How does this “heat exchanger” work? It is connected to a hydronic loop off a steam boiler

ericmhurtado

Hello,
I’m trying to understand my heating system and all of the components in an effort to improve efficiency and thus save on my fuel bill so many thanks and any advice is well appreciated.
I have posted a pic of the “heat exchanger” and have tried to research its function and I’m hoping someone can please help me understand how this work? Again many thanks.

EBEBRATT-Ed

Need more pictures of the boiler and the piping especially the boilers left side. Obviously it's a hot water zone off a steam boiler. What issues are you having?

This system is described in one of @DanHolohan's books available on this site. I believe it is in "How Come"

JUGHNE

Need to see all the pipe connections between it and the boiler, so a couple more pictures.

Does this make the hot water for your faucets or do you have a separate water heater tank?

ericmhurtado

Thanks for the prompt responses. Here are some additional pics. I have a separate electric heat pump water heater for dhw so I’m not using the tankless coil.
I’m really trying to learn how this system works but I am having issues with the high limit shutting off before reaching set point so maybe learning more about this system will help me solve this issue.

neilc

if you follow that pipe with the red circ on it, where does it go to?
post a picture of that end.

ericmhurtado

That’s the return pipe for the hot water zone and it goes to the “heat exchanger” and I’ll try to post a pic of it later

Jamie Hall

You are using quotes around "heat exchanger". Looking at it in your pictures, I expect that, in fact, it is a heat exchanger. If so, it contains tubes or other hardware which separate the water from the boiler from the water from the hot water heating system, but allows the heat from the boiler water to get into the heating system water. The separation of the water is necessary since the boiler is running at much lower pressure than the hot water part of the heating system.

I note that you also have a domestic hot water coil connected on that boiler.

ericmhurtado

Does the heat exchanger work like an indirect water heater?
My hydronic loop seems to work like this: boiler water passes through a high limit aquastat and then a circulator seems to pump it through the heat exchanger to heat up the hot water in the system. This is where I get stuck. Does the red circ coming from the return pipe push the hot water in the exchanger to the supply part of the system?

Jamie Hall

Basically, yes. The two piping circuits -- boiler through exchanger and back to boiler being one, and the house through the exchanger and back to the house being the other -- are physically separated; water does not go from one to the other. But they are in contact inside the heat exchanger, so that heat does go from the boiler water side to the heating system water side.

It is not simply a tank!

ericmhurtado

Thanks Jamie. If the system is steam heating and there is a call for hot water baseboard heat, could the circulator run hot water past the high limit aquastat set point of 180? Does the high limit shut off the boiler at 180 even though system is still calling for steam heat?

Jamie Hall

No -- the way these things work is an "or" sort of thing. If the system is calling for steam heat, the boiler will fire for steam -- that's from the steam heat thermostat(s) through the steam safeties. On the other hand, if the hot water loop is calling for heat, and the water temperature is below 180, the boiler will fire to bring the water temperature to 180. The circulator for the hot water should run whenever the hot water thermostat is calling for heat, regardless of boiler water temperature. If the boiler is making steam at the same time, the hot water in the hot water radiators will be a lot hotter than 180, but there's no harm in that.

ericmhurtado

Thanks Jamie. Any chance that the hot water can “flash” to steam in this setup?

Jamie Hall

No. Assuming, that is, that all the safeties are working as they should. The boiler water temperature will never get over steaming temperature -- 215 at 1 psi. The circulating water in the hot water system will be less than that -- although it may be very hot indeed.

ericmhurtado

Thanks Jamie and everyone else for their helpful advice. I feel more knowledgeable about the hydronic loop side of my heating system after this discussion.