SuperHot boiler issues

JohnJay1

Hi everyone,
Hoping someone can help. We replaced an old Burnham boiler with a new Super Hot. The boiler is piped the same as the old boiler just adding an air separator. It's 1" piping supply and return to the boiler as was the old one. It's a one pump system feeding infloor radiant with a mixing valve. 

First question, the boiler seems to overheat and retain heat for a while before it cools down. It's set at 180 and once boiler reaches set point, it continues to climb to 200 or so and stays there for a while. It discharged relief valve also every so often. Zones are flushed of air and pump is working and sized the same as last boiler. 

Second question, where would be the best place to wire the high limit aquastat for the infloor temp?

Thanks in advance 

EdTheHeaterMan

Do you have the Saturn series or the Mini? What size boiler? How was the size selected? Did the installer select the boiler based on the heat loss of the building? The above is probably too late to ask but, a 200,000 BTUh boiler can heat a building that only requires 50,000 BTUh. A 50,000 BTUh boiler can also heat that same building, and it will do it more efficiently.

As far as placing an additional aquastat to regulate the boiler temperature, that is not a bad idea, but why not just lower the one that comes with the boiler to 170° or 165°?

How old was the Burnham boiler? If it did not last more than 25 years, then there is a possibility that the piping design was incorrect and was part of the reason for a premature failure. That said, You mentioned that the new boiler is installed the same way, so you can expect the same result.

I wonder if it can be made better before it is too late?

JohnJay1

The old boiler was 96k Btu and lasted 25 years. Was actually still going strong when replaced. The new boiler is 100k btu. 

It is sized correctly for the home. 

The radiant aquastat is a high limit aquatstat set to 140 to shut down the boiler in the event that the mixing valve fails. The boiler has its own on board limit control set at 180.

EdTheHeaterMan

I wonder how the boiler return temperature will ever get above 135° if the limit is set to 140°. Did you read the yellow highlighted portion of the manual I posted? Does your original piping to the Burnham boiler and the new Super Hot boiler have some bypass or other accommodation to maintain the recommended 135° minimum return temperature to prevent flue gas condensation?

My recommendation would be primary secondary piping. Others might recommend a Hydraulic separator. That is where you might locate the secondary limit if so desired. The ides is to keep the boiler water at a high enough temperature (like 150° or higher) to maintain a 135° return temperature with a convienent path for that 135° water to get there. If your only return water to the boiler is from the low temperature loop return to the mixing valve, then you will never get a high enough return temperature to prevent flue gas condensation.

@hot_rod has the best ideas on this subject and the proper parts and material to implement them economically. You might need to add a circulator for the system and keep the existing circ. for the boiler loop. A buffer tank may also be in order. The problem you are explaining sounds like the older Burnham boiler had a higher water content than the newer Super Hot Mini boiler. That smaller boiler may be the reason for the large overshoot... there is less water to absorb the stand-by heat after the burners shut off and the circulator pump stops.

You may just need to have the circulator post purge for a few minutes after the burners shut off. You can do that with a reverse aquastat.

JohnJay1

Bypass is installed yes. Thanks for the info. I think you're right regarding the smaller boiler. I will try adding another pump on the boiler loop. 

JohnJay1

However, the pump is still running during the overshooting the call for heat is still active but the boiler has just cycling.

JohnJay1

It's a 15-58 at the moment. Do you think a larger pump would help?

EdTheHeaterMan

JohnJay1 said:

Bypass is installed yes. Thanks for the info. I think you're right regarding the smaller boiler. I will try adding another pump on the boiler loop. 

here are some diagrams that may help explain the wrong and right way to add a bypass to protect the boiler from cold water
# 1 is not going to protect the return water temperature
# 2 will protect the return water temperature
# 3 is the diagram that shows how to get 2 temperatures from one boiler and is the source of the diagrams i doctored up. This is not the only way to do it, That is why I was hoping @hot_rod might offer some suggestions.

Is it possible to post some photos of your piping design... to see if there is an easy way to get you where you need to be. I'm all for doing it at the lowest cost possible. Others here, well they like the gadgets and bells and whistles that you may not actually need but are pretty cool to play with.

EdTheHeaterMan

JohnJay1 said:

However, the pump is still running during the overshooting the call for heat is still active but the boiler has just cycling.

You could just lower the limit on the boiler from 180° to 160°. then the overshoot will end up being lower also. I still believe that you need some type of hydraulic separation from the system pump and the boiler pump in order to protect the boiler. The manual from Super Hot is not as specific as other manufacturer boiler manuals. I guess they assume their customers know more about this stuff than the average residential plumbing and heating company.

hot_rod

You need a valve or assembly that can measure return temperature at the boiler and respond. A thermostatic mix valve is a simple, effective method.
Also a second valve for mixing the radiant.
There are a number of other way to do it with variable speed pumping or a motorized 4 way valve.

https://www.caleffi.com/sites/default/files/coll_attach_file/idronics_19_na.pdf

JohnJay1

Hi everyone,
I was wondering if anyone would be kind of enough to forward me a piping schematic. I have a mid efficient Super Hot MG 100 boiler serving a radiant infloor heating system.
The old boiler was piped with one pump and mixing valve on system. The new boiler works ok until only one zone is calling for heat. Then the boiler overheats and retains the heat for quite some time before it begins to cool. If more than one zone is calling, it seems to be ok. The old cast iron boiler wasn't an issue.
Also, can someone advise where the best place would be to wire in a HL aquastat for the radiant supply?

Thanks in advance !

Mad Dog_2

Can you take some good pictures and post them?  Mad Dog 🐕 

EdTheHeaterMan

hot_rod Bob Rohr offered you 2 designs and I offered you one that will work. There are more designs that may work for you. Can you post a photo of your existing design, there may be an easy fix with minimal repipe that will fix the problem

From your explanation of over heating when only one zone is operating, sounds like there is not enough flow thru the boiler whenever there is only one zone operating. This will lead to longer burner off cycles during a call for heat. the high limit will shut the burner off but there is insufficient water flow to dissipate that heat unless there is more than one zone open. Your bypass may not be configured correctly to manage the heat. You may need to add a buffer tank in order to compensate for the lower water content of the replacement heater. There may be another issue. Hard to know without seeing what you have.

Maybe you can draw a detailed piping schematic of your boiler room piping with pipe diameters listed. What ever way is easiest for you to show what and where all the components are.

By the way, both of your Discussions are related to the same problem. Perhaps @Erin Holohan Haskell can merge them together in order to keep the confusion to a minimum.

Also, can someone advise where the best place would be to wire in a HL aquastat for the radiant supply?

. What would be the purpose of this high limit? Would it be to stop the burner in the event of a water temperature above a given set-point temperature? Would it be to stop a circulator from pumping heated water to a low temperature zone in the event of a mixing valve failure?

What you want to LIMIT would determine where to locate it on the wiring schematic.

Also What temperature would this limit be sensing? The supply water direct from the boiler supply outlet?... The supply water to the zone as it leaves the Mixing Valve MIX port? Or is it measuring the minimum return water temperature to the Boiler Return to insure the minimum 135° water temperature is always present in order to prevent condensation of flue gasses?

We need more info in order to help you. OR... if you are ready to start from scratch, we can offer a low price design, or a medium price design. or even an expensive design that has all the bells and whistles to measure and adjust the low temperature with injection pumping kits, and hydraulic separators that will insure proper return temperature to the boiler and buffer tanks to solve the problem of short cycling.

What are your budget constraints and your goals?

Mr. Ed

Erin Holohan Haskell

@JohnJay1, I've merged your two discussions into one here.

EdTheHeaterMan

@JohnJay1... Can you follow up with a picture(s) or diagram? I believe we can help, but we need more info.

Without the additional information... I can only speculate on the best location for the limit control. As far as physical location, I would say the best place is within the first 12" of pipe exiting the MIX port of the mixing valve. As far as the place to cut in the wires on the limit control I would say to break the burner circuit in series with the flame roll out switch and the existing limit control. See the diagram below and the enlarged section in the attached file

The Red circled section is the auxiliary limit location. This will stop the burner only if the tempering valve fails and the water temperature to the radiant mix port exceeds the temperature setting. This will also allow the circulator to continue to operate as long as there is a call for heat.

Hope this helps

JohnJay1

Thank you for your feedback. I did wire into the vent spill switch. I also ended up piping the boiler primary/secondary. I will post pics of before and after.