Pressure Anomaly in Two-Furnace System

Does the electric boiler have any form of booster or circulating pump, or non-return valves, etc.?

Yes. The electric boiler was turned off during the wood fire test.

There are three circulator pumps in the system, labeled below. #1 (red) is the electric boiler pump. #2 (blue) is the main circulator for the wood boiler/tank system to heat the house. #3 is behind the wood boiler/tank, and is the circulator referred to in the data: a switch on the front of the boiler is set (or manually operated) to cause this pump to circulate the hot water from the furnace into the tank when it reaches the target temperature (150 degrees, usually).

Did you factor in the new added volume of water into the size or your expansion tank?

If you greatly increased the system volume the expansion tank volume has to be proportional to the change.

The expansion tank is a pretty big one, shown below in the rafters. Not quite as big as the storage tank but maybe half the size. Worked fine for years, never a problem. We did try draining the expansion tank to solve the problem - had no effect. At least, the pressure dropped while it was draining, but came back up to 20-21 psi minutes after, and the dripping/blowoff continues.

sorry about the crappy pictures - borrowed camera.

I think you're on the right track. Why else would there be water dripping from the pressure valve and out the blowoff pipe at 20 psi.

There is a valve that does the automatic filling: it's old, part of the old system, and may be broken. That seems like the first thing we should try.

I wonder, though, why the pressure on the electric furnace isn't affected in the same way.

Are the two systems sharing any common line connections or are they completely independent of each other?

If they are connected you may just have an old pressure gauge that is stuck or gets stuck at a certain point and just does not read any higher because the needle cant move any further. That is fairly common.

The systems are connected. There's a valve to cut the tank out of the loop if electric is running alone. Otherwise, heat circulates to the tank as well as the zones on thermostat.

The pressure gauge on the wood boiler has always worked just fine - I read that thing every day when I put the fire in, so I know what's normal or not. Resting pressure should be 10-12 psi when it's cold. Pressure increases with temperature, never exceeds 30 psi at the usual operating temperature around 150-160 degrees. Nudging over 30 at 180 it will blow off if the circulator pump is not engaged to send the heat to the tank.

This evening I decided to try heating the tank with the electric and then shutting it down. I let the electric run for three hours. Not only the tank was hot, the pipe from the tank to the wood boiler was also hot. The only cool pipe was from the circulator which was off, at the back of the tank. The pressure on the wood boiler went up to about 24 psi at the hottest. The pressure on the electric boiler is steady at 12 psi.

The valve on the wood boiler's pressure relief system keeps dripping - it's never done that before either. Doesn't make sense - it should just go down the blow-off pipe, normally. Now it's doing both.

If two gauges are connected to the same system and have far different readings one or both is damaged or broken.

My suspicions are that you have a bad gauge or two. Its more common than most people realize and is partly why having working pressure relief devices is so important.

Well, I had another look at the valve involved in refilling and where it is placed in the line. "Path of least resistance" is what I'm told, and it looks to me that any excess pressure from this source has a more direct path straight down to the tank and therefore the wood boiler system. To affect the electric boiler, it would take a side route that goes down but then turns up into the bottom of the boiler.

The other thing that occurs to me, is that the electric boiler has something internally to regulate incoming or internal pressure. This thing reads 10-11 psi whether it is stone cold or boiling - that sure isn't how the wood boiler gauge works. But the man who installed it said that's how it should be.

I'd be wondering if the electric boiler's pressure regulator is driving up pressure in the wood boiler to maintain itself at 12 psi, except the pressure difference is still there when either or both appliances are off. So my best guess at this point is, that refill valve is broken.

Can you give us a fairly accurate plumbing schematic to represent the system as its actually plumbed?

Unfortunately no CAD system here. And I gave the camera back to its owner.

What I did do this morning is RTFM. The electric boiler is a "low pressure boiler" by design, and in all schematics it has its own expansion tank. This is something I had not considered because the installer said it doesn't need one. I just talked to the top guy who is very experienced and a smart designer, and he said we'll be right in to install one.

I sure hope that works! I'll talk to him about the refill path as well.

Well there it is! The answer. The guys installed the usual expansion tank for the electric furnace and drained the big expansion tank one more time. The pressure went to 12 - 15 psi and stayed there as a resting pressure.

I've got a roaring fire in right now and my reading is 19 psi pressure at 160 degrees: perfect!

The position of the little expansion tank is scribbled on in the photo. Works a charm.