Confused about heat-exchangers, pressurized system vs non-pressurized system

[336mlk]

I am now confused. First a little bit of what I thought was the case so please correct me if I am wrong. I thought you could not mix non-pressurized systems with pressurized systems unless you used something like a heat exchanger.

So now my confusion. The house I bought has a Central Boiler 5048 Classic which I gather is a non-pressurized system. The oil boiler in my house is a pressurized system. The outdoor wood boiler connects directly to the rear of the oil boiler (There are valves and pumps in between so I can close it off when I need to).

The thing is being I'm a noob at this I came to understand that you needed a heat exchanger and I thought the heat exchanger was just somehow inside the oil boiler. I now came to the realization that a heat exchanger is outside the boiler. The oil boiler is a Buderus. So how exactly does this work since the water from the wood boiler is feeding directly into the oil boiler. Wouldn't that change the pressure in the oil boiler or if I'm topping off the wood boiler outside isn't that affecting the oil boiler?

The reality is this is how the previous owner (who built the house) had it running for the past 10 years so what do I know. So my question is more about understanding how this works and not really if it is correct or not. I just was under the assumption that you couldn't mix pressurized and non-pressurized That being said I posted on some other heating boards and it was mentioned that I really should install a heat exchanger.

Thanks

 

[BadgerBoilerMN]

Many things can be done but shouldn't. The Buderus boiler was made for a closed system under pressure.

At the very least it will void the warranty, only available to the first owner.

All hydronic systems should be maintained most especially the heat transfer fluid. I always isolate solid fuel open boilers from all radiation if possible.

 

[jebatty]

I thought you could not mix non-pressurized systems with pressurized systems unless you used something like a heat exchanger.

A question I normally would answer "you can't mix without a heat exchanger," except that for six years (and about 10 years by a prior owner) I did operate the two without a heat exchanger, but not exactly as you describe. My OWB maintained about 2 psi pressure by using a bladder. The boiler was on level ground with the one story house and above the level of the hydronic baseboard. That 2 psi was sufficient to "pressurize" the non-pressure heating system.

The house also had a pressurized propane boiler, maintaining about 15 psi. So, to do this there were several hand operated valves to shut off the pressurized system and open the OWB supply, and the OWB would heat the system. Or, shut off the OWB supply and open the pressurized system.

I think it is possible to operate the two systems without a heat exchanger depending on the elevations and/or having slight pressure in the non-pressurized system, like that described.

 

[336mlk]

Jebatty, yes add now "someone else" who is not using a heat exchanger. Like I mentioned the person who built the house I am living in set it up and it has been running like this for 10 years as well. For my situation the OWB has a cirulator pump with valves on the supply and the return lines. The supply line from the OWB goes directly into one of the "extra" connections on my boiler. The return line of the OWB comes out the drain line of the oil boiler.
(The drain line from the oil boiler has a T fitting so one path goes back to the oil boiler and the other path goes to hose valve for draining and filling the boiler) . When I want to use it what I will do is bring my OWB up to temp first and then start the OWB circulatory and open the supply and return valves. This will start to bring the water into the oil boiler thereby bringing up the temperature of the oil boiler. Initially there is a drop in temp on the oil boiler because the water that comes in first is the cold water that has been sitting in the lines underground so the oil boiler fires up when this initially happens but within a minute the hot water from the OWB has reached the oil boiler and from there on out heats the oil boiler. The oil boiler never kicks on again unless the temp drops below it's thermostat level witch usually means I ran out of wood.

The OWB is about 100 feet from the house going uphill. There is no bladder as you mentioned and there are no other devices in the path other than the circulator pump and the close off valves. The supply and return valves and pump are located next to the oil boiler so when I close those valves I am literally cutting out everything from the OWB.

So effectively it seems like I am adding water to my pressurized oil boiler and removing it at the same time so I guess this is what is keeping my oil boiler from being over pressurized by adding too much water. I suppose if one "forgot" to open the OWB return line, but started the pump and opened the supply line there would be a serious problem.

 

[maple1]

Any idea what the elevation difference is between the OWB & the oil boiler?

Every 1 foot of elevation will make 0.4psi. So, for example, if the OWB is 20 ft above the oil boiler, and is open/unpressurized, that will create 8psi at the oil boiler with zero at the OWB. So it is possible to connect them without an HX, depending on elevations. But it is not a good idea to do that, as it will promote corrosion inside the entire system - unless a very thorough & comprehensive anti-corrosion maintenance program is in place, which typically requires regular water testing and treatment with chemicals.

 

[336mlk]

Hey Maple, yes if I had to guess the boiler is probably at least 20 feet higher than the oil boiler. It actually might be a little more. So that kind of explains some things thanks! As part of the OWB maintenance a anti-corrisive solution is supposed to be used.

 

[leon]

You have a water circulator, NOT a pump.

Are you sure you do not have a steel expansion tank
somewhere in the ceiling above the boiler??

As the CB is above the oil boiler, I can see why they
did that as the water weight above oil boiler is greater.
Apparently he was treating the water in its complete volume
with the oil boiler as well.

But that is still no reason to do that as you should have
a steel expansion tank above the oil boiler if the oil boiler
is shut off from the OWB.

 

[336mlk]

Oh yes anywhere I said pump I meant circular. Sorry about that. Yes there is an expansion tank on the oil boiler.