Need expertise on Bosch Greenstar Combi 151

RLawrence41

Hi folks,

I have a Bosch Greenstar Combi 151 (wall mounted unit). It's a little over a year old. I'm a pretty heavy duty DIYer, but I may be in over my head here.

The unit failed to heat the water or the house when the heat came on in the fall. We have radiant heat tubing (black Onix tubing) in the floor. Apparently, this tubing deteriorates slowly, and is clogging up the heat exchanger in the boiler.

I have had 3 certified installers here. The original installer has left the state. The other two knew about the problem, but have had no solutions to offer. What's more, the local Bosch distributor doesn't want to talk to me, the home owner. They only want to talk to their certified installers.

One of these certified technicians did install faucets that allow me to flush the boiler. I have done this, and shut off the heat zone. The domestic hot water is working great.

So, here is where it gets wonky. The only solution other than replacing the radiant heat tubing is to isolate it from the boiler. A brief look at heat exchangers told me that these are very expensive. Consequently, I have built a custom heat exchanger. (Please don't laugh. I've very proud of this accomplishment.) See photos.





Having said this, my system is still not working.

The problem is that I am getting heat in the send pipe from the boiler, but it is NOT making it TO the heat exchanger. Here are a couple of shots of the heat exchanger plumbed in, and the overall system...




I have pictures galore! It's been a challenge for me to truly understand this system and map it out. So, if something else is needed, I probably already have a photo.

One more note. It was clear that separating the radiant heat from the boiler took the only significant check valve out of the circuit. I found that the boiler was heating the return line and not the send line. So, I have installed a check valve on the return side since the above photo.

The heat now seems to be going in the right direction, but it makes it to the top of the heat exchanger plumbing. It does NOT make it into the heat exchanger.

My questions:

1. Is the internal circulating pump for the boiler strong enough to push hot water through this circuit? The only thing on the circuit now is the heat exchanger tank.
2. Is there the possibility of an air lock in the heat exchanger preventing the flow? It seems like the Spirovent would eliminate that, but this is where I bow to better expertise than mine.

Any help would be GREATLY appreciated. I don't feel like I can call another certified installer or Bosch for help.

EBEBRATT-Ed

you piping going to the hx is way to small for starters. I don't think you pumps are capable of moving water against the head pressure in the hx coil

We need to know how much heat you need to move

also is the boiler water going through the coil or the shell?

RLawrence41

Boiler is going through the shell. I figured the tank would heat the coil better than the other way around. But right now, I am only worried about getting heat to the tank. The radiant side is a separate concern.

RLawrence41

According to an online BTU calculator, I need 66,976 BTUs. The radiant heat tubing is for the first floor of our house. The Taco circulator has done the trick for years.

Ironman

I can’t follow your piping from the pics. Can you draw it out or post better pics?

Also, that small (1/2”?) piping at the HX ain’t gonna cut it. Too much resistance to flow.

pecmsg

What’s with the 5 gal soda syrup can?

RLawrence41

These start from the bottom of the boiler, so you can see the connections. The bioler supply runs closer to the wall to get to the back of the heat exchanger. The return line is in the front, and has the feeder system and filter. Shutoff has the green handle.






These are prior to completely pluming the heat exchanger in, but may give you a better idea of the leads. The boiler side feeds the tank from the back, and returns to the boiler on the right top.






I have been worried about the size of the inlets as well. In case these were too restrictive for the pumps, I installed the bypass valves on both sides of the heat exchanger. But as I say, the heated water doesn't seem to make it into the tank even with the bypass valves completely shut off.

I just took another look at the setup. I drained the tank and refilled it. There's a convenient pressure relief valve in the top of the tank. I lifted that until I got water coming out. Warm water started to fill the tank, but was quickly overtaken by the cool water from the filler system. Once full, it doesn't seem to warm up.

Ironman

You’ll need to eliminated the bypass; water takes the path of least resistance and all your flow will go through the bypass.

I still can’t follow the piping.

RLawrence41

Well the bypass is shut off anyway, and I expect to keep it that way.

Does this help?

RLawrence41

I HAVE HEAT!!!

Here is my addition to remedy the situation with no heated water making it to the tank...



After some of our discussion above, I realized that the problem is not so much the size of the inlets. It's the fact that the pipes surrounding the boiler are so large! The pipes coming off the boiler are 1 1/2 in copper. It graduates down to 1 1/4, and finally down to 1 inch for most of the run. I never really understood the reason for these large pipes, other than a vague notion that you want some capacity surrounding the boiler. But I realized that these pipes probably hold more water than the 5 gallon heat exchanger! That convinced me that the pump is necessary. Fortunately, I already had one.

RLawrence41

After all of this, I felt the need to document my system. In case it might be helpful to others and our discussion, here is my diagram...

Ironman

I’m glad you got it going.

Your entire problem was that you didn’t have a circulator in the main loop. I suspected that but couldn’t tell from your pics.

The boiler’s internal circulator only moves water through the closely spaced tees; it won’t pump the main (primary) loop. It’s called primary/secondary piping which is a means of providing hydraulic separation between two loops while heat is transferred between them.

RLawrence41

One more thing. Thanks all for being here, and for taking the time to comment. It really was great to have some people to talk to about this problem. I was definitely pressing the limits of my plumbing knowledge.

RLawrence41

Thanks Ironman. Yep. Isolating the radiant heat meant that I moved the existing circulating pump to the outside of the heat exchanger. So, the only thing pumping into the heat exchanger was the internal pump in the boiler. My initial question was whether that pump was strong enough; but after thinking it through, it doesn't matter how strong that pump is. It would have trouble moving water from those big pipes into the tank.

Ironman

Back to your original issue with debris from the Onyx tubing: I’d pipe the Onyx side to the reservoir side of the can, not through the coil as your diagram shows. That way you can open it up and clean the debris out.

RLawrence41

LOL! I've gone back and forth on that question. Actually, there are cleanout faucets on the radiant feed and return. But, JUST IN CASE, I designed the ports on the heat exchanger so I can turn it 180 degrees and switch the tank and coil feeds if I need to. Just prior to installing, I decided that I would get the most heat by filling the tank and letting it heat the coil. I plan to insulate the tank, but wanted to get the system working first.

Ironman

You might wanna look at putting a Caleffi DirtMag on the return line from the floor to filter out debris before it gets to the HX.

https://www.supplyhouse.com/Caleffi-546306A-1-NPT-5463-Series-DIRTMAG-Dirt-Separator-with-Magnet

RLawrence41

Thanks but there was one already on the boiler side. (It's in the drawing and you can see it in the photo.) That clearly did not do the job. I think I will rely on flushing this system periodically for now. I don't see the debris as a problem for the heat exchanger.

RLawrence41

I have a new issue with a pressure relief valve on this system. Should I put it here or start a new discussion?

Ron

HeatingHelp.com

https://forum.heatinghelp.com/discussion/comment/1822348#Comment_1822348

Please put it here @RLawrence41. Thanks.