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Bosch Greenstar Combi Not Able To Maintain Boiler Temperature

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I have a Bosch Greenstar Combi that my heating & cooling contractor installed in 2018. It supplies our DHW and hot water to our Stadler radiant in floor heating plumbing via three Grundfoss zone pumps. The boiler is a floor standing model KWB35-3. It is not fitted with the Bosch FW 200 accessory heating control unit.

I believe that this unit has not worked right since it was installed. It is not able to maintain boiler temperature. I have it set to 3 on the boiler temperature dial, which should be in the neighborhood of 130°F. It can’t hold this temperature. It will try to hold it, but will start to ramp up to about 145°F, at which point the flame turns off and stays off until the boiler temperature falls to about 95°F. When it goes over temperature, I do not hear the flame fan decrease in RPM, and indication that the flame is not being reduced. The DHW side temperature also fluctuates since it’s connected to the boiler via a plate heat exchanger.

However, there are times where it is able to control the boiler temperature. Usually when all three radiant floor loops are calling for heat, but not always. When it is working right, I can hear the burner fan speeding up and slowing down as needed to maintain the set boiler temperature.

I have had the installer come out several times and have a Bosch representative come out to look at it. The contractor doesn’t know what’s going on, and the Bosch rep never got back to me or the contractor as he had promised. I would like to get to the bottom of this and am looking for advice on what I need to do in order for the boiler to maintain the set temperature. Thanks for any suggestions!

Comments

  • Big Ed_4
    Big Ed_4 Member Posts: 2,809
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    What is that storage tank used for ? Do you use the plate heat exchanger for Hot water ?

    There was an error rendering this rich post.

  • hot_rod
    hot_rod Member Posts: 22,271
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    Sounds like either a control or sensor error
    Does  that control do any data logging, many do.
    That could help determine when the system is falling behind and if it has thrown any error codes

    I would do a deep dive into the control section of the manual
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • MrFireball
    MrFireball Member Posts: 9
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    Big Ed_4 said:

    What is that storage tank used for ? Do you use the plate heat exchanger for Hot water ?

    The storage tank was intended to be used as a dump load for a solar system to preheat incoming DHW. It's not connected. The Combi has a built-in plate heat exchanger for DHW.
  • MrFireball
    MrFireball Member Posts: 9
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    hot_rod said:

    Sounds like either a control or sensor error
    Does  that control do any data logging, many do.
    That could help determine when the system is falling behind and if it has thrown any error codes

    I would do a deep dive into the control section of the manual

    It's never thrown an error code. I put a temperature data logger on all of the inlets and outlets. Here's a graph of the heating side.
  • hot_rod
    hot_rod Member Posts: 22,271
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    The boiler may be ramping down when flow is low. Most boilers watch supply and return temperature and modulate the firing rate based on the manufacturers minimum flow rates. So with only one zone calling the boiler may be ramping down.
    Is it piped primary secondary, or as the manual indicates?
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • GGross
    GGross Member Posts: 1,072
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    I think this is the model that actually has a low loss header built inside the boiler cabinet
  • yellowdog
    yellowdog Member Posts: 157
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    why do people accept getting blown off after someone comes to their house as acceptable? you hear about it so much on this forum. get the original installer and a bosch rep back in the house to figure it out and make them give you an answer. from what i can see of the piping, the plunger should have been removed from the low loss header in the boiler.
  • fenkel
    fenkel Member Posts: 162
    edited February 2023
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    what are you zone btu requirements?
    sounds to me like your zones might be too small.
    was a heat loss caculation performed before the installation?
    can you provide it ?
    The Kwb35-3 is a 131k btu boiler ( this is a very large boiler), with a turn down ratio of 3.7 to 1
    so each zone needs to be at least 36k btus or a combination of zones has to be at least 36k btus otherwise it will short cycle.. you can turn down the boiler output Display 1.A, BUT, you need to know your total BTU requirement before you make an adjustment of this nature. I think I was informed in tech class, you possibly can cut output up to about 50% of rated btus ..
    I've enclosed a picture of one of our installs. Each zone was about 26k btus, we used the FS 100 KBR28-3 (turn down of 4.1 to 1, minmun output of 24.6 BTUs).100K btu boiler , we decided to add the buffer tank to slow down the short cycling even more.. We timed it and it does maybe 3 cycles to 5 cycles per hour... depends on zone heatlosses..


    hot_rod
  • MrFireball
    MrFireball Member Posts: 9
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    GGross said:

    I think this is the model that actually has a low loss header built inside the boiler cabinet

    Yes, that is correct.
  • MrFireball
    MrFireball Member Posts: 9
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    yellowdog said:

    why do people accept getting blown off after someone comes to their house as acceptable? you hear about it so much on this forum. get the original installer and a bosch rep back in the house to figure it out and make them give you an answer. from what i can see of the piping, the plunger should have been removed from the low loss header in the boiler.

    I had that concern too and checked with a thermal camera. The plunger was removed from the low loss header.

    You make a valid point, and the installer did spend a lot of time trying to figure it out, but said he was not getting the support he needed from Bosch. So here I am, the homeowner, trying to figure it out before I call another contractor to look at it. If I can't fix it, at least I want to be at a knowledge level where I can talk to and understand the service personnel.
  • MrFireball
    MrFireball Member Posts: 9
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    Thank you! I think you are on to something. Yes, zone btu loads per zone were done by a heating contractor and I have them. The winter heat load of the entire house was 34677 Btuh. The original system was a simple 80K btu combi-coil hot water heater for DHW and the coil supplying the radiant heat. After 23 years of service, I decided it was time to have it replaced. Originally the smallest Greenstar was specified, but the installer said he was unable to get one and moved up a size, saying it would be okay and do a better job of supplying our DHW needs...

  • fenkel
    fenkel Member Posts: 162
    edited February 2023
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    Sorry to say, but your boiler in 3 times what you need for heating... the contractor was right on for heating the DHW needs. I did a quick calcution on head load and i don't think you can turn the output down low enough to work... you only need about 30% of capacity for maximun heating needs.. that would be with all the zones running at the same time.. you mighty try it at 50%, but i wouldnt go any further than that. it possibility might work better..
    Could you take off the front cover and take a picture of the low loss header and post it?.. just to see if the temp sensors is installed correctly.. also there is a white line on the header and if its split or missing , there's a chance the contractor removed the plunger.. if the plunger has been removed, then its functing in p/s mode. The boiler is cycling alot because the heat cant be moved away from the probe on that header. causing it to shut down the boiler because of high temp. that could also explain why you have temps over 135 degree setting.. in short, the boiler is making more hot water for zone heating than the system can move...
    A buffer tank would be a better option.. at least a 20 gallon buffer tank. I'd look at a Vaughn s20d4B, under a grand in price.. or make one up out of a water heater..
    Bosch is a good product.. we havent had any issues so far... from what we've encountered, most are installation errors by the installers( contractors)..
    bosch also doesnt have the higher turn down ratios (10 to 1) , like some of the other boilers..Over sizing is one of the biggest problems we run into.. a boiler with a higher
    Modulation factor, might work better, but again, it would need to modulate down to about 13K btus...
    GGross
  • MrFireball
    MrFireball Member Posts: 9
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    fenkel said:

    Sorry to say, but your boiler in 3 times what you need for heating... the contractor was right on for heating the DHW needs. I did a quick calcution on head load and i don't think you can turn the output down low enough to work... you only need about 30% of capacity for maximun heating needs.. that would be with all the zones running at the same time.. you mighty try it at 50%, but i wouldnt go any further than that. it possibility might work better..
    Could you take off the front cover and take a picture of the low loss header and post it?.. just to see if the temp sensors is installed correctly.. also there is a white line on the header and if its split or missing , there's a chance the contractor removed the plunger.. if the plunger has been removed, then its functing in p/s mode. The boiler is cycling alot because the heat cant be moved away from the probe on that header. causing it to shut down the boiler because of high temp. that could also explain why you have temps over 135 degree setting.. in short, the boiler is making more hot water for zone heating than the system can move...
    A buffer tank would be a better option.. at least a 20 gallon buffer tank. I'd look at a Vaughn s20d4B, under a grand in price.. or make one up out of a water heater..
    Bosch is a good product.. we havent had any issues so far... from what we've encountered, most are installation errors by the installers( contractors)..
    bosch also doesnt have the higher turn down ratios (10 to 1) , like some of the other boilers..Over sizing is one of the biggest problems we run into.. a boiler with a higher
    Modulation factor, might work better, but again, it would need to modulate down to about 13K btus...

    Okay, it's nice to have an answer that makes sense. I really appreciate it! Yes, the plunger was removed from the low loss header. The installer told me he had removed it and I confirmed it with a thermal camera. I've attached its picture per your request. I looked at the maximum heat output setting, 1.A, and it's set at 100%. I'll try dialing it down to 50% and see how that goes, though, as you suggested, I doubt it will do much since I can't reduce the minimum heat output setting to keep the turn down ratio the same.

    Besides adding a buffer tank, I was thinking of adding a finned radiator with fan loop to increase the heat load. This system is in a well insulated basement (ICF construction) of a ranch style home, so heat dumped into the basement will eventually work its way upstairs.

    One thing that puzzles me is how can a large boiler supply DWH at a low flow rate? Does the pump supplying the plate heat exchanger modulate the boiler water to maintain DHW temperature? My DHW temperature bounces up and down with the boiler temperature and the installer installed a mixing valve in attempt to keep the DHW under control.

    The installer will be coming in a couple of days to do its 5 year cleaning. I'm starting to regret having gone this route. Should have stuck with the combi coil hot water heater...
  • fenkel
    fenkel Member Posts: 162
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    i read you post, Questionj...has the DHW exchanger been cleaned? has the filter been cleaned in the DHW? both of these can cause some varibles in the output temp of the DHW..
    Its a good practice to have the tempering valve installed in the DHW system to prevent scolding water.
    I'd get the DHW exchanger cleaned per the owners manual. ( we clean these yearly, espically if water is hard is present) it may take care to the veriable temps in DHW.
    the picture has the correct install of the temp probe. I've seen this moved which can cause temp issues.
    if you going to add the fan loop, you'll need to add it to a current zone, one zone might work correctly and the other would still be short some BTUs... in short, you still have some short cycling going on.
    what you could also do, is run both zone at the same time.. this would put in at 34k btus, which would take of two modulations. @ 50% heat output.
    Try the 50% on the heat output. I did a rough calcuation and can up with the modulation at variables of about 16K btus, this would be just about perfect for yor heating zones. it should help alot... your still going to have some cycling, but not as much....
    Yes you can controll the heat output in the system .. functions 2.b and 0.E
  • MrFireball
    MrFireball Member Posts: 9
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    Yes, I flush the DHW side heat exchanger with vinegar every year. I need to check the filter, but I'm not concerned since it's been this way since new and we have good hot water flow. Turning 1.A down to 50% may have helped increase the cycle time, but I've not spent enough time observing it and turning on other loops to see if it helped. Thanks again for helping me figure out what was going on with my Greenstar!
  • MrFireball
    MrFireball Member Posts: 9
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    With 1.A set at 50% and the boiler temperature set to 3, the burner comes on and the boiler temperature rises, at about 1°F/2.5 sec, to over temp (134°F). The burner shuts off and comes back on after the boiler temperature drops to about 91°F, and the process repeats itself. During this process, the burner fan runs at a constant speed, there is absolutely no modulation of the fans speed. Their literature claims: "A fully-modulating fan helps maximize efficiency by ensuring that the unit operates with optimal combustion. The fan automatically increases or decreases speed according to the amount of gas coming into the boiler, meeting heat demands and saving energy." Not mine, it's being a simple bang-bang controller.
  • fenkel
    fenkel Member Posts: 162
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    you'e only using one modulation to heat your zones.. demand will increase the speed of the fan. remember this boiler has only 3.7 modulations and each mod is about 36K, we lowered the output it to 50% of that total so, ever 18 k should be a modulation.. if both zones are runiing, the fan might speed up... will you btu usuage you'll be on low speed almost all the time. The question is, has the boiler reduced its cycling time? does the boiler run longer now than it did before and is the heat curve slower to ramp up and also to ramp down? does the home get cooler or has the heat changed in the areas being heated? the main goal is to reduce the cycling down.
    does the circular pump for the zones run during the temp reducing from 131 to 91?
  • Bscott
    Bscott Member Posts: 1
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    Did you find the cause of the problem?
    I have the same problem on my Bosch combi 151p