Radiant Heat / Cast Iron Boiler Corrosion Problem

ALH_4

A stainless steel indirect water heater might fit that bill. Crown Megastor, Triangle Tube Phase III (inner tank is stainless), Heat Transfer Products Superstor are possibilities.

Unfortunately heat exchanger performance is highly dependent on the temperature difference across the exchanger. With 165°F in your floor loops, you dont have a lot of delta-T to work with. An indirect water heater may not be able to keep up depending on the hx design, as they are generally designed to heat relatively cool water. To my knowledge, <a href="http://www.viessmann.ca/web/canada/ca_publish.nsf/AttachmentsByTitle/dbl-vitocell-v-300.pdf/$FILE/5167-410-v2-VitocellV-300-TDM.pdf"target="_blank">Viessmann</a> is the only manufacturer that publishes detailed heat exchanger performance data for their indirect tanks. You may be able to use that data (conservatively) to infer what another tank might do based on heat exchanger areas.

If you use a flatplate heat exchanger, you don't necessarily need a stainless buffer. Put the buffer on the boiler side of the HX and you are fine. Some have used electric water heaters for this purpose.

Bob_103

Corrosion in Radiant System with Cast Iron Boiler

I'm looking for suggestions about how to correct a problem with my heating system. Here is the situation:
* Pex Tubing does not have 02 barrier
* Weil Maclain CGi5 Gas fired Boiler (133 Input, 112 output)
* Radiant system requires 160F setting to radiate from under floors.
* Significant Rust in water loop tubing
* System was newly installed in 2002
* Boiler must be set for 210 Hi limit otherwise output drops below 160 in-between cycles.
* Boiler is constantly going off on hi limit during operation.
* Boiler vent is approx 20ft long to outside wall.
* House has 5400 feet of 1/2" pex to cover 2300 s/f on two floors, with 2x4 construction and many windows.

Questions:
Q1: Can the boiler output be restricted (can gas jets be capped?) to limit output and keep the boiler running longer before hitting hi limit?
Q2: How should I correct 02 problem? Rust inhibitors? Inhibitors w/Propylene Glycol? Indirect S/S tank? S/S Heat Exchanger Plate?

Any help would be appreciated.

Thanks.

ALH_4

Lots of problems

You have some major issues. First of all, you need to add a heat exchanger between that non-barrier tube and the boiler. Second, you need to protect the boiler from low operating temps. Third, you should have some sort of outdoor reset control.

The fourth issue is that I assume you have pex stapled directly under the subfloor. This installation method will not provide you with good heat transfer between the tube and the floor. The boiler cycles because the system cannot get the heat from the tube into the floor. This is why products like Thermofin, Joist Track, Raupanel, Warmboard, Climate Track, etc were developed.

It sounds as if someone was really cutting corners when they designed your heating system. The problem is they cut more than just the corners and did not include features your system requires. You should have barrier tube, heat transfer plates, boiler low temperature protection, and an outdoor reset control. Unless you have access beneath the floor, you are stuck with the staple up tube. The rest can be fixed.

Bob_103

Lots of problems is right...

As installed currently, the mixing valve feeds back into the return so that should address the low-temp issue. The pex was installed underfloor and heat transfer plates were used.
Q1: Can you suggest a heat exchanger? Q2: You are correct about why the boiler cycles, but it may help if the heat output of the boiler were better matched to the house too. Can the gas nozzles on a CGi5 be capped to reduce output capacity? Q3: Does an outdoor reset control just lower the high limit? I set the limit high so the output water temp will not drop below 160 before it kicks back on again.

Thanks for your help with this.

Unknown

why is it so important that the output not drop below 160 before it kicks back on again? You shouldn't need 160 all winter long.

A boiler reset control can outsmart an aquastat.

Sounds like you might need a buffer tank or something as well if the boiler is really oversized.

Bob_103

If the temp drops below 160F, heat transfer radiation thru the 2nd floor carpets really slows down and does not keep up with losses thru the walls/ceilings. You are probably right though...that lower temps are not required on warmer days.

ALH_4

Carpet and radiant

Carpet kills radiant. Does the system send 210°F water directly to the floors? Higher operating temperatures also increase oxygen diffusion through your pex. The carpet is the root of the system's problems except the oxygen barrier issue. The heat cannot get through the carpet quickly enough, and therefore the boiler probably bounces off high limit. Supplemental heat (baseboard or panel radiators) is the only way to get this system to operate properly.

Flatplate manufactures nice brazed plate heat exchangers to take care of the oxygen diffusion problem. You will need all non-ferrous materials on the system side where the oxygen will destroy any iron parts.

Bob_103

Carpet is tough! A mixing valve keeps the temp under 165. A Flatplate sounds like a good solution for the O2 problem....but I'll still need to find a S/S Buffer Tank to stop the short cycling.

Does anyone make a S/S buffer tank with an integral heat exchanger that would perform as well as a flatplate?