combi bosch zwb28-3 dhw issues

sam33

Hi,

There was reduced DHW output. Essentially almost luke warm. I cleaned the main exchanger (the one with the burner) which was real bad. That helped some. I replaced the DHW temp sensor and DHW plate exchanger. That also helped some. The sensor had tons of scale on it. The exchanger not so much that I could tell. However, lots of scale throughout the system on the o-rings and seals on the heat side.

The behavior now is if I set the DHW temp to max (140 degrees) and turn on the shower (low flow), boiler DHW temp runs at 129 degrees. Temp at the mixing valve is ~112 degrees (6 feet from boiler). I've got the mixing valve set to hottest which should allow 150 degrees. If I turn off the shower, boiler temp rises to ~145. If I then turn on the shower, that hotter water will flow through the system. Mixing valve temp will increase to ~118 or so initially but then drops. The boiler temp also slowly drops back to 129 degrees. Is this normal operation? Seems to me that with one shower open, the boiler should be able to maintain 140 or close to 140.

I've got a new diverter valve flapper on order and plan to inspect the diverter valve and input DHW filter. But I'm wondering if the scale as fouled the main exchanger. Any thoughts?

thx!

Edit: The system provides ~1500sq ft radiant heat via a Rehau system. I think its Raupex pipe. Two zones upstairs, two zones down. Two bathrooms.

sam33

To add, it's 10 years old and the main exchanger has been cleaned once before.

Here's a photo of the exchanger before I cleaned it.

sam33

Picture of corrosion on temp sensor

sam33

Seemed to be slowly getting worse after all the previous work. I put in a new diverter valve paddle and no improvement although the old one was really scaled.

Intplm.

Have you used something like this to descale the inside of your unit?

As to the temperature of your shower? You do not mention what type or brand of shower valve you have. I'm guessing it's a single handle that is a pressure-balanced valve? They are designed to offer a lower temp to help prevent scalding. Replacing the temp sensing device on that shower might also help but the temp settings can only be adjusted so high. Some are designed so that they cannot be adjusted to a dangerous scalding temp.

sam33

I have not used a descaler, but I was thinking about it. The boiler also provides for radiant heat. I'm wondering if the pex is going to be scaled as well.

It's a pressure balanced valve, but I've tried other faucets as well that don't have any scald protection.

thx

Big Ed_4

Nasty , you need a filter on the hot water inlet first off . There is a fine cone strainer on the hot water inlet . I would replace the flow sensor , it comes with all the seals that you need … Good Luck

sam33

Replaced the flow sensor, right hand block with new diverter motor, and the pump.

Boiler setting is on 140. Boiler DHW temp gauge will read ~124F with a faucet on. There's a drain valve right at the boiler DHW out. If I pull some water from there, water temp reads 115F. I already put in a new temp sensor.

If I cut back the flow at the faucet, the boiler slows down (fan, etc) and the boiler water temp gauge stays the same. If I open up another faucet, the boiler(fan, etc) cranks up and boiler water temp stays the same.

Maybe a bad PCB?

Big Ed_4

Does the boiler sense the call of water and run up to 199* and you lose hot water? You may have a dirty plate heat exchanger on the boiler side .

sam33

If I turn on the faucet for a few seconds and then shut it off, the boiler heats up and eventually the DHW temp gauge shows 150F. If I turn on the faucet, the DHW temp gauge creeps back down to 124F. If I turn on the faucet and leave it on, the boiler DHW gauge creeps up to 124F, but doesn't go any higher.

One faucet on and the boiler shows 0.8 GPM, Two faucets and 1.7 GPM. At the 0.8 GPM, the boiler shows it's operating at 50%. At the 1.7 GPM, the boiler shows it's operating at 100%. In either case, water temp is 115F.

I've replaced the DHW plate heat exchanger, DHW temp sensor, the flow sensor and turbine adaptor, right hand block with new diverter motor and paddle, and the DHW circulator pump. I've also cleaned the boiler fire side exchanger fire chamber.

Unfortunately the installer didn't plumb in any valves to make flushing the main exchanger with descaler easy. System pics in this thread https://forum.heatinghelp.com/discussion/comment/1821087#Comment_1821087

hot_rod

Those aluminum block boilers need to be filled with good, low hardness water, and a can of inhibitor to protect the aluminum. Some of them ship with the conditioner to add.

It clear that you have hard water from the pictures. So you need to pump in a descaler, run for a day, flush, add good water and conditioner.

A kit like this comes with the pump and hoses, you can buy additional chemical at any hardware store, citrus acid, I believe.

Screenshot 2024-12-01 at 10.52.20 AM.png

Teemok

The term "boiler" temperature is the hydronic supply water temperature. DHW water is completely separated by the flat plat heat exchanger. Typically when a call for DHW production is created with an open faucet it is sensed by a flow switch or sensor and the boiler's control responds. What ever boiler heating temperature set-point it was making for hydronic space heating is immediately changed to 180F or above and water is shunted to the flat plate via a diverter or 3-way valve. This DHW production water supply set-point can be changed in many boilers but is not a great idea for good DHW production. Maybe your programing is not right or your control is not sensing a DHW call. 140F boiler supply is too low to make adequate DHW with a small flat plate HX. That combi is in need of a through clean up.

Edit: I see that you cleaned it and nearly rebuilt it. Confirm the settings are right. I would be looking to prove the flow sensor is being recognized. Even new ones can jam. The turbine flow sensors send a pulse signal. The broad should respond with DHW mode changes: elevated set-point and diverter on. If it doesn't check flow senor wiring and confirm a signal to the board. If those are good, I'd suspect the board.

sam33

The DHW is set for 140F on the boiler. When a DHW faucet is on, the DHW output temp on the boiler is 124F. However, if I bleed out some DHW right at the boiler via a drain valve, the water is 115F. That's only 8 inches away from the boiler. It seems strange that the boiler says DHW is 124F, but the water is 115F. It's a new sensor and I don't think it reads different than the old one.

The boiler ramps up when I open two faucets. It's clearly sensing more flow and it shows that it is. One faucet is 0.8 GPM, and two is 1.7 GPM. The boiler operates at 50% at 0.8 and 100% at 1.7 GPM. What doesn't make sense to me is why doesn't it ramp up to 100% when there is only one faucet open and the DHW output temp (124F) is less than the 140F set point.

I've been watching the temps on the heat side mixing valves. It looks like heat side doesn't see water temps over 115F either which kinda makes sense. Hard to say since I have the fw200 and outside temp sensor which I guess lowers boiler output temp based on some curve.

Based on the scale I found on other parts, the main exchanger has got to be scaled up.

I guess the next cheapest thing to try is descale. I did put the old temp sensor and paddle in vinegar overnight to see if scale would come off. Nothing happened.

Teemok

You said you have an external mixing valve. Could be a bad check valve letting cold water in to back flow in the hot where you are sampling 115F. Without service valves it's a bit harder to diagnose.

With a faucet open the boiler water (heated by the flame) temperature should ramp up to at least 180F at first regardless of what the DHW setting. It's fairly easy to prove that the boilers aluminum heat exchanger flows properly. Your boilers hydronic supply sensor might be telling a lie limiting supply output to the flatplate.

Big Ed_4

Your photos show a lot of mud on the boiler side . a sign of a slab leak or bad Pex… Boiler circulator ? There is a cap on the end of the circulator to check its rotation.

sam33

I've been trying to figure out what is going on with the display temp being different from output temp. I've found that the DHW temp on the display is always wrong. The boiler is set for 140F DHW. Input water on the cold side is 54F. I turn on one faucet with 1.2 GPM flow rate (both measured at the faucet and reported by the boiler, so flow turbine is good) and let things stabilize. DHW temp on the boiler display is 126F. Actual DHW temp 1 foot off the boiler before mixing valve is 116F. With this flow rate, the boiler shows it is operating at only 50% capacity. If I open another faucet, flow rate is 2.4, boiler reports it's running at 100% (and you can tell because fan speed, etc increases), but temp doesn't change.

I went through and tested the water temp sensor resistance and it jives with actual water temp. I let the boiler get hot, kill power, and then run the DHW while also testing water temp and resistance as cold water flows through the circuit. I think the DHW sensor is good. With the boiler on, I also tested actual water temps as it heats up versus what the display shows and the display is consistently wrong at all temperatures.

So it seems it's getting good data, but the brain thinks the set temp is maybe 126 and not 140.

The manual says not reaching DHW temp might be due to a bad code plug, but I've got to think it has a bad PCB. A code plug is $50 so I ordered one.

I'm working on getting it descaled.