Bosch Combi - heat ramp up - can it be quicker?

Polanka

Combi 151, also for DWH. Original install was to all baseboard heat system. Kept the house warm (South Jersey, temps are not brutal but high teens happen, and it's a loose house), but the clicking/snapping noises of the baseboards was too much.

Since there was also central ac with decent ducting, I decided to install a hydronic coil, and it worked. Recent cold temps has me looking a bit more closely at the set up, since the house is cooler than the set temp (it dropped to 64 the other night, while set for permanent hold at 68.

When the stat calls for heat, the boiler starts up, as does the air handler blower. If it has been off-cycle for a bit, the temp in the boiler might be 150, depending on how long it's been cooling off. Once fired, the circ pump turns on, and water starts flowing to the hydronic coil, which sits on top of the A/H. The boiler temp drops a bit , sometimes down to below 140, during the first 2 minutes, at which point the burner goes into high gear and the temp starts to climb.

Monitoring the boiler output through the menu, it starts at less than 50%, and after 2 minutes starts to ramp to 90 or even 100% until the set temp is reached. The boiler dial is set to 6, not max. This gets the unit up to 177-180 degrees, which works fine to get the house up to temp, at least when it's 30 degrees outside.

When I had it set to max, the boiler temp would go to 191, but then the unit would ramp down, even though the stat was calling for heat. Air handler was still going, but the boiler seemed like an overlimit temp had been reached. Flame would shut down completely, circ pump turned off, and the boiler temp (slowly) would drop all the way below 150 before firing up again. This can take a good 10-15 minutes, meanwhile the AH is sending not so warm air to the house. Setting the boiler temp back down to 6 seems to prevent this from happening, so I suspect the unit is detecting an overheat condition.

Two questions I'd like to find answers to: 1) can I get the boiler to ramp up more quickly when the first call for heat goes out, and 2) what would be the likely culprit causing the boiler to simply quit when set to the max temp? I could see that the most recent error code set was a d3, which can mean 'temp high limit defective.' BTW, I've never seen an error code on the screen, so this could be years old for all I know. Assuming a sensor is bad, there seems to be several temp limiting sensors on the unit - is there a likely culprit, and would 191 degrees be enough to trip it normally?





The manual indicates that the boiler temp dial allows temps up to 194 degrees, but the specs say the max boiler temp is 187, which seems contradictory.

Polanka

Ironman

The problem is not with the boiler, but the AHU. You need to install an aquastat on the water line at the AHU that will delay the blower until there's hot water in the coil.

Taco makes a couple of control for this. Or, if you're tech savvy, you could use an ETC and a relay to accomplish it.

Polanka

OK thanks! Is ETC electronic temperature control? I agree the AHU starting up pulls heat from the coil, but the boiler is completely capable of oversupplying the coil (if it wanted to!) so why does it wait two minutes to fully come on line - is it doing self-checks? Once it fully amps up, after a few minutes it dials back to a lower output. Somewhere around 50% output seems to be the sweet spot that balances the heat generated with the capacity of the system to absorb it (at 32 degrees outside).

Any recommendations on Aquastats? I don't have a well obviously, so I suppose a style that lays on the output line near the hydronic coil output (taped tightly, whatever, etc) is the best option. Just to delay the blower coming on. I needed that anyway, so I'll try that to see if it resolves the issue. Would I just switch that out of the circuit in the AC season?

Thank you.

Ironman

You can limit the boiler's maximum firing rate in the control.

The ETC would be the aquastat and the relay would be to allow the ETC to control the fan in heating but not a/c.

If the integration of the ETC and relay with the AHU is beyond your skill set, then you should consider Taco's AHU control.

Polanka

Thanks Ironman. Most reasonable priced solution I can find is the White Rodgers 1127, which is a 'strap on' type sensor, and can be used to 'make' the electrical connection at any temp from 100-240F. This saves me from having to do any plumbing to install a well for example. I am completely comfortable with electrical work, but even with the Taco, which seems nice, I'd still need an aquastat, so I will start there. Another relay seems likely though, in the future. I appreciate your help.

Polanka

Temp control installed now on the out line of the hydronic coil (not actually an aquastat since it doesn't touch water) and that is really working well. Also, dialing back the boiler to only allow a max 70% keeps it from temp overshoot - and then cycling all the way down to 135 degrees, finally then back up. Still trying to figure out why it takes 2 full minutes upon heat demand to really start heating up, and also why it takes a 20 minute siesta on space heating whenever hot water is demanded. This house, at 30 degrees outside, can have a 2 degree dip in the inside temp of the house. Yes it leaks like a sieve. Not comfortable. According to the Bosch setup/install guide, it should only wait 1 minute to get back to space heating, but no, it takes 20 minutes. The setting is indeed 1 minute. I am at a loss to speed that up, so we just don't use the hot water unless the house is fully up to temp. Probably have to go to a DHW tank setup if we can't get it to not shut down for such long periods of time.

Ironman

All mod/cons go through that 2 minute (or so) trial period to prove proper combustion. That can't be changed.

Domestic water heating is given priority over space heating for a selected time that's adjustable in the control. The 1 minute setting is the change over delay, but the tank temp sensor has to be satisfied first.

fenkel

you may need to do a system reset to get this to work correctly.. are you sure you don't have the boiler set space heating disabled for 20 minutes after DHW is drawn?
check service functions:
3.A
3.b
3.E
3.F
Set all to default settings...

Polanka

Hi,

3.A. is 00, auto cycle time is disabled, which is the default
3.b is set to 01, which the manual says is the best for forced air heating - 03 minutes is default
3.C is 06 degrees, default is 18. I reduced this range to 6 in an effort to keep the boiler from overshooting the high limit. Not sure if this change had any noticeable effect.
3.E is set to 20 minutes, which is the default and the lowest setting possible
3.F is set to 01 minutes, which is the default, according to the installation manual.

Finally though, the setting for 1.F, Pump mode, is set to 00, indicating the boiler is used only for central heating, but obviously we use it for DHW as well. The manual says to never change this setting on a Combi, and that the default setting is 03 - which is Pump mode 3: A DHW tank pump is connected on the system side after the low loss header.

I don't have a DHW tank, but the default 3 does not seem correct anyway. Pump mode 1 seems correct, that "A 3-way valve for DHW tank loading is connected in boiler circuit before low loss header (see also service function 2.A). 2.A is set to 24, which is the default and which also says this should never be changed for a combi.

I have never change 1.F setting. If it was changed it was changed by the installers - a local plumbing company, and they of course knew we had no separate DHW tank. Could this setting be mucking everything up? The book does say the setting default is 3, but maybe that's just for a boiler, not a combi, but 03 doesn't seem right. That is why I had thought of resetting the whole unit.

Polanka

By the way thanks for your responses. Very helpful. Ironman, I hear you on the 2 minute hold time, that makes sense. And this morning, I think the time to go back to space heating was only about 4 minutes, much better than 20 minutes, and which is consistent with 240 seconds as set in service function 2.A. This function supposedly only works in pump mode 01, and my unit is set to Pump mode 00 (setting 1.F: Pump mode is 00). Overall the best improvement besides topping the max output to 70% was to limit the minimum output to 45%. This is the level it will run at while proving (the two minutes before full burner activation). I realize that collaring it this way is probably not good for gas efficiency, but otherwise she is not doing what we need. I may go to a separate DHW tank. If I did that, would it be expected that the supply of hot water to the DHW tank should come from a new zone/circuit off the main boiler and not the DHW section?

fenkel

Finally though, the setting for 1.F, Pump mode, is set to 00, indicating the boiler is used only for central heating, but obviously we use it for DHW as well. The manual says to never change this setting on a Combi, and that the default setting is 03 - which is Pump mode 3: A DHW tank pump is connected on the system side after the low loss header.

Why? do you have an external Indirect hot water tank? if not your system is not set correctly... Your heattronic controller is sensing its a combi boiler and thus its timing out( cause its confused as to what it should do). I would set it to default 03... if the pump after the heat loss header is used for heating, it shouldn't have anything to do with DHW. please upload pictures of that pump and also take a pictures of the wiring for that pump.. please send the wiring of that pump all the way to the boiler.. take off the cover to the wiring block as well.. possible the installer may have been mistaken on the install setup...

Polanka

Thank you. There is no external DHW tank - I meant that we are using the combi for the DHW. The external pump after the header is used only to circulate water for space heating, nothing to do with DHW.

Not sure these images loaded right, but the circulator pump is a Taco and is the only pump other than what's inside the Bosch. There is a single water circuit, no zone valves. The pump power wires (armored cable) runs into the wiring block (I did not take the cover off), but inside it is connected presumably to the black plug 120VAC line feed from the board. The pump runs when the boiler tells it to, which generally lines up when with the boiler is firing.




The only reason I didn't change the 1.F code to 03 is the warning "System damage or boiler malfunction!"
I don't want to damage the unit of course. Question is, why would the installers have changed that code, if indeed they did?

fenkel

NOTICE: System damage or boiler malfunction!
▶ Never change the setting of this service function on
combi boiler ZWB..-3A appliances...

this may be why your boiler is acting up. Never change that setting, yours is changed, boiler malfunction.... Id put it to the correct setting..
Or at least call Bosch tech support and quiz them on this..
From past experience, its very possible that this setting change can confuse your boiler...
its a fork in the road that the boiler computer cannot manage. its either go right or go left.. boiler cant do so.. so its shut down or times out..

Big Ed_4

On the front plate of the boiler there are two bolts that hold it to the boiler , Bottom left and and the second top right next to the limit sensor , which has two red wires going to it . Make sure the bolt are snug . Some came though with loose bolts . it there is leakage top right it will throw off the sensor ...

Polanka

Hi Big Ed - we just had the boiler's annual inspection service on Tuesday, and the tech took off that cover and cleaned it. I think that's probably OK but I will check those bolts. I know there is a service bulletin about them needing to be torqued correctly. I wish I had asked him about setting 1.F!

Hi Fenkel - I just went on to Bosch's website and looked at the on-line service manual for the Greenstar 151 combi. Interestingly it has a different write up for the settings at 1.F than my print copy left with me by the installers:

Service function 1.F is not used
NOTICE:
System damage or boiler malfunction!
▶ Never change the setting of this service function on combi boiler
ZWB..-3A appliances.

Now there is no guidance as to what the default setting is supposed to be, just a warning to not change it! Also on-line there is a separate service manual for non-combi boilers, and in there it says the default is still 03 - the exact same language as is in my paper service manual. So, I wrote an email to Bosch tech support. Not being an HVAC pro, I wonder if they will balk at answering what the setting is supposed to be now, but if they do I'll call the plumbers that installed it. I'll report back with findings.

Much obliged to you all.

Polanka

Polanka

I went ahead and did a full factory reset (after writing down every setting). The factory setting for 1.F is indeed 00. Bosch' response is pending.

Big Ed - I checked those 2 bolts, they are tight.

Polanka

Bosch tech came back and said that for the combi, 00 is the correct setting for 1.F. So much for the '03' that the manual indicates. I suggested they be a bit more clear about defaults (I am sure they were like 'yeah ok f-u' but at least I know now the unit is set correctly.

fenkel

How does your boiler work since the reset?

Polanka

Hi Fenkel - it is working better. Now that I know that it takes 4 minutes to return to space heating duty after literally any draw on the DHW, I am resigned to that. The outdoor temps have not been lower than 26 degrees since the reset - not that I am wishing for that, but I wanted to challenge the system at sub 20 degrees, and it seems like that situation will have to wait until next winter. Anyway, she keeps a nice steady 68 degrees, or 69 if that's what we want. I don't set back to 66 anymore, because it seems to struggle to get back to 69 in the early am, when it naturally is coldest outside.

Big improvement though is that I can run it at 180 degrees, and it's not overshooting and shutting down.

I have limited the unit to 75% max, and 40% minimum. When it fires up, it starts at the minimum (default is 34%) but set at 40%, that is where it starts up, and that usually is enough to make enough heat to get the AH going (which has an external temp sensor on the outlet pipe of the hydronic heat exchanger controlling the fan on).

Now to resolve the DHW - water is cold at first (of course, clearing the pipes), then warm, then scalding hot (145 degrees!), then lukewarm, and then after 2 minutes, finally settles to the set temp of 106.

Thanks for all assistance - it is very much appreciated!

Polanka

fenkel

Do you have a mixing valve installed in dhw system?
This device will take the 145 degree water and reduce the temp down to what you set it to..
The hot and cold you describe is called "sandwitch effect"  a very common issue with combi boilers...
You can try to reduce the flow rate out of the dhw system...sometimes this will help..

Polanka

I just bot a Watts under sink "Guardian" for the kitchen - to clamp down the 145 degree temp spike. That should help a lot with the kitchen. Probably could do something similar for bath/shower. But am considering throwing in the towel and getting an indirect tank. Nothing nicer than a shower that is warm right away, and doesn't require constant adjustment of the valves!

Polanka

JJJJace

Press the ECO button till it is green... Even though you do not have the dial to ECO it will keep the boiler from choosing DHW over sending hot water to heat for the from the boiler...

This should fix the issue for good...

Page 51 of manual...

ECO button
The default setting is DHW priority; the ECO button is not lit.
In order to switch between DHW tank priority and alternating
operation, press the ECO button.
• DHW priority
First, the DHW tank is heated up to the set temperature. The
appliance then goes into heating mode. Therefore, it can happen that
space heating is interrupted for a while and the room temperature
may drop.
DHW tank priority guarantees highest DHW comfort.
• Alternating operation
The appliance switches between space heating mode and DHW
mode. This prevents the room from cooling off too much.
Alternating operation guarantees an even room temperature with
somewhat less DHW convenience

JJJJace

The Bosch Combi is a beast and will heat water relentlessly.... I have the floor model and it produces hot water for showers from 6 am till 8:30 when the whole family is out the door while providing heat to two hydro coils in 2 air handlers... Sure way to test your output is in emissions test...
Should get to 200*




13 Flue gas test
13.1 Emissions test button
The following appliance outputs can be selected by pressing the
emissions test button until it lights up:
Fig. 80 Press the emissions test button
• = Maximum set output in heating mode ( Maximum heating
output, page 55)
• = maximum nominal output
• = Minimum nominal output

hot_rod

Is there a system sensor connected and installed on the line near the system pump.
When you have primary secondary piping there is some temperature blending in the closely spaced tees. Just because the boiler indicates 180/doesn’t always mean the system is seeing that.
A system sensor tells the boiler the actual temperature going to the AH.
I’d run the supply as low as possible to cover the heat load on a design day.
The boiler becomes more efficient as you run lower supply temperature.

If 160 f covers the load, no reason to set it to 180.

The design of the AH usually defines the required temperature and gpm to get the output you need.