Boiler pressure range. Air intrusion.

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Tangent

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There are two issues I observe in my boiler heating system: (a) excessive pressure variation and (b) air trapped in the system.

The distance from the lowest to highest point in the system is 8 feet, thus the minimum required pressure would be (8 / 2.31 + 3) ~= 6.5 psi. The operating pressure varies from 12 psi (water T = 110 F) to 21 psi (water T = 180 F). The expansion tank is new, and tapping the tank with a stick in various spots sounds like it is half full. The pre-charge, before installation, was measured at 12.5 psi. The tank size is 4.4 gal, the same as the previous one, and the size recommended by the sizing calculator on the Amtrol web site. There are no vents near the radiators. There are no leaks from the T/P release valve or at the boiler drain valve, or anywhere else as far as I can see.

I discovered the prior expansion tank was water-logged, and thought the new tank would fix the excessive pressure variation. I spoke with tech support at Spirovent, who informed me that my installation has the expansion tank in the wrong location – it should be located on the suction side of the circulator. I also noticed that the the backflow preventer appears to be in the wrong location: in Watts documentation it is before the pressure reducing / feed valve.

The system has been flushed this year, and in the past by professionals. It will run fine for a couple of weeks, then air will start creeping in. The Taco Hy-vent was replaced last year during the flush, and this year a Spirovent was added. This year, the air came back after 3-4 days. Tech support at Spirovent suggested I close off the Taco hy-vent, since it was possible a negative pressure condition existed and air was entering via that device. That seemed to help somewhat.

A photo of the system is attached. It's not pretty – a number of different contractors have repaired the system over the years.

My questions are:

(1) Should the feed valve and tank pre-charge be lowered to 10 psi, since 12 is much more than required?
(2) Should the expansion tank be moved to a location before the circulator?
(3) Should the order of the backflow preventer and feed valves be changed?
(4) Should a new feed valve be installed? It seems OK in manual mode.
(5) Why are there pipes from the backflow preventer into the concrete slab? That section runs hot, and I don't know where it leads.

Thanks in advance for your comments.
 

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Dana

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What is the elevation difference between the expansion tank and the highest point on the system?

The pump is pumping toward the boiler, and the expansion tank should be reasonably close to and on the input side of the pump, with no ells or tees on the stretch of straight pipe between the tank and pump (if possible.)

The backflow preventer is normally on the other side of the auto-fill valve, between the auto-fill and the shut off. If the outflow pipe is getting hot it means as the system pressure increases it's probably spilling system water into the outflow- not a good thing! Has it always been configured like that way? This is the normal configuration:

B911S.jpg




If the tank is undersized for the total volume of water in the system the system will see large pressure swings with system temperature. If you're only seeing a 9psi swing between 180F and 110F I would't sweat it too much. It's highly unlikely that the system actually NEEDS to run at 180F and setting it up to run cooler can result in measurable fuel savings, as well as keep the system well within the overpressure trip point of the pressure relief valve (usually 30psi.)
 
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Tangent

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The system has been configured this way for perhaps 20+ years. It's in my parents' house, so I never inspected it closely until recently. The contractor whose father installed the system recommended replacing the boiler since it was 30 years old.

The distance from the center of the expansion tank to the highest point in the system is 5 feet.

Since the old expansion tank had a ruptured bladder and the pressure range observed with that tank was also 12-21 psi, I expected the new tank would do a better job at regulating pressure. In other words, the new tank and the old broken tank are both about as effective as having no tank.

If the new tank is moved from point "B" in the photo to just below point "K", that would introduce two 90 degree elbows before the circulator, so I'm not certain that would be a net improvement.

I've heard different arguments on the ideal aquastat set-point temperature. The InspectAPedia site mentions that the higher the set-point, the more efficient a hydronic system runs, but the pressure needs to be kept in a safe zone.

BTW, any idea why the tubing (labeled "U" in photo) from the feed valve runs into the concrete slab?
 

Dana

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With only 5' of vertical the system would be fine with 9-10 psi, which would be enough to suppress macro-boil on the heat exchanger plates, but if the expansion tank is correctly sized and correctly pre-charged you should just set it to 12 psi.

If the new tank was pre-charged to 12.5 psi and the system is hitting 21 psi it may be very slightly undersized or it may be just fine, if have a LOT of water volume in the system (big old fashioned radiators, not baseboards.)

You could re-plumb some of the near boiler to move the pump to a different location &/or orientation to make the expansion tank more functional as a shock absorber for the pump impeller, but if you're not going through pumps every few years with it configured as-is it may not be "worth it".

As long as lower temps on the boiler aren't causing it to short-cycle (for a cast iron boiler that means any burns less than 5 minutes, or more than 5 burns per hour), lowering the operating temperature of the boiler reduces distribution losses, and combustion efficiency improves modestly. As long as the entering water temp at the boiler is over 130F there is zero condensation risk to the boiler. But at 130F EWT there is some risk of increased flue condensation, primarily if the flue is oversized, and runs on the exterior to the house. With a right-sized flue it's not a problem.

I can't really hazard what the sub-slab pipe is doing.

If the boiler is more than 15 years old I'd personally be inclined to rip it out and start over with a right-sized condensing boiler. But if you're keeping the boiler it's worth re-plumbing all the near-boiler plumbing to something that's actually rational and correct, and worth tracing the sub-slab pipe to figure out where it's going. If there are leaks in the system down that buried line causing the auto-feed to be constantly adding water into the system, that would be bringing in air & creating corrosion problems as well. To test for leakage, close the gate valve (labeled "P") and see if the system loses pressure over hours/days.

Installing an air scoop on which to hang the tank directly in line with the pump 6-12" of straight pipe away would every likely fix all air issues. Make all the ells twists & turns on the output side of the pump where it can't contribute to cavitation conditions.
 

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Dana, you've brought up several interesting points.

When I turn off the auto-fill valve, the system loses at most 1 psi @T=180F within 72 hours. There are pipes below the slab in the bottom floor. Since I recently closed the cap on the (1-yr old) Taco Hy-vent, the gurgling noises in the pipes have subsided. I assume the Spirovent is working.

The radiators are finned baseboard. Heating zone 1, bottom floor: 630 sq. ft., ~ 132 feet of 3/4" tubing. Zone 2, top floor: 1500 sq. ft. ~ 308 feet of 3/4" tubing. Boiler capacity: 5.5 gal. Est. type M pipe capacity: 11.9 gal. Est. system capacity: 17.4 gal. The boiler is a Utica M150AGB. The expansion tank is an Amtrol EX-30. Based on Amtrol specs, that tank can handle a 45 gal system when max T = 200F. Unless my calculations are wrong, the tank is more than twice as large as required. Since the expansion tank is not located near the fill valve, I wonder whether the tank pre-charge should be set to the pressure at the point of the tank instead of the fill valve.

Turns out the copper tube running into the slab is a return line for zone 1. The two return lines above the boiler are both for zone 2. My bad for assuming 2 zones implies 2 return lines.

Short cycling was the reason I started looking into this project. A few HVAC techs have tried to fix that problem by replacing all thermostats, replacing the aquastat, bleeding, cleaning, and more. Finally, they told me not to worry about it. For example, in testing zone 1 only, here is the pattern when there is a call for heat: Circulator turns on and stays on. Burners run until T=180F; Burners off for 4-5 mins, T=170F; Burners run for 1-2 mins, T=180F; repeat until t-stat is satisfied.

To clarify the backflow issue: the auto-fill valve and backflow preventer are in reverse order (from manufacturer's recommendation) in the pipeline, but they are both oriented correctly directionally (inlet/outlet).
 
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Dana

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They buried return line is worth replacing with something that is visible/serviceable, as is correcting the order of the backflow preventer/auto-fill. A copper pipe in contact with the slab/soil it's losing quite a bit of heat to the subsoil, which probably isn't something you'd intentionally spend money heating. That pipe is also prone to higher corrosion from ground moisture.

A 1 psi delta loss over 72 hours is somewhat in the noise of measurement accuracy. Leave the valve closed for a month and see it it continues to drop, which would indicate an actual leak somewhere. As long as it's not dropping below 10 psi when the boiler is at idle it's not a big deal.

Without doing the math myself expansion tank sounds reasonably sized, and based on it's behavior the system would never be in danger of tripping the 30 psi limit on the PRV.

Regarding the short cycling- the beast is simply oversized for the radiation, and ridiculously oversized for the heat load. How much baseboard length (not tubing, but baseboard) is there on each zone?

A 150,000 BTU/hr input 80% efficiency boiler would be oversized for the heat loads of at least 19 out of 25 houses out there, and 99 out of 100 houses in the ~2130 square foot range. Most reasonably tightened up 2130' houses in New England would have a heat load of ~32-45,000 BTU/hr @ 0F, and still under 60,000 BTU/hr @ -20F. If you have some mid-winter fuel bills, a ZIP code, and the exact meter reading (or fill-up, if propane) dates and quantities we can easily establish an upper bound (unless there's a wood stove or mini-split covering a big chunk of the heating load.)

With some WAGs on the range of answers regarding radiation per zone, here's how it roughly breaks down:

The short-cycling is a function of how much (or rather how little) radiation there is in the system, and the limited amount of thermal mass. The boiler is delivering heat into the system at a rate of about 120,000 BTU/hr, and even your bigger zone probably has something between 100-150' of baseboard (?). At 180F entering water temp (EWT) and a 20F delta-T the average water temp (AWT) is about 170F. With an AWT of 170F most fin tube is only emitting about 500 BTU/hr per running foot, so you have about 60,000 BTU/hr (= 1000 BTU per minute) of excess. There's something like 15 gallons of total water in that zone + boiler, or about 125lbs, plus maybe another 20lbs of water-equivalent thermal mass in the cast iron, for 145lbs water-equivalent thermal mass. At 1000 BTU/minute of excess heat the temperature is swinging about 1000/150= 6.7 F per minute on the larger zone. So if the differential between high/low on the boiler controls is 20F you have at best ~3 minute burns. On the smaller zone with probably something half the radiation you'll have 90,000 BTU/hr of excess (1500 BTU/minute) and even less thermal mass, with burns on the order of a minute or so.

You could spend a couple hundred on a retrofit heat-purging economizer like an Intellicon 3250, which could be set up to where it would let the temps swing between 140-200F , which is 3x the differential delta-T. That would buy back some fuel savings and cut the numbers of burn cycles by half, but the fundamental problem would remain. It's simply not worth the trouble at this level of oversizing, especially on a boiler from the Reagan era. (The Utica M150AGB is probably from the mid or late 1980s? There may be a date of manufacture or date code on the nameplate). It's not really worth spending much money on tweaking an oversized boiler that old for modest efficiency improvement when there are now a few cheap modulating ~80K-in stainless steel condensing boilers out there with 10:1 turn down ratios, modulating down to ~7500 BTU/hr at condensing temperatures. If you set up one of those correctly you could suppress cycling on call for heat from a zone with as little as 30-35' of baseboard, and it could run long nearly continuous burns through the winter months on home with 99% design heat loads as low as 20,000 BTU/hr or as high as 75,000 BTU/hr.

If the total radiation on the system is only about 150-180' the biggest cast-iron boiler that won't have cycling issues even when both zones are simultaneously calling for heat would be about 90-110,000 BTU/hr input. Right-sizing the cast iron boiler with a 45-60K input beastie might have cycling issues on the smaller zone, and the installed cost would be about the same as a right-sized modulating condensing boiler, and the fuel use 20-30% higher. (The purchase price of a pretty-good cast iron boiler that size is actually higher than some of the stainless 10:1 80K mod-cons.) Swapping out the 150K beast with a right sized mod-con would reduce fuel use by 35-50% while improving overall comfort (once the outdoor reset control function gets tweaked in.) The mod-con won't last 30 years, but you'd be able to buy it's replacement on the fuel savings- they seem to only be getting better and cheaper over time.
 

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Wow! That's incredibly comprehensive and will take some time to digest.

The mfr. date is ~ 1985, so the boiler has served a useful life, in spite of improper sizing, installation, and maintenance.

Measuring only the finned sections of the baseboards, the lengths are: 32' for zone 1 ; 102' for zone 2.

The boiler gas consumption for 2015 was ~ 448 CCF, costing $685, with an estimated 4573 heating degree days (balance point = 60F) for my zip code. The median degree days during the past 11 years was 4316, so let's say the expected consumption is 423 CCF, costing $647. The last tech that serviced the boiler did not leave a printout of est. AFUE.

If I can save 40%, about $259 per year in heating costs with a new boiler, assuming near zero repair costs on the old boiler since many of the controls were replaced recently, how would that compare with the amortized yearly cost of the mod-con?

Don't laugh, but is it safe / feasible to disable the middle burner in order to operate more efficiently with the remaining two?
 
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Dana

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Unless it's 2x6/R19 house with l0w-E windows or you keep the thermostat at 65F rather than 65-70F it's better to use base 65F HDD.

It's often possible to de-rate the boiler pulling burners, but it's really not worth it.

Without a ZIP code I can't estimate the 99% outside design temp very easily.

Typical BTU content of natural gas varies, but it's roughly 102,800 BTU/ccf. It's better to use a mid-winter billing periods only rather than a whole heating season, since other uses such as hot water heating or solar gain during the shoulder seasons adds an error factor.

(448 ccf x 102,800 BTU/ccf)/ 4573 HDD= 10,071 BTU/HDD.

With 24 hours in a day that's 420 BTU per degree-hour of boiler input fuel. In an 80% efficiency boiler that becomes 0.8 x 420= 336 BTU/degree-hour of boiler.

Heat load grows approximately linearly with temperature below the balance point. So if your 99% outside design temp is 0F and the balance point is 60F, thats 60F heating degrees, and the implied load is:

60F x 336 BTU/degree-hour= 20, 160 BTU/hr @ 0F.

That's a bit less than 10 BTU/hr per square foot of conditioned space, which implies a very tight well insulated house with either high performance windows or lower than typical window area for new construction.

That make the existing boiler nearly 6x oversized for your actual load too, so it's as-used AFUE is probably much lower probably in the 65% range, and your heat load proportionally lower than that too. That implies either a house that is WAY better than code, or maybe you actually WERE in Belize for the month of January with the thermostats turned down to 50F? :) Can you describe the house a bit for me?

The biggest boiler anybody should have installed on 134' of total baseboard would have an output of maybe 80,000 BTU/hr, which also would have been ridiculously oversized for your actual loads. It's guaranteed to short cycle forever with 120,000 BTU/hr of boiler output going into 32' of baseboard that's emitting less than 20,000 BTU/hr. Even a boiler with 50,000 BTU/hr of output would cycle on that zone, but would be fine on the other (or with both calling for heat simultaneously), despite being more than 2x oversized for the load.

A really simple boiler swap for something more appropriately sized would be about $5-8K, depending on the greed & competence of the installer. The UFT-080W (aka WBRUNG 080W ) lists for under $2K from distributors, in the same range as 2 & 3 plate cast iron boilers. There will be another $300-400 in pumps & valves etcs, and HVAC conractors need to eat, keep the business going pay the insurance too, so add on another $2.5-3.5K, not more unless it's complicated, or they're re-zoning the place. If you are adding an indirect fired hot water tank that would add another $800-1000 to the installed price.

Assuming the old boiler's AFUE was 65% and you get the mod-con to run 95% (which you would, if the load is that small), you'd cut the fuel use by about a third. It would take quite a while (maybe never) for it to "pay off" on fuel savings alone with a load that small, but the already sunk cost of replacing the controls probably would have been better applied toward a replacement boiler of a more appropriate size, or even a condensing hot water heater based solution. (How are you heating hot water?)

Until you're ready to take that plunge you'll have to live with the short-cycling. If you can boost the high/low temperature limit differential you'll get longer burns and somewhat higher efficiency out of it, but that's about it.
 

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For zip, let's say 06114 (not my real zip, but equal in HDD).

Can you describe the house a bit for me?
A 1969 raised ranch. Older parents primarily live in top floor zone, and keep it heated to 70F, 24/7. The bottom zone is used primarily in non-winter, and is kept at 60-62F to keep pipes from freezing (yes, a waste, but some of the heat moves up). It's more of a tract house, so they cut corners, like running heating pipes below slab in 1st floor. There is extra insulation in the attic, but not more than in new construction. There are sheets of foam board(?) insulation between the vinyl siding and the original wood siding, nothing modern. About 40% of the windows are original Andersen 2-pane casement with no seal between panes, the rest are newer sealed 2-pane, but lower end.

Water heater (with tank) was replaced last year with an atmospheric unit with 0.68 energy factor. We hadn't planned on replacing the boiler at that time, in order to combine the two systems. Wish I had read this forum earlier.

There is no differential adjustment on the Honeywell aquastat. I chose thermostats with differential adjustments (non Honeywell), but that's really not the same.

What is the life expectancy of a mod-con? Or are they too new to estimate?
 

Dana

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Hartford's 99% outside design temp is +6F, so it's more like 55 heating degrees, and an even lower design load, not that it matters. For 1% of the hours in an average year it's colder than +6F, sometimes much colder, but it doesn't matter. If you design with even 10% margin you're covered. ASHRAE recommends 1.4x oversizing, or 40% margin.

The fact that part of the house is below grade, and the fact that there is a layer of continuous foam both contribute to the relatively low BTU per hour per square foot ratio. A 2x4/R13 wall with R5 of exterior insulation meets IRC 2015 code-min for zone 5, and that's roughly what you have. Non-sealed double pane casement windows with wood sashes typically come in at around U0.50, which isn't as low as current code-min, but it's not terrible. Since the upper floor is all above grade (thus lossier) and is kept at 70F, it's probably closer to use base-65 F heating degree days.

Mod cons of various types and quality have been around since the 1980s. Heat Transfer Products (HTP), the manufacturer of the -080W has been in the condensing boiler game since very early on. The current line up is something like the 4th or 5th generation product. What's new about the UFT series is the stainless fire-tube heat exchanger that can operate efficiently with a 10:1 turn down ratio. (I'm not sure who makes that heat exchanger- probably not HTP. Many condensing boiler companies buy third party heat exchangers of known quality & reliability rather than risking capital on a manufacturing line-up for a new design on this most-critical component.)

Stainless steel heat exchangers have a good track record, and are less sensitive to system chemistry than models with aluminum alloys. You should be able to get at least 15 years out of a stainless mod-con with only minor annual maintenance, and 20-25 years isn't an unreasonable expectation. The standard warranty from HTP / Westinghouse -080W is 10 years, but extended warranties are available.

That boiler is already designed for ease of hooking up & controlling an indirect hot water heater, with a separately piped port coming out the bottom, with the space heating port coming out the top. But since you have a brand new hot water heater and very low fuel use it's not something you necessarily need to jump on right away even if you decided to swap out the boiler sooner.

With 102' of baseboard on the most-used zone it should be possible to tweak it for very long burns while serving that zone. The total heat load of that zone is probably on the order of 10,000 BTU/hr @ +6F, so there would be some cycling. If the load at +6F is 10,000 BTU/hr the baseboard only needs to deliver about 100 BTU/ft-hr, which it could do even with 110F AWT. Going much below that temperature with fin-tube baseboard becomes problematic due to the non-linear response of low-rise convectors at low temp, so it might work better with a fixed temperature output of say, 120F at a delta-T of 15-20F, rather than trying to squeak another 0.5% efficiency out of it with an outdoor reset curve only to have less stable room temps during warmer weather. (But these are details to be worked out only after you've decided to actually make that move.)
 

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Thanks for the excellent analysis and feedback.

There are a few issues I can have fixed immediately, and your data and calculations provide all the guidance required for selecting an ideal replacement boiler.
 

Tom Sawyer

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A whole lot of discussion for a non issue. Yes, ideally the expansion tank should be on the suction side of the pump but it's not going to make any difference in your system if you're only pushing water up eight feet or so. It's pipe the way everyone piped boilers twenty five years ago and it's fine. The fill/back flow preventer has zero effect on the system as the fill only works once upon initial fill and if the system is tight, never works again. The pressure should be about 12 to 15 pounds and apparently it is so that's not a problem either. The only real issue I see is that the boiler is a 30 year old, oversize, inefficient piece of junk...
 

Dana

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The 3o+ -years-buried return line is an issue worth addressing sooner than later, as well as the screwy reversal of the backflow preventer & auto-fill once you've opened it up and already in re-plumbing mode on the return line, but NONE of this is an emergency.

If the tee off the backflow preventer pipe is sometimes bleeding hot water down the drain only to be filled back up by the auto fill, that could be an issue. But turning off the gate valve ahead of all of it should be able to determine whether that (or other leakage) is actually happening. If it was a BIG issue the system would be losing a lot more than 1 psi over 72 hours when fully isolated and it's not, so...

If a boiler swap is contemplated in short years it can all wait.
 

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No, pulling/ plugging burners will cause the boiler flue gasses to condense an you don't want that to happen
 

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There are two issues I observe in my boiler heating system: (a) excessive pressure variation and (b) air trapped in the system.
For Over Comes From This Air problem We Need To Takes Advice From The Our Boiler Technician.
 
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