Bosch Greenstar Combi 151 Not Maintaining Temp

hibby50

Hey All, I realize this question/issue has been posted many times before, but I've read through most of the existing info and can't figure out my problem.

I have a Bosch Greenstar 151 that does DHW and Baseboard heat. It has an fw200 mounted in the boiler, and is controlled by a nest thermostat. The issue is the boiler is not keeping up. I woke up this morning and heat was set to 65 with the nest upstairs reading 63.

I have changed the min outdoor temp on the fw200 to 32 degrees, and the outdoor temp on the fw200 is reading 30, but the supply temp is still only at ~130-140F. I see it spike up to 160 sometimes, but its rare and obviously not often enough. Max temp is 186 with design temp at 180. Heat loss was set to 50% (probably the default) I moved it to 40% since its an old leaky house.

The dial is set to comfort mode.

Bottom line is ive been messing with the settings and I can't figure out why the supply temps are so low most of the time causing the boiler not to keep up.

I appreciate any help. Thank You!

hibby50

Also worth noting I've thoroughly bled the system from the boiler (shut off the recirc valv and bled with a hose) while using the bleed function several times

Ironman

Get rid of the Nest! It's the absolute worst stat that could be used with a modulating boiler. It fights with the design and logic of the boiler. Get a regular hydronic thermostat, adjust the CPH to 1 and set it at one temperature and leave it there.

Jamie Hall

Most likely the problem is that your Nest is commanding a fairly deep setback temperature, and the Greenstar is controlled by an outdoor reset -- and the two are simply not compatible. If you are using a deep setback, you will have to set the Greenstar for a higher operating temperature to recover in a reasonable time, and disable the outdoor reset, or tell the Nest to get lost. The latter -- telling the Nest to get lost -- will save fuel.

hibby50

Ironman said:

Get rid of the Nest! It's the absolute worst stat that could be used with a modulating boiler. It fights with the design and logic of the boiler. Get a regular hydronic thermostat, adjust the CPH to 1 and set it at one temperature and leave it there.

Thanks so much for the responses guys! Do you have any thermostat recommendations? One that works with wi-fi/google would be fantastic since I have central air on it too.

Also, there are only 2 t-stat wires running to the nest from the boiler, will I need to wire up more?

Jamie Hall said:

Most likely the problem is that your Nest is commanding a fairly deep setback temperature, and the Greenstar is controlled by an outdoor reset -- and the two are simply not compatible. If you are using a deep setback, you will have to set the Greenstar for a higher operating temperature to recover in a reasonable time, and disable the outdoor reset, or tell the Nest to get lost. The latter -- telling the Nest to get lost -- will save fuel.

How would I go about disabling outdoor reset and setting a higher operating temperature as a temporary solution?

Jamie Hall

That would be in the control menus and options on the boiler -- and I'm not familiar enough with the Greenstar to tell you to push this or push that. On the other hand, telling the Nest to stop trying to be so smart and just hold a constant temperature -- is in the Nest's menus and not that hard to do. I'd go that route.

hibby50

Jamie Hall said:

That would be in the control menus and options on the boiler -- and I'm not familiar enough with the Greenstar to tell you to push this or push that. On the other hand, telling the Nest to stop trying to be so smart and just hold a constant temperature -- is in the Nest's menus and not that hard to do. I'd go that route.

I think thats the True Radiant setting in the nest your talking about, I'll give that a shot.

I'm doing research on the thermostats and not finding anything I like, I see the Nest supports opentherm for modulating boilers, and they sell converters in europe to convert opentherm to the bosch protocols (or there's a make your own with a board I could try out). But I really don't wanna do all that. I'll try turning off the True Radiant on the Nest to see what happens in the meantime while I research t-stats.

Big Ed_4

With running an outdoor reset you can run heat without a thermostat .. You can program any setback on the Bosch control . Just a simple thermostat will work best , just for a high limit and a sense of control ..

hibby50

I understand the nest is the most likely culprit here, and I am planning on replacing it, still doing some research on what I want to replace it with, but I have some more info that is suspect.

All of the nest learning stuff has been turned off, and I turned off the true radiant feature as well (though I don't think this was doing anything since the nest has no schedules). I woke up this morning and again the hose was cold with the supply temps topping out at 130.

I've found that if I reset the boiler, it starts pumping out 170 supply temps for a few days before falling back into this 130 degree supply situation. As far as I can tell the nest is operating as a simple on/off Tstat right now, and the dial on the fw200 is set to comfort. Is there something I'm doing wrong? Or is it still definitely the nest screwing up the system.

Jamie Hall

No, this time I don't think it's the Nest. You've managed to quell most of its unfortunate tendencies!

This time I think that something, somewhere, is amiss in the boiler programming. If the Nest is calling for heat, and the boiler isn't ramping up to provide it (though it's clearly capable of doing so), I'd start looking around at some of the almost equally perverse programming features (intended, of course, to save energy) on the boiler.

And honestly, I don't play with those things, so I have no idea where to start.

hibby50

So I think I found the fix.

I went into the service menu of the combi and saw the outdoor reset was only commanding 130 degrees,

Then I want into the fw200 info page and saw the max temp for the combi was 130. I turned the dial on the combi from max to 6 and it changed to 167, then put it back to max and it went to 190.

Maybe my knob is physically broken somehow, but immediately the boiler started cranking out heat.

EdTheHeaterMan

You don't need to throw the NEST away. You can program the NEST to a smaller setback. Your boiler will react the same to any thermostat with a setback feature. So changing from NEST to say Honeywell or ecobee will have the same result. The boiler can't catch up as quickly when the boiler is set up for maximum efficiency.

The Nest will eventually learn to start very early to get the temperature back up to the desired "Occupied" setting. The NEST will work better with a common wire attached since the NEST battery recharges when the thermostat is not calling for heat when only 2 wires are used. The lower temperature boiler water will make for long cycles where the thermostat will be calling for heat and therefore the battery won't be getting a charge.

There is one thing that will help with your dilemma. I think there is a setting on the BOSCH that increases the boiler temperature if the call for heat lasts too long. I may be mistaken on that point

Big Ed_4

What is the outdoor temperature reading ? Is it with range ?

hibby50

Outdoor temperature reading is 30. Everything seems to be working well after I flipped the knob from max to 6 and back to max

EdTheHeaterMan

Ironman said:

Get rid of the Nest! It's the absolute worst stat that could be used with a modulating boiler. It fights with the design and logic of the boiler. Get a regular hydronic thermostat, adjust the CPH to 1 and set it at one temperature and leave it there.

hibby50 said:

So I think I found the fix.

I went into the service menu of the combi and saw the outdoor reset was only commanding 130 degrees,

Then I want into the fw200 info page and saw the max temp for the combi was 130. I turned the dial on the combi from max to 6 and it changed to 167, then put it back to max and it went to 190.

Maybe my knob is physically broken somehow, but immediately the boiler started cranking out heat.

Are you going to get rid of the NEST? according to many on this site, they are the cause of all evil!

Ironman

Setback works contrary to the logic of a mod/con with ODR. It does NOT save energy, but actually uses more while sacrificing comfort.

Bosch makes it very clear in their training not to use a Nest with their equipment.

No Ed, the Bosch does not have a boost feature that increases SWT with time. That feature is generally not found on European equipment, but American because we've been fixated on using it with bang bang forced air for decades.

hibby50

EdTheHeaterMan said:

Get rid of the Nest! It's the absolute worst stat that could be used with a modulating boiler. It fights with the design and logic of the boiler. Get a regular hydronic thermostat, adjust the CPH to 1 and set it at one temperature and leave it there.

So I think I found the fix. I went into the service menu of the combi and saw the outdoor reset was only commanding 130 degrees, Then I want into the fw200 info page and saw the max temp for the combi was 130. I turned the dial on the combi from max to 6 and it changed to 167, then put it back to max and it went to 190. Maybe my knob is physically broken somehow, but immediately the boiler started cranking out heat.

Are you going to get rid of the NEST? according to many on this site, they are the cause of all evil!

Ironman said:

Setback works contrary to the logic of a mod/con with ODR. It does NOT save energy, but actually uses more while sacrificing comfort. Bosch makes it very clear in their training not to use a Nest with their equipment. No Ed, the Bosch does not have a boost feature that increases SWT with time. That feature is generally not found on European equipment, but American because we've been fixated on using it with bang bang forced air for decades.

I am planning on getting rid of the Nest given all this new information. I just want to find a solution that has smart home features integrated like I mentioned. I work with computers and it seems there are some ways to convert the EMS protocol that Bosch uses with heatronic to the opentherm protocol nest is compatible with using custom circuit boards and software. I need to do some more research on that.

I also saw the BCC100 Bosch makes but I'm unclear whether or not that's compatible with the system.

The interim solution while I explore other optioms  is pretty much a heavy outdoor reset curve and the nest operating as a simple on off switch with no setback or scheduling enabled.

The important thing is the house is comfy now, but long-term I want to make sure it is energy efficient and extending the live of the boiler.

hibby50

In case anyone is curious here's what I'm talking about, it's pretty cool! https://github.com/proddy/EMS-ESP

EdTheHeaterMan

Go it. I'm not up to date on the Bosch most recent stuff. My experience with training on new stuff ended about 4 years ago. I'm retired now and it is time for the youngins to take over the digital world.

Give me a good mercury-filled T87F and I'm comfortable!
I even liked the TM11A2 art deco series 10 thermostats with R W B terminals connected to the stack relay.