Backup oil-fired boiler in series with a GARN, or something else?

[foxt]

I finished the install of my GARN last January, and for convenience sake had piped it in series with my existing oil-fired boiler. I just read JimK's post about the efficieny gain he has seen after taking his oil-fired boiler out of the loop completely. So, sitting here late in October, I am wondering if it would be worth it for me to repipe my setup and take the boiler out of series?

The house is served by a manifold loop with secondary piping feeding all of the zones. That manifold loop circulates through the boiler. The GARN is connected to the manifold loop via an hx that is plumbed in to return side of the boiler (see simplifed, crude, amateur drawing). Here's a simple explanation of the controls ... When the GARN falls below my set point and the house calls for heat, the manifold circ comes on and the boiler fires and the calling zone circ also comes on. I shouldn't have much loss through the hx because nothing is flowing through the GARN side of it. When the GARN is at or above set point, the manifold and GARN circs come on. I am sure I have loss through the boiler which is being heated by the GARN in this mode.

When I plumbed this, I did it in such a way so as to make it easy to modify it by connecting the boiler to the manifold loop via closely spaced tees:

In this setup, when the GARN is at or above set point, it operates just like above. When the GARN is below set point, the boiler needs to supply the manifold loop, and here is where I am in over my head. How do I need to control that new boiler circ? And does this setup have the potential to boost my efficiency enough to warrant the repiping? Is a parallel configuration a better alternative?

Tom

 

[Gooserider]

Parallel is better, but I don't know just how much heat you are losing on the way through the boiler to be able to tell if it's worth changing the plumbing... Perhaps you could do temperature measurements on each side to tell how much heat is getting dropped across the boiler when the manifold circ is running? Combined with info on your flow volume, that would tell you how much heat is being lost in operation. It might also be worth looking at how much the temp of the water in the boiler drops when the circs aren't running to see what the stand by loss is - if the boiler has a large volume and cools more than the rest of the system, I'm sure it doesn't help to dump a big slug of cold water into that manifold before the hot gets there... Lastly, how much flow resistance does the boiler have? Will replacing the boiler with a pair of tees drop the system resistance enough to let you downsize or use a slower speed on the manifold circ (and therefore use less electric)?

If you do go parallel, the control circuit should be pretty straight forward - have the boiler control the boiler circ, it's probably already got the wiring for it that will turn on the boiler circ when the boiler is up to temp. Let the set point controller turn the boiler on as at present, or if you want to get fancy have a couple relays so that if the setpoint controller turns on the Garn circ it turns on the manifold circ, but if it turns on the boiler, have the boiler circ control turn on the manifold circ (In either case have the manifold circ starting turn on the zone circs)

Just the way I'd do it, the pros might have a better approach...

Gooserider

 

[Jim K in PA]

Tom - as Goose said, use the oil furnace control relay to turn on the furnace circ. Depending on what type of relay you have on the oil furnace, you may have all you need. If not, it's not hard to add another relay to turn it on. Make sure it's set up for cold start (jumper on the ZC/ZR terminals).

Does your oil furnace have an automatic flue damper? Mine did not, so if the boiler casting was warm, it created it's own draft, sending my hard earned heat up the chimney when I was circulating through the furnace.

With my recent repiping, I am not set up for an automatic switchover. However, it is pretty easy to do the handoff. Swing two ball valves and flip a switch, and I am on Dino-heat. Reverse to go back to wood. My bypass works both ways in isolating the heat exchangers. If I am on oil, no flow through FP HX. If on GARN, no flow through oil furnace.

I had a number of issues to address when I did my repiping, but I think I would still have done it even if the only thing I accomplished was bypassing the furnace. YMMV.

 

[Gooserider]

I will say that I have a definite preference for some form of automatic switchover to the backup heating system - If something happens of the disastrous sort that keeps you out of the house for an extended period, I figure it is far better to have the backup able to automatically kick in and keep the pipes from freezing.... If that isn't practical for whatever reason, I would say the next best think is to put up a BIG HIGHLY VISIBLE sign pointing to instructions on how someone non-technical that has never seen your setup could make the switchover... (After you have the instructions done up, have a couple folks do a "dummy test" to make sure they can follow them...) At least that way you could call a neighbor or friend and ask them to deal with it.

Gooserider