Optimizing WM oil boiler + OWB setup, and do I have a leak?

[BPardy]

Hi everyone,

I'm hoping that you kind folks here can help me out with some suggestions on aquastat (and other) settings on my boiler system. I would like to keep fuel usage down while maintaining boiler life expectancy as much as I can. This house I've lived in for two and a half years has a seriously complex (to me) heating setup so I would greatly appreciate any knowledge you can share. I'm also slightly suspicious that I have either a leak in the hydronic system somewhere or a huge gap in my understanding of how it works, if not both. Please pardon the length of this post, I wanted to include as much potentially useful information as possible.

The main questions I am trying to answer, for this system in northwest Vermont where we can easily get down to 20F below zero for weeks at a time, are:
1) Should I disable the low limit on my oil boiler and run it cold-start?
2) Should I adjust the "economy / number of zones" setting at the oil boiler controller from its current setting of 1?
3) Should I enable the "thermal targeting" feature on the oil boiler controller?
4) Any suggestions on optimal aquastat set point and diff settings to control heat transfer from the outdoor wood boiler and breaking the thermostat call for heat to the oil boiler (so that it does not fire until the OWB has cooled enough to require the oil boiler to come back on)?
5) Does my system have a leak somewhere?

The heated portion of this home is nearly 6000 square feet, made up of 8 zones (in-slab radiant pex tubing for a basement apartment, a two car basement garage and a one car main floor garage; four zones of radiant pex under hardwood floors and tile on the main floor of the home; and one zone of baseboard heat for several rooms upstairs). Domestic hot water comes from a Weil-McLain Gold Plus 80 indirect water heater set up as a priority zone. The radiant is managed by a Wirsbo ProPanel 212 with a Wirsbo ProMix 212 running two low temperature radiant loops off of the main hydronic loop, and this system has an outdoor temperature sensor that seems to appropriately reduce the oil boiler's high temp cutoff when outside temperatures are warmer and the radiant demand is lower. The various circulators and thermostats are wired in through multiple Taco relay boxes. The in-slab radiant circuit runs around 85-95F, while the second underfloor radiant circuit runs around 105-125F. I do not know the temperature the upstairs baseboards run but I assume roughly the same temperature as the oil boiler itself shows at any given time.

The primary heat source is a couple year old Weil-McLain WGO-4 oil boiler in the basement with the rest of the equipment, no tankless coil. The control on it is a FuelSmart HydroStat 3250-Plus, currently set with a high limit of 190F, a low limit of 140F, the economy/number-of-zones setting is at 1 and thermal targeting is off. WM describes this as a cold-start boiler, but the installers did not configure it as such by disabling the low limit on the 3250-Plus. As I understand it this is a not a condensing boiler and so I have concerns about allowing it to cool such that return water ends up below 130F, based on what little I understand about these things. The ProMix 212 unit is my only sensor showing me return temperature to the boiler when the outdoor wood boiler is not engaged.

As a supplemental heat source I have a Sequoyah Paradise E3400 outdoor wood boiler. The OWB runs at atmospheric pressure, with its hot water circulated into the basement through pex lines to a heat exchange plate with the other side of the plate plumbed in to the main hydronic loop. The circulator that brings water from the OWB in to the basement is controlled by a manual switch, and the incoming copper has two Honeywell L6006C aquastats on it. Currently the first, on temperature increase to roughly 175F, with a diff of 10F, enables the circulator on the closed indoor (main system) side of the plate exchanger, allowing heat from the OWB to transfer to the rest of the heat system. The second aquastat, on temperature increase to about 185F, with a diff of 5F, breaks the circuit carrying the thermostat call for heat going to the oil boiler, effectively preventing it from coming on at all (the indirect WH may have a separate call for heat circuit, I have not tested/traced that). The manufacturer of this OWB, long since out of business, specifies this unit as capable of heating 10,000sqft. I don't think anything really serves as a "dump zone", if the OWB is hot and no heat is called for by the home, the water just recirculates in the secondary loop going to the plate exchanger and the primary oil boiler system circulator does not run.

By way of history on this system, soon after my first attempts to use the outdoor wood boiler a couple winters ago, the WM WGO-3 oil boiler previously installed developed cracks in the cast iron, leaked all over the basement floor, and was replaced with partial warranty coverage from WM with the WGO-4 in there now. A little over a month ago a thermostat went out in the upstairs garage, freezing the pex tubing in the slab in multiple spots, which burst and leaked hot water onto the garage walls and down through the slab into the basement garage. After much jackhammering and patching of the pex tubing that garage loop was shown to have no further leaks via air pressure testing at 70psi holding overnight, and has been replaced with a closed glycol setup running on the other side of a heat exchange plate.

The reason I suspect I may have an additional leak somewhere in the system is that when plumbers had the main valve from the municipal water supply turned off and much of the cold water system drained to perform some work unrelated to the boiler, I noticed that the pressure gauge on the oil boiler had dropped down to near-zero, though the low-water cut-off had not kicked in (I turned off the service switch out of concern and left the boiler off until the municipal water supply valve was opened up again, and it promptly returned to the 15-20psi it usually shows). My understanding of the pressure reducing auto-fill valve is that it should only be feeding water to the hydronic system at initial startup, or if there is a leak that it needs to make up. There is a shut off valve upstream of the auto-fill valve that I can turn off to re-test the boiler's behavior when no automatic make-up water is available, but I have been hesitant to try that out of concern that another pressure drop could create a vacuum that sucks air in to the rest of the lines. Am I just understanding this wrong, and that draining the home's cold water supply for other work is expected to reduce the pressure in the boiler? I figured there would be a check valve after the auto-fill that would allow the boiler side to hold its pressure. The plumbers I had doing the unrelated work did not seem concerned like I was.

The behavior I see from the oil boiler is that when multiple zones call for heat, or when the temperature outside is very cold, it tends to come on and run frequent short cycles of 5-10 minutes on, then 5-10 minutes off, heating itself up from 180F to 190F. I am under the impression that greater oil efficiency can be achieved if the boiler runs longer, less often, but perhaps this is not relevant to my setup. If I can safely allow it to run as a cold-start I think I want it to do so, and that that would enable longer burn times.

The guy we purchased the home from, who built it, told us that all these pieces worked together seamlessly and he was able to go through 10 cords of wood a winter, but I tend to doubt that as when we moved in the aquastats on the OWB line were set such that they would disable the oil boiler thermostat circuit when the incoming water was 100F but would not enable the circulator transferring heat into the house system unless the incoming water was 180F. My initial attempts to use it before noticing and understanding that problem left me with no hot water and no heat in the home, and I believe re-firing the old oil boiler when that happened was a huge contributor to the cast iron cracking on me. The OWB's controller that operated the draft inducer fan and damper solenoid was also placed in a location where smoke, creosote, and condensation from the OWB repeatedly burned it out after three days of use, and after going through about eight of those $45 Love Controls TS2-010 controllers I eventually replaced it with a weatherproof controller wired in about 8 feet away on a fence. So I am not sure if it is even reasonable to expect that all these parts can work together as they appear to be designed to do. The ideal state I would like to see is that in winter I can run the OWB pretty much constantly and have it serve almost all of my home heat needs, instead of going through 300 gallons of heating oil every 6 weeks as I do without running it.

I can provide photos of this set up if it would help anyone to understand it and help me figure out the best way to configure all these aquastats for fuel efficiency. Thank you to everyone who reads this whether you can offer any advice or not!

 

[Dana]

That's a lot of questions. But...

If the WGO-4 is a cold start boiler, go ahead and set it up for cold start. The start up temp is less important than the entering water temp when the system is flowing, which should be consistently above 140F for the bulk of the time when there is a call for heat.

Minimum burn times between 5-10 minutes and 5-6 burns/hour isn't an efficiency disaster, but giving it a much bigger temperature swing between high and low limits would improve that. Shorter than 5 minute burns and 10+ burns and hour would be a disaster.

With the system operating at a normal pressure, go ahead and turn the isolating valve off, then observe whether it slowly drops off over a day or two. If it's normal 12 psi at at the low, limit, 15 psi or more at the end of the burn, but drops to 10psi at the low-limit 13psi-hot over a few days, there's a leak.

Even the WGO-3 is ridiculously oversized for the heat loads of more than 90% of homes in VT, and the WGO-4 even more so, but without a careful Manual-J heat load calculation on your place it's difficult to determine the actual heat load. The bigger the burner, the more likely it is to short-cycle, and the ~25% increase in burner output may be part of what's behind the cycling artifact. The fact that it satisfies the thermostats even running at a 50% duty cycle means it's guaranteed to be more than 2x oversized for your heat load, since it seems likely to be on the order of 2x oversized for the radiation, and the radiation is (more rightly) oversized for the actual heat load with some margin.

 

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[BPardy]

That's a lot of questions. But...

If the WGO-4 is a cold start boiler, go ahead and set it up for cold start. The start up temp is less important than the entering water temp when the system is flowing, which should be consistently above 140F for the bulk of the time when there is a call for heat.

Minimum burn times between 5-10 minutes and 5-6 burns/hour isn't an efficiency disaster, but giving it a much bigger temperature swing between high and low limits would improve that. Shorter than 5 minute burns and 10+ burns and hour would be a disaster.

Thank you very much. I will give this a try,

With the system operating at a normal pressure, go ahead and turn the isolating valve off, then observe whether it slowly drops off over a day or two. If it's normal 12 psi at at the low, limit, 15 psi or more at the end of the burn, but drops to 10psi at the low-limit 13psi-hot over a few days, there's a leak.

I've just tried this, with the system at 15psi and about 192F. After about 8 minutes with the isolating valve off, system pressure dropped to just over 10psi (during which the system cooled by about 10 degrees). I re-opened the valve and within 30 seconds it was back to 15psi. I suppose that is a pretty sure indicator of a leak somewhere, and not somewhere kind enough to spew water out where I can see it, else I would have seen plenty of it by now.

Even the WGO-3 is ridiculously oversized for the heat loads of more than 90% of homes in VT, and the WGO-4 even more so, but without a careful Manual-J heat load calculation on your place it's difficult to determine the actual heat load. The bigger the burner, the more likely it is to short-cycle, and the ~25% increase in burner output may be part of what's behind the cycling artifact. The fact that it satisfies the thermostats even running at a 50% duty cycle means it's guaranteed to be more than 2x oversized for your heat load, since it seems likely to be on the order of 2x oversized for the radiation, and the radiation is (more rightly) oversized for the actual heat load with some margin.

Along with the documentation of the lengths of the pex runs through the radiant system, the prior owner left a copy of some partial heat loss calculation worksheets that show the main floor at an outdoor design temp of -30F to have a heat loss of just under 32,000 Btu/hr. The plate on the WGO-4 shows it capable of 145,000 BTU/hr. I doubt the basement apartment, garages, and upper floor could have enough heat loss on top of that to make this anything but wildly oversized. I should have asked more questions when the WGO-4 was installed, but I was so happy to have WM honor the warranty that was only days from going beyond 10 years I took whatever they had to offer.

Thank you again.

 

[Dana]

If it's dropping 5 psi in under 10 minutes that's a very significant leak!

One place where a leak might not be obvious if it's leaking through one of the plates of the heat exchanger into the atmospheric pressure tank, which would be at least reasonably easy to fix. A leak in the slab radiant could take quite a bit of sleuthing to fix if the water is only flowing downward, under the slab.

A design temp of -30F is WAY below the 99th percentile temperature bin of any town in VT, but might be possible at some significant elevation. The ACCA's short list's coldest 99% design temp is in Barre, at -11F.

 

[BPardy]

If it's dropping 5 psi in under 10 minutes that's a very significant leak!

One place where a leak might not be obvious if it's leaking through one of the plates of the heat exchanger into the atmospheric pressure tank, which would be at least reasonably easy to fix. A leak in the slab radiant could take quite a bit of sleuthing to fix if the water is only flowing downward, under the slab.

A design temp of -30F is WAY below the 99th percentile temperature bin of any town in VT, but might be possible at some significant elevation. The ACCA's short list's coldest 99% design temp is in Barre, at -11F.

Thanks for the idea about the heat exchanger. I wasn't even considering that as a possible leak site, though after turning off my OWB circulators and closing valves that isolate the heat exchanger, then isolating the auto-fill valve, I still noticed a decrease in system pressure down to 10psi, though it seemed slower (or I am getting more tired and less patient). It again returned quickly to 15psi when I reopened the valve that feeds the auto-fill.

How big is very significant in this case? When the main floor garage slab pex burst on me before I had a gusher spraying water onto the walls (as it broke through the slab upwards in one spot, the other four freeze-burst points were deeper in the slab), nothing subtle about it as it pulsed out. I just remembered my municipal water meter (an analog Neptune T-10, 3/4") and watched it for a while. It looked like about half a gallon of use was measured over about half an hour, but that could have been refrigerator ice makers or our tenant. I saw and heard one quick burst of about two gallons used in a minute or two, which also appeared as usage on my water softener (it displays "softening" when it detects water use, at least above some threshold flow). Would a gallon-ish an hour of make-up water sound roughly correct for a leak that would drop system pressure that quickly?

I'll have to take some more careful measurements of time and usage, and probably valve off each zone one at a time to see if I can isolate a zone, then hopefully a single radiant loop in that zone, to find one that allows the system to hold pressure with the auto-fill isolated. We should have some 50F+ days next week where I can get away with shutting down big chunks of this system for a while.

Barre is probably a fair comparison. We're colder than Burlington but closer to the lake and sea level than Barre.

Thanks so much for your time helping me out.

 

[Dana]

If a leak that size were fully exposed it wouldn't be a drip, but a small geyser, probably similar to the one you described.

If the atmospheric pressure tank or wood boiler has a built in overflow somewhere on that side of the system, check to see it if isn't putting out a quart or three per hour, which would make the heat exchanger the prime suspect. If any buried or slab plumbing has a history of freezing up it's more likely that the leak is going into the ground.

 

[BPardy]

If a leak that size were fully exposed it wouldn't be a drip, but a small geyser, probably similar to the one you described.

If the atmospheric pressure tank or wood boiler has a built in overflow somewhere on that side of the system, check to see it if isn't putting out a quart or three per hour, which would make the heat exchanger the prime suspect. If any buried or slab plumbing has a history of freezing up it's more likely that the leak is going into the ground.

Following up on this one.

It looks like I have two leaks. First, one of the two radiant pex loops in the basement garage, and second, minor water seepage from the automatic air bleeding valve just above the boiler.

If I close the two valves going to the supply/return for one of the pex loops in the basement garage, and also close off the valve supplying the automatic feed/pressure reducing valve that adds supply water to the hydronic system, my boiler holds pressure at 15psi. As the water in the boiler heats up (via the outdoor wood boiler, heat exchanger, and circulators), pressure in the boiler starts to go up as it would be expected to. The pressure reducing valve then starts to warm up, as it is no longer feeding cold supply water into the system. Previously my boiler was showing 15psi even when up to 190F, now it hits 20psi at 175F.

If I open the return valve on this presumed bad pex loop, boiler pressure quickly drops down to 15psi, and the pressure reducing valve gets cold to the touch as it feeds cold supply water into the system to maintain the 15psi at which it is set.

I don't really want to go through another round of jackhammering a garage slab to find and fix a pex leak, so I may just get this bad loop drained and abandon it, unless I find someone with a thermal imaging camera that's up for a challenge. I will definitely have the automatic bleeding valve replaced soon though.

Thank you again for your help, Dana!