Thermal Shock?

I have a question about thermal shock in hot water boilers.

First is a picture of how the hot water heating system was converted and setup during the early seventies.



The house was originally heated with a coal boiler that would gravity feed through an open hot water system. The system has large iron radiators, one for each room. The installer of the oil boiler converted the system to a closed setup with a compression tank, and installed one B&G series 100 circulator. The pump was wired to run constant. When the thermostat called for heat the boiler would run until the heat requirement was fulfilled.

In 2006 a corn fired boiler was added to the system and piped as shown in the following diagram. Boiler specs here: Boiler Specifications - Installation Instructions - ldjamaizeingheat.com



The original pump on the oil boiler was moved to the supply side and is still wired to run constant. A B&G gravity flow control valve was added near the corn boiler to control water flow thru the corn boiler and the second B&G series 100 pump was installed to inject hot water into the system when the thermostat for the corn boiler received a call for heat. The pump near the corn boiler only runs when its respective thermostat receives a call for heat. The Honeywell triple aqua stat on the corn boiler controls the new pump. It turns the pump on if there is a call for heat, shuts the pump off when the boiler temp drops below 140 degrees, and cycles it accordingly as long as there is a demand for heat.

Currently the only thing limiting thermal shock to the corn boiler is the Honeywell triple aqua stat. When the return water entering the corn boiler cools the boiler water inside the unit near 140 degrees the circulating pump near the corn boiler is stopped. Is this an adequate way of controlling thermal shock and condensation in a steel tube boiler? Should a mixing valve and bypass line be used instead so actual water entering the boiler is above a certain temperature? Also will the bypassing of hot water and mixing with return water increase the time required to heat the home significantly? How fast the temperature will rise in the home from a set point.

 

I would believe the corn boiler was purchased to save fuel on the heating boiler. The way this is installed is defeating that purpose.
See this link from the manufacturer of the corn boiler.
Hydronics Drawing for Installation with an Existing Boiler System - ldjamaizeingheat.com
This will keep the heating boiler from running unless the corn boiler goes down and will save much wasted fuel.

 

It should be pointed out that the corn boiler and oil boiler have separate temperature controls. One thermostat for each unit located in the same location in the same room of the house. When the corn boiler is used as the primary heat source the oil burner thermostat is set a few degrees below the setting of the corn boiler thermostat. This way the oil burner remains off until the corn boiler fails to provide the requested heat.

Just for curiosity sake, why if ever would anyone continuously circulate water between the two boilers like the link you posted shows? If the hot water never goes out to the system radiators what is the point?

Do you have any comments concerning the use of the triple aqua stat to control the water temperature in the corn boiler versus using bypass piping and a mixing valve to mix return water with hot boiler water to control water temperature in the boiler?

 

That is why I gave the link to the drawing for fuel savings. What you are talking about the oil boiler still maintains temperature and uses even more fuel bringing the water temperature up to 140º to protect the corn burner.
You are so concerned about thermal shock on the corn burner what about thermal shock on the oil boiler?
The triple aquastat will try to maintain temperature all the time which means burning fuel any time the thermostat calls and/or the return water temperature drops below the setting on the triple aquastat.
Piped like the link the corn boiler will keep the oil boiler hot also so the oil boiler does not have to run. Besides it the the only drawing the manufacturer shows in conjunction with another boiler why would you not follow there directions?
A simple bypass works well with oil and gas systems it should work well with this application. I admit I did not read the I&O manual. Are they concerned with thermal shock?

 

I just read the FAQ from the site and they are concerned with low return water temps. They also suggest primary secondary or a dump zone.
A cast iron radiation system normally works with low water temperature. This system could heat maybe with water on the high side of 140º. Since it should be P/S piping maybe a system bypass or thermic valve makes sense here. It will not affect the system flow rate due to p/s piping.
It is a shame they do not put the I&O online. Sounds like they have reverse action built in from your original post. The way you have it piped the the oil boiler has a triple aquastat and will run to maintain temp in the oil boiler. Why a constant run system pump? What is the radiation load to the heat loss? Is the boiler making DHW?
Did you install a triple wall all fuel chimney or construct a masonry chimney?
Looks like a nice product as compared to some I have seen.

 

Actually the oil boiler does not have a triple aquastat (I guess I should have mentioned that originally). The aquastat on the oil boiler is only there to prevent an over heat situation (it has a high limit setting only). My understanding it is used to turn the oil burner off if water reaches the maximum set on the aquastat.


Not trying to be confrontational, just trying to make sense of the way they piped this setup.

First lets just look at the first drawing no corn boiler. In that setup they had the circulator running continuously. I think (I don’t know) it was an alternative to removing, re-piping, or dividing the system into separate zones with separate controls and circulators. It was a simple and cheap way to convert the gravity system. Running the circulator continuously was the only way to make sure hot water reaches every radiator. If the circulator turned off when the thermostat reached it set temperature, only the room with the thermostat would be at the set temperature. Other rooms would be either colder or warmer depending on their location in the system. This is the only reason I can see for running the system pump continuously. The boiler is not used for DHW so it never had a need to raise its water temperature more then required to reach the set temperature of the house. So I suspect in the spring and fall it may have only heated the water to 80 or 90 degrees and then turned off. This is all just speculation on my part.

Now still sticking with just the oil boiler setup, was that a proper or ideal way to run the system? I don’t think so, but again I really don’t know. Did this setup cause the oil boiler to get thermal shocks from cool water returning from the system? I would think it did, depending how hot the water is leaving the boiler. The fact it lasted over thirty years either means the boiler handled the shocks very well or the thermal shocks from cold return water were mild. It did heat the home, but I suspect it is not the most cost efficient way.

Now fast forward to the present, the owner buys a corn boiler. His cost for corn is so cheap to the point it is like buying fuel oil at less then 50 cents a gallon. He say’s hey all I need to do is pipe it like shown in the second drawing. Cheap and simple, but then realizes the manufacture of the corn boiler requires the return water be no lower then 140 degrees (to protect from condensation of boiler gases and thermal shock). Again being cheap he says how about I just stop the cold return water from entering the boiler once water in the boiler reaches that low point. To do this he uses the control on the triple aquastat (that came free with the boiler) that is normally used for maintaining minimum water temp when using the boiler for heating DHW. So he does it that way.

This is why I asked the question, is that even an acceptable way to reduce or prevent thermal shocks to a boiler. I suspect it will help stop gas condensation which leads to excessive corrosion of boiler tubes, but not thermal shock damage. Again I truly don’t know maybe water mixes enough at the entrée point so that the possibly more fragile tube weld points never see a large differential in temperature. If not I will see if I can convince the owner to spend the necessary money for a quality bypass and mixing valve setup to maintain minimum return water temperature.

This being family, it is kind of hard to go there and say hey you screwed this all up or did it wrong. Especially when he saves so much money burning corn, and I am sure I will here “well it heats the house just fine”. But after reading about thermal shock I also don’t want to see him shorten the life of the corn boiler if a bypass and mixing valve is required. I really don’t know if one way or the other is better, acceptable, or just an idea not implemented. I understand a manufacture almost always knows the best way to use their product, but sometimes they fail to or can not provide details on how to use their product in the wide variety of systems installed in older homes. This then leaves some decisions and ideas up to the installer who may or may not understand everything. This is my reason for checking up on this install and hopefully correct mistakes.

It would be great if the funds were available to redesign the system to be more efficient and heat only occupied areas of the home but currently that is not possible. For now I will settle with just trying to protect the current investment he has in the new corn boiler.


Not sure why you asked about the chimney, but it has its original clay liner, I am told it is in good condition. Even before he installed the unit I had heard about how corrosive the exhaust is from burning corn, so he did make sure the chimney was OK.

 

This is partially right. It will also bring the oil burner on if the temp is below the setting with cold return water.
Thermal shock is caused by quick temperature changes. Constant circulation reduced thermal shock issues.
They call for a triple wall vent system. They don't even want it in a clay lined chimney. It is against code to share the chimney with the oil boiler.

 

Ok.................................................................................................................................................................................................

 

rbeck your comments and concerns are welcome but seem to keep diverting from my original question. All of your remarks are taken seriously and will be investigated in more detail, but at this time I wish to just discuss thermal shock and wondering if someone had ever seen a system controlled in that manner. There seems to be plenty of information available about those other items but I have yet to see or talk to anyone about controlling water temperature in the manner (and reasons) described previously.

Thank You,

WB

 

Probably to reduce thermal shock. Remember that being a corn boiler, the heat source can't simply be turned on and off... there's ALWAYS a fire in there. Yes, it modulates, but there is always fire.

Why would the hot water never go out to the radiators? All of the distribution piping remains intact and uses the system pump to take the water out of the oil boiler (HOT) and to the system...

By running constant circulation, and using the existing boiler to provide HYDRAULIC SEPARATION between the distribution system and the corn boiler, the cool water returning to the oil boiler from the system won't directly enter the corn boiler. It is being 'buffered' by the mass of the oil boiler, and the water in it. One thing that I find odd with the drawing on the web page though... MOST modern boilers don't have an 'extra' set of tappings that can be used the way they are showing.

The way the system is currently piped, if the oil boiler is not firing, cool return water will pass through it, and right into the corn boiler.

Also, the system pump was moved... why? Now there are technically two pumps in series... no point in that I can see.

 

Let's talk about some alternatives... I think this drawing would serve a few objectives:



The pump on the system loop would run constant. This would provide even heat throughout the home.

The pump on the corn loop could be controlled, but probably should run constant, per manufacturers recommendation. IF it were controlled, I think the idea of using the triple A'stat has some merit... there are a couple reasons why it might not be necessary to control it at all though...

1. The water returning from the system will be tempered by mixing with the hot output of the corn boiler before entering the oil boiler.

2. As this water passed through the HOT oil boiler, it will pick up some additional heat there before passing to the corn boiler.

3. If the pump on the corn boiler is allowed to stop, there MUST be a 'dump zone' available to deal with possible overheating issues that will arise. This 'dump zone' can be the home distribution zone... and controls arranged to override the home's thermostat and push the excess heat out of the boiler into the radiators.

The pump on the 'bridge' (pointing upward in the drawing) could be controlled by the thermostat in the home.

There are many details missing in this drawing, mainly all the controls... it's for concept and discussion only... but it's a start.

It would be a huge help if there were a PDF I&O manual available for the corn boiler.

 

An addition... there needs to be a check valve on the return line from the closely spaced tees on the system loop, before the supply from the corn boiler joins it.

 

NJ Trooper your design makes sense, simple to control, something the owner I suspect would like. I suppose using the low setting on the triple aquastat to try and control thermal shock to the corn boiler is apparently just wrong. It was done that way just to cut installation cost. It just struck me as strange the way he was using the triple aquastat and was curious if others have seen this before.

The setup already has a dump circuit, like you stated the fire just doesn’t go out when the call for heat is gone. For now I will just talk to the owner, maybe this spring he will make changes. If I remember I will drop back in then and give an update.

Thank you and Happy New Year everyone.

 

I wouldn't say 'wrong' exactly... but perhaps not thought fully through. It's sometimes difficult to envision ALL of the various operating conditions, and properly design for them.

That's why I came back and noted the extra check valve in my drawing... I realized it when running the 'conditions' in my haid. I realized that _some_ of the flow between the boilers would flow through the upper loop without that extra check valve, causing constant heating of the radiators.

Rather than rely on the low limit for the pump control as a PRIMARY control, it is probably not a bad idea at all to leave it wired up as a SECONDARY control, where if say the boiler ran out of corn, the fire would go out, and shut the pump down.