Replacing NG boiler and piping a bypass in a converted gravity system

mariusz6100

I have an older natural gas boiler (1990) for radiant heating (cast iron radiators). The system is a converted gravity system. The current boiler (AO Smith HW-160P, chimney vent) produces a lot of carbon and it looks like the heat exchanger may be wearing out.

I am thinking about replacing it with a new boiler (Burnham 205E) and I wanted to get input on the piping. I attached the exiting piping diagram with my new proposed diagram and I wanted to get your input to see if I am missing something.

I am also thinking replacing the Filltrol expansion tank with an Autofill Valve and the Air Scoop with Spirovent. Lastly, I am thinking about adding ESBE Thermostatic Mixing Valve on the return side.

Thank you in advance for your feedback!

SuperTech

Put the circulator in front of the expansion tank and air eliminator.  The expansion tank is the point of no pressure change when the circulator is operating.  Pumping away from it will help with air removal.  The bypass is a good idea. An outdoor reset controlled mixing valve like the Taco I-series would be nice. I would even consider piping it primary/secondary to keep the return temperature above 130. 

Why did your current boiler soot up? Is your chimney/venting ok?

Jamie Hall

If your boiler is producing a lot of carbon, has anyone cleaned the heat exchanger and the burners and adjusted the combustion with instruments? You may not need a new boiler so much as a thorough tuneup...

mariusz6100

I did not have combustion adjusted. I called to see if the heat exchanger could be cleaned and I got a quote for $900 based on the pictures I sent. This was just to open it up and try to clean, and the tech said it may not even be salvageable. I figured in that case, a new boiler may be a better option. I had the chimney swept a couple of years ago, and It should be functioning, but may be worth checking again.

Here is the picture of the heat exchanger and the coils from inside the boiler, and the current set up.

Jamie Hall

Looks like the poor old girl hasn't been maintained -- they should be cleaned at least every two years, and yearly is better, and the combustion adjusted.

Sorry about that -- I'm thinking your tech. is correct...

EBEBRATT-Ed

I would change the boiler.

The pump needs to go between the outlet of the expansion tank and the new boiler. For boiler protection you need a three way valve with a sensor. The sensor gets mounted on the return pipe to the boiler, that is the only way to protect the boiler.

The three way valve (there are 2 types mixing and diverting) you need a mixing valve (2 inlets and 1 outlet)

If you dont sense the boiler return temp it will not protect the boiler

mariusz6100

Is the Taco I-series mixing valve a better option than ESBE Thermostatic Mixing Valve or Caleffi Thermostatic Mixing Valve?

I also have a question about temperature. I have 130 at the air scoop (one of those big Purgetrol from AO Smith that is attached to the supply output from the boiler and has the temperature gauge). I have 110 right after the air scoop and 96 on the return line. The system is a single zone with 9 cast iron radiators. What should target temperatures be?

426hemi

That boiler is shot that’s all not soot it’s corosion 180-190 output from boiler 150-160 on the return ideally I’m surprised you get any heat at 130 output 

EBEBRATT-Ed

@mariusz6100

The idea is to keep the return going to the boiler above 130 and not being below 130 for an extended period of time

jumper

Good advice above. I suggest making bypass full size and leaving valve open. Butterfly is best for fiddling.

I believe that the old AO Smith type boiler requires circulator going full time. Was it?