How to add thermal purge to a boiler

HeatingHelp

This discussion was created from comments split from: Energy Kinetics: Oil vs Propane?.

MikeAmann

EBEBRATT-Ed said:

@EdTheHeaterMan

I used a strap on aquastat (reverse acting) on my boiler on the return line (one zone). When the thermostat call ended, and the boiler was sitting there with 180–190-degree water the aquastat would keep the pump running until the water dropped to 130 deg. Any lower and I would get some flue gas condensation on the next call for heat. The temp in the house used to overshoot 1-2 degrees . Hardly noticeable

Is a relay needed to wire that in? Got a diagram?
I want to do the same for the same reason.

EdTheHeaterMan

Mike, what is controlling your boiler? An aquastat relay like the L8148 or the L8124 or the L7224 or the Hydrostat? That is not maintaining a minimum boiler temperature, It is simple!


Wire a L6006C strap on aquastat on the returm pipe to the aquastat relay this way.


Connect R on the L6006C to L1 on the Aquastat relay
Connect B on the L6006C to C1 on the Aquastat relay
Set temperature on L6006C to 140° on fixed differential models and the circulator will continue to operate until the return temperature drops to 135°


On adjustable differential model set differential to 5°

MikeAmann

@EdTheHeaterMan @EBEBRATT-Ed
That's simple - L6006C works on line voltage. No additional relay or transformer needed!
And I have the super expensive Honeywell R8182D with Intellicon HW+ Economizer.
I have the LO limit disabled, but it still controls when the circulator is allowed to operate.

That unit saves me 31% every year by holding off the firing of the burner until the water really needs to be heated. I am glad you mentioned about >>> when the thermostat call ended, and the boiler was sitting there with 180–190-degree water... I have seen that happen more than a few times, so this will be a perfect solution to putting that heat into the house, instead of up the chimney.

MikeAmann

the aquastat would keep the pump running until the water dropped to 130 deg.
Looking at the diagram, I will assume that the added PURGE aquastat would then have to be a "switch closes on temp rise" version. That would then mean that the switch would open on a temp fall being the set point minus it's differential. That is what is needed to create this PURGE function, correct?

Left over from the old wood stove, I happen to have a L4006B 1023 aquastat. 100-240F and it's differential appears to be non-adjustable. And it is the immersion type. I think that I can use this aquastat. Can I strap the sensing bulb to the return pipe using a couple of worm type hose clamps (and some heat transfer grease) and wrap it up in insulation? And of course I would make a bracket to support the rest of the aquastat. Or would anyone have the parts to convert this aquastat to strap-on?

jringel

How are you keeping the zone valves open?

MikeAmann

jringel said:

How are you keeping the zone valves open?

The whole house is just one loop - one circulator and no zone valves. It's as simple as it gets.

MikeAmann

It's too much fabrication to do what I suggested above.
So unless someone says that this won't work, since I also have the immersion well that was paired with the aquastat, I am going to tee into the 1" copper return pipe and use a female adapter and length of pipe to set the end of the well into the flow of the return water - the same as one would do to tee in when adding an LWCO.

hot_rod

Overkill😏a strap on sensor would work fine, maybe even a quicker response less brass or copper to transfer through

MikeAmann

The only piece that I had to buy was the female adapter. I had all the rest.
So when I got out of work I couldn't help myself and started the project.
Luckily there were no hiccups and I got it completed in one night.
It's working great and it only cost me $12 and and my time.











Standby losses are now greatly reduced. I love it!
Another added benefit to doing this is now I will get longer burner on times which is not only better for the burner, but will go a long way towards reducing flue gas condensation and chimney clay tile deterioration.
Thank you @EdTheHeaterMan.

Why do I keep doing things like this to this 56 year old boiler that is original to the house?
Because the water always comes out looking like this:



So I will then assume that the water jacket is in very good shape.
And I already know that the combustion chamber is in fantastic shape.
So I will keep it going, hopefully for the next 20 years.

EdTheHeaterMan

I like the label on everything. this way when someone else needs to work on this, it will be idiot proof. But I remember someone saying that we are making better idiots now a days. And Mike, If you are a avid follower of the Holohan writings, you will know that putting arrows on the outside of the pipes is useless... the arrows need to bo on the inside of the pipes for the water to see, then and know which way to go.

MikeAmann

Again, I just observed a CFH to the aquastat. Boiler water was sitting at 105.
Burner ran for 23 minutes up to the 180 HI limit. Circulator started at around 140.
About 1 minute after the HL was reached, the CFH was satisfied. But now, with the
thermal purge function added, the circulator kept running all the way down to about 135.
Greatly reduced standby loss. I LOVE IT.

EBEBRATT-Ed

I am glad what you did worked. Next time I would put the aquastat well in the run of a tee if you can in place of an elbow

Steamhead

Nice job, @MikeAmann . That's certainly one way to do it.

However, if one is going to replace the aquastat relay on a single-zone system, the Beckett AquaSmart has the thermal purge built in. It's time-based rather than temperature-based, but it works well in my experience.

And for replacing those super-expensive R8182 controllers, Beckett makes a combination AquaSmart/GeniSys package. I put one on my boiler some time ago:

https://forum.heatinghelp.com/discussion/176551/nice-job-beckett

MikeAmann

@Steamhead
I have seen that thread before.
If I had to buy new controls, then I would consider that combination.
For me, the best part was being to make use of items that I was already storing.
Now instead of collecting dust on a shelf for years, these parts can finally save me some $$$.
I love making that happen. I have SOOOOO MUCH CRAP!

MikeAmann

Update: I LOVE LOVE LOVE thermal purge!
I have a programmable thermostat and I have it set to begin setback at 5am. I work 2nd shift.
It just so happens that the burner usually runs at about 4:50.
I watched it last night and when the thermostat stopped the CFH, the boiler temp was 165 F.
Without thermal purge, all of that heat would have been lost up the chimney.
Instead the circulator kept running for the next half-hour until the temp got down to about 135 F.
Greatly reduced standby losses. Thanks again @EBEBRATT-Ed

RascalOrnery

Can someone help me understand how allowing the boiler to be drawn down to such low temps won't cause problems with oxygen dissociation (spelling?) - also in a similar way how cold start boilers don't have problems with flue gas condensation during the initial first minutes until it's over 140 or whatever the temperature is?

To me allowing a boiler to drop to such low temperatures just creates longer burn times (which could be helpful for efficiency) and takes longer to satisfy domestic and heating loads. I understand 'heat loss through the chimney' is a concern but how is boiler health not effected by such low temps?

thanks

EdTheHeaterMan

RascalOrnery said:

Can someone help me understand how allowing the boiler to be drawn down to such low temps won't cause problems

Flue Gas Condensation will always happen on a cold start boiler. It is part of the physics of the combustion process and the cold metal that causes the flue gas to condense. You just don't want it to be there for extended operation. Let's say that the burner operates for 8 minutes before it gets up to the high limit on a small water content boiler system. for the first 2 to 3 minutes that cast iron will have condensation droplets on the surface. For the last 5 minutes of burning the flue gas temperature will be hot enough to burn off that condensation and the cast iron surface will be dry. Very hot and very dry. Then the burner stops as a result of the high limit setting even if there is still a call for heat.

That cast iron will have only the draft of the chimney drawing cold air thru the boiler past the hot cast iron. No chance of flue gas condensation here, there is no flue gas without combustion. Then the boiler temperature drops and the burner starts again. The boiler temperature is only 10° below the limit setting and therefore no condensation. This cycle can happen more than once but no more condensation.

Eventually the call for heat is completed and the circulator can stop and leave the 170° to 200° water there while the chimney draft pulls air across the heat exchanger or you can operate the circulator and put a portion of that energy in the house, and less of it in the air that is going up the chimney. Either way there is no flue gas when the burner is off, so no condensation at any temperature above 80° in the boiler. That won’t happen with just air until the boiler reaches temperatures in the upper 40°s. I don't see that happening in the winter… Do you?

Bottom line. Flue gas condensation gets burned off pretty fast on a properly designed system. Purging heat into the home or DHW is only going to save $$$ and will not have ill effects on the boiler.

Steamhead

EdTheHeaterMan said:

......... That cast iron will have only the draft of the chimney drawing cold air thru the boiler past the hot cast iron. No chance of flue gas condensation here, there is no flue gas without combustion. Then the boiler temperature drops and the burner starts again. The boiler temperature is only 10° below the limit setting and therefore no condensation. This cycle can happen more than once but no more condensation.

With a stack damper, there won't be any draft when the burner is off, so the cast-iron will cool even more slowly.

RascalOrnery

That does make sense, but then it seems to me to negate the whole rationale behind 'primary/secondary piping' or 'bypass loops' to "protect the boiler" , I can understand protecting the boiler from thermal shock but flue gas condensation is also brought up in the various resources I've read.

As long as the boiler setpoint/HL is above condensation temperature why would it ever matter if there's condensation? It will always be "burnt off" right? So I'm confused why it's regularly mentioned as a danger.

I'm also understanding all of this from a 'non condensing boiler' standpoint.

Are cold start boilers also very low water volumes usually?

Panheadsforever

Beckett heat miser

EdTheHeaterMan

https://forum.heatinghelp.com/discussion/comment/1793972#Comment_1793972

It appears that no one answered @RascalOrnery's query. "As long as the boiler setpoint/HL is above condensation temperature why would it ever matter if there's condensation? It will always be "burnt off" right? So I'm confused why it's regularly mentioned as a danger."

If you replace an old, large water content, coal converted boiler that operated by gravity (no circulators) with large cast iron radiators, It would not be uncommon for that system to have over 100 gallons of water in it. Now remove the boiler and replace it with a new smaller water content steel or cast iron boiler that has less than 5 gallons of water content, your system may have over 80 gallons of total water content. In that situation with the boiler supply connected to the old supply main and the boiler return connected to the old system return main, you could be circulating anywhere from 70° to 125° water for as long as 45 minutes before the call for heat is satisfied. you may never get the system water temperature above 130° in the fringe seasons. That would be unacceptable. The flue gasses would condense for the entire cycle because the water on the other side of the cast iron or steel would keep the combustion side of the metal beliow the flue gas condensation temperature for more than 3/4 of the burner operating time.

That is why there are bypass and primary secondary piping arrangements for large water volume system. That allows the boiler temperature to get hot enough to eliminate Flue Gas Condensation.

Back the the post thermal purge, no matter what the system water temperature is, the boiler water temperature above 130° on shut down will allow heat to be wasted up the chimney. Thermal purging into the last zone that calls for heat will have a savings. Any heat that does not go up the chimney will be money in the home. It's just that simple, when ever you can take heat from an idol cold-start boiler, and put it in the DHW tank or a room in the home. that is heat not wasted.