combi aspect of it. It’s got to be that size for DHW because you’re operating a just-in-time system. You could install a buffer tank or just live with it. When it breaks, you can replace with a Combi boiler with a better turndown ratio or move to a DHW storage system (like an indirect tank or standalone hot water tank). It is not surprisingly the contractor was frankly clueless about the heat loss, that’s a major problem in the industry. 

For a small building that well built, you have many more options for heating too - a small heat pump would thrive and provide AC, but it’s such a small load that you could potentially do electric resistance water heating. The trade off is operating cost vs install cost. As the heat load decreases, operating cost is much less relevant. These choices are rate and location dependent. 

Something like this is tiny and doesn’t require a primary/secondary loop. 

https://www.ecomfort.com/Electro-Industries-EMB-S-9/p18277.html

You’re also in the size range where you could run the central heating off a tank type water heater, provided that it’s designed for that. 

Yeah Im definitely pot committed to this setup for the time being at least. I really appreciate you taking the time to break down your thoughts like this for me too. I will get the circ pump hooked up to the boiler and let it control it, just makes sense.

I didn't really dive into the tech details on radiant heating and boilers and heat loss and primary and secondary along with all those other details until it was time to install. When we were shopping around this was more of a "wouldn't that be cool to install" type of thing. We even installed an 18k twin cassette heat pump in the building too, mostly for cooling or backup heat if the propane tank is out. But I don't regret going with the hydronic at all because the winter time comfort is far superior, even if the boiler is short cycling and way oversized for the zone heating aspect of it because every room is getting very evenly heated.

It'll last as long as it lasts, even if I can't eliminate the short cycling. I will definitely be tweaking things to see if I can make it more consistent though.

BoilerNovice7

Update here. I have reduced supply and design temp on the boiler room controller (CRC200) from 123 degrees down to 98 degrees.

I have also reduced the thermostat temperature from 72 down to 65 degrees yesterday which seemed to help a bit as well.

I haven't actually had the time to sit there and time the cycles yet however it seemed to be firing much les frequently at a cursory review in between working on finishing the building today.

I will report back with actual cycle times and delta t etc once the changes have a day or two to set in.