Chimney vs. Direct Vent vs Power Vent for oil boiler

[thomase00]

Here is the situation...

I have a 33 year old Weil-McLain oil-fired hot water boiler in the basement vented into a MASSIVE brick chimney 13 feet away in the garage. The chimney has problems: The fireplace flue liner is collapsed and the boiler flue liner is not in the best shape. Given the collapsed fireplace flue liner, we were considering a propane fireplace insert, but we don't like the look of the 70's fireplace facade and started to investigate demoing and redoing the facade. One thing leads to another and we discover that the chimney is poorly constructed, leaning away from the house, and starting to pull the framing of the wall away from the house.

After talking to several masons, I am becoming convinced that the masonry chimney should be taken down entirely (the boiler flue is the only thing using it currently).

Assuming the chimney comes down, I will need an alternative venting solution for the boiler. My options are:

1.) Keep going with the existing boiler and have a steel chimney installed up the side of the house. I'd probably want a chimney chase built around it as well. This would be a bit closer to the boiler than the current chimney (maybe 8' away).

2.) Install a power vent for the current boiler. The boiler is located at the back side of the house, right near where the stairs for my deck meet the ground. I wouldn't want a power vent so close to the deck stairs, so I'd like to have it as far away as I can get away with. The problem is that I have a walkway along the back of the house 2' to 3' from the foundation. On the plus side, the basement is partially above grade, the power vent would be plenty high, AND we would need only need to cut through sheathing and siding rather than through the foundation wall.

3.) Get a new higher-efficiency direct-vented oil boiler. Again, I wouldn't want the vent coming out right where the boiler is because it's too close to my deck stairs, but I don't know how far a direct vent pipe can go or how many elbows it can have. Also, my boiler service tech says he doesn't like direct vent because it might not be suitable given the direction of the wind relative to the house.

4.) Get a new higher-efficiency direct-vented or power vented oil boiler and move the WHOLE system to the opposite side of the house. If it is vented out the side of the house, it will be far away from the deck and walkway. However, the install cost is MUCH higher if everything is moved.

4.) Get rid of the oil tank, and get a buried propane tank with a new high-efficiency gas boiler. I'm not as concerned about venting this out the back of the house because its just CO2 and water. However, when you compare the highest efficiency gas boiler to the highest efficiency oil boiler (95% vs. 87% AFUE), it looks like the per-gallon price of propane needs to be about 75% of the price of oil in order to make up for the lower BTU content. Currently, propane is more like 80% to 90% the cost of oil.

I'm leaning toward option 1. I'm assuming if/when I get a new high-efficiency oil boiler in the future, it can still use the new chimney. However, I've heard of people abandoning perfectly functional chimneys when they install a new boiler with side-wall venting as an option.

Also, I have an indirect water heater running off the boiler, so it WILL be running occasionally in the summer time when the yard is in use.

Opinions?

 

[Dana]

It's almost certain that the existing boiler is ridiculously oversized for your space heating load. At 33 years of age it probably can't even hit it's nameplate combustion efficiency no matter how well tuned, and at more than a 1.7x oversizing factor it's unlikely to have EVER hit it's AFUE numbers. At 33 years it could probably still be nursed along at low efficiency for a decade or more, but it doesn't owe you a thing, and could start to leak at any time (though most won't.) With the number of burner hours this thing has seen the turbulence generating features on heat exchanger plates have been eroded/corroded on both the fire side and water side, and the heat transfer efficiency impaired.

Even though the oversizing factor is probably high enough that it could still heat the place even when the heat exchangers have worn smooth enough that it only musters 55-65% efficiency, there's not much point in investing ANYTHING into keeping it going. A stainless chimney liner sized for the oversized boiler would likely be oversized for a boiler sized small enough to hit it's AFUE numbers, but with more information on the system & boiler that could be assessed if you really want to.

It's probably time to bit the bullet and replace it with something right sized (meaning 1.2-1.4x oversizing factor) or at least as right-sized to the heating load as possible. It's difficult to right-size oil burners for the lower heat loads of reasonably tight reasonably insulated houses- even the smallest oil burners tend to be oversized for those houses. So the first step toward optimizing the equipment choice is to get a handle on the actual space heating load at the 99% outside design temperature, which can be done reasonably accurately & quickly using fuel-use against heating degree-day (HDD) date over a winter period between fill up. An explanation of how to do that can be found here.

If the burner tech tagged it with the measured combustion efficiency during the last tune-up, use that as the efficiency number for the fuel use heat load calc, otherwise use the manufacturer's AFUE or nameplate efficiency numbers.

For Andover, MA, use 0F as an outside design temperature, and base 65F HDD for 2x4 framed house, or 60F HDD for a 2x6 framed house. Better yet, run the numbers both ways (as outlined in that blog bit.)

Other relevant heating system factors are the number of zones, and the amount of radiation/baseboard on each zone. The model number of the existing boiler, the nameplate input & output numbers, the nozzle size currently installed in the burner are all useful for making judgments on what way to go here.

With some houses you will be better off just mothballing the thing and installing ductless mini-split heat pumps for both heating & cooling. That's a tougher call, but a description of the house would help.

Propane is almost always the worst option, since in this state propane is usually a micro-monopoly, where the tanks is owned & maintained by the propane delivery company who reserves the sole right to filling it, usually with a ridiculously steep "removal fee" should you opt to go with a competitor. (Not sure if that's how it works in Andover, but it's important to find that out.) The price per delivered BTU is also higher with propane than oil, despite the marginally higher efficiency, but it's easier to right-size a modulating condensing propane boiler in low to medium load houses, and the local air pollution factor is lower than with oil.

 

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[thomase00]

It's almost certain that the existing boiler is ridiculously oversized for your space heating load. At 33 years of age it probably can't even hit it's nameplate combustion efficiency no matter how well tuned, and at more than a 1.7x oversizing factor it's unlikely to have EVER hit it's AFUE numbers. At 33 years it could probably still be nursed along at low efficiency for a decade or more, but it doesn't owe you a thing, and could start to leak at any time (though most won't.) With the number of burner hours this thing has seen the turbulence generating features on heat exchanger plates have been eroded/corroded on both the fire side and water side, and the heat transfer efficiency impaired.

Even though the oversizing factor is probably high enough that it could still heat the place even when the heat exchangers have worn smooth enough that it only musters 55-65% efficiency, there's not much point in investing ANYTHING into keeping it going. A stainless chimney liner sized for the oversized boiler would likely be oversized for a boiler sized small enough to hit it's AFUE numbers, but with more information on the system & boiler that could be assessed if you really want to.

It's probably time to bit the bullet and replace it with something right sized (meaning 1.2-1.4x oversizing factor) or at least as right-sized to the heating load as possible. It's difficult to right-size oil burners for the lower heat loads of reasonably tight reasonably insulated houses- even the smallest oil burners tend to be oversized for those houses. So the first step toward optimizing the equipment choice is to get a handle on the actual space heating load at the 99% outside design temperature, which can be done reasonably accurately & quickly using fuel-use against heating degree-day (HDD) date over a winter period between fill up. An explanation of how to do that can be found here.

If the burner tech tagged it with the measured combustion efficiency during the last tune-up, use that as the efficiency number for the fuel use heat load calc, otherwise use the manufacturer's AFUE or nameplate efficiency numbers.

For Andover, MA, use 0F as an outside design temperature, and base 65F HDD for 2x4 framed house, or 60F HDD for a 2x6 framed house. Better yet, run the numbers both ways (as outlined in that blog bit.)

Other relevant heating system factors are the number of zones, and the amount of radiation/baseboard on each zone. The model number of the existing boiler, the nameplate input & output numbers, the nozzle size currently installed in the burner are all useful for making judgments on what way to go here.

With some houses you will be better off just mothballing the thing and installing ductless mini-split heat pumps for both heating & cooling. That's a tougher call, but a description of the house would help.

Propane is almost always the worst option, since in this state propane is usually a micro-monopoly, where the tanks is owned & maintained by the propane delivery company who reserves the sole right to filling it, usually with a ridiculously steep "removal fee" should you opt to go with a competitor. (Not sure if that's how it works in Andover, but it's important to find that out.) The price per delivered BTU is also higher with propane than oil, despite the marginally higher efficiency, but it's easier to right-size a modulating condensing propane boiler in low to medium load houses, and the local air pollution factor is lower than with oil.

I calculated a load of about 90k BTU using the Heat-Calc Residential 4.0 Manual-J software.

http://www.hvaccalc.com

I did this a few years ago when I wanted to double check estimates that I was given for a new AC system. It calculates both heat loss in the summer and heat load in the winter.

I used a winter outdoor temp of 1F (I think this is the 99% number for Lowell) and an indoor temp of 68F. Also, this doesn't include the load for my partially finished basement (15 ft of baseboard) although I don't often heat the basement. It also didn't include the indirect hot water load. I entered a TON of info into the software regarding insulation, windows, walls, floors, attic, etc. The only thing I wasn't sure about was air exchanges per hour. I wouldn't say that the house is particularly well built, probably average at best.

The house is a 2180 sqft colonial. The 1st floor has 72' of baseboard plus a Twinflo II K84 kickspace heater. The 2nd floor has 62' of baseboard. House was built in '79. I haven't paid much attention to how often and long it cycles in the dead of winter. I used 885 gallons of oil in the past year and the boiler is tagged as 81% efficient. Obviously, I have no idea what the standby losses of the system are.

 

[thomase00]

I haven't yet done the load calculation following the method in your blog post. From reading some of your other posts, I expect that your method will result in a much smaller load than the Manual-J. Is it the gist of what you're arguing that the Manual-J is bogus?

Also, my Megastor MS-40 indirect heater has a min boiler BTU output of 102k. I'm not exactly sure what that means, but I don't want to compromise my hot water recovery. I've seen some of your posts talking about using wastewater heat recovery, but the waste PVC pipe for my primary shower is behind a finished basement wall.

 

[Dana]

Something is totally amiss with your manual J. There are a few reasons your whole house heat load is nowhere near 90,000 BTU/hr at a temperature difference of 67F:

1: To start with, most 2200 houses with a full basement would come in under 50,000 BTU/hr at that temperature difference, tighter houses with insulated basement walls would come in around 35,000 BTU/hr

2. With 885 gallons of use in the past year, it would require a boiler that runs at more than 200% efficiency to have heated the place. From 1 May 2016 through 1 May 2017 the airport in Lawrence logged 6020.1 heating degree days. (base 65F).

882 gallons over 6020 HDD is o.147 gallons per HDD. At 138,000 BTU/gallon that becomes 20286 BTU/HDD source fuel imput. But at 81% efficiency that means at most 0.81 x 20285= 16432 BTU /HDD went into the house- the rest went up the flue.

In a 24 hour day 16432 BTU /HDD bcomes (/24=) 685 BTU/degree hour.

The 1F is 64F below the presumptive 65F heating/cooling balance point, so the absolute highest the heat load could possibly be (assuming no standby or distribution losses during the non-heating season, and no hot water use in any season) would be 64F x 685BTU/F-hr=

43,848 BTU/hr.

But you probably use at least 100 gallons/year for hot water, and there is also idling & standby losses to consider. Your real heat load is under 40K, and probably something like 35-36K. The smallest oil boilers out there start at about 60,000 BTU/hr out, and some vendors' smallest are over 70K.

Using only winter time periods heating degree days would be more accurate, since that's when the errors from solar gains and hot water use are the smallest. If you're on a regular fill-up service they usually stamp a "K-Factor" on the billing slips. A K-factor is HDD per gallon, so it's simple arithmetic to convert that into BTU/HDD, and BTU/ degree-hour to come up with a heat load. If you have a few K-factors from mid to late winter fill-ups, what are those numbers?

3. There is a total of 150' of baseboard, plus a kickboard heater. Even if you ran the boiler at 180 high-limit /170F average water temprature through the baseboard you would get about 500 BTU/ft or 75,000 BTU/hr for the whole thing. The K84 toe-kick heater puts out 8500 BTU/hr @ 180F EWT which only brings it up to 83,500 BTU/hr. If your heat load is actually 90K @ 1F it wouldn't be able to keep up at anything below 5F. The biggest non-modulating burner you should EVER hook up to that radiation would have an output of 84K, but something like ~50K out would be better.

So, for the best accuracy run the fuel use heat load calculation using both 65F and 60F for a base temp using a nearby weather station data on degreedays.net, ( I used station "KLWM, which is the Lawrence Municipal Airport) and only mid to late winter fill ups.

 

[Dana]

I haven't yet done the load calculation following the method in your blog post. From reading some of your other posts, I expect that your method will result in a much smaller load than the Manual-J. Is it the gist of what you're arguing that the Manual-J is bogus?

Also, my Megastor MS-40 indirect heater has a min boiler BTU output of 102k. I'm not exactly sure what that means, but I don't want to compromise my hot water recovery. I've seen some of your posts talking about using wastewater heat recovery, but the waste PVC pipe for my primary shower is behind a finished basement wall.

Manual-J isn't bogus when done correctly, but most people (especially newbies) use unduly conservative R value and air leakage assumptions, and insufficient discounting for occupants, 24/7 plug loads, etc. The instructions require that you use AGGRESSIVE assumptions, and give creditt whenever/where ever possible to things like drapes, etc. An aggressive Manual-J on my place in comes in only about 5-10% higher than a fuel-use calc.

Indirect water heaters don't have a minimum boiler output. The numbers are in the spec only to indicate that's what it takes to hit the specified water heating rating. A standalone gas fired 50 gallon tank typically has 28-36K of burner output, and even the smallest oil boilers run about twice that. Even with the smallest oil boiler behind it you'd get substantially better hot water performance out of a 40 gallon indirect- enough to run a continuous 24/7 shower(!). Size the boiler for the heat load, size the indirect for the biggest tub you need to fill. If the indirect can fill your tubs now, it will still do it with a 50K output boiler.

 

[thomase00]

Wouldn't HDD be a better measure if it integrated the difference between the indoor and outdoor temperature continuously over the day rather than just using the average of the low and high? Better yet, calculate relative to my programmable thermostat which is higher when we are home and awake and lower when we are out of the house or asleep.

What I really need is a continuous temperature vs time curve...

 

[Dana]

Degreedays.net uses more sophisticated integrative HDD calculations, not high+ low/2 methods when the weatherstation data is good enough (which includes all airport weather stations.)

The additional accuracy of more sophisticated HDD or thermostat setting modeling doesn't change the peak load numbers by very much. Only if the house were allowed to cool off to less than 60F for most of the day (actually reaching those temperatures, not just a thermostat setting) would it affect the fuel use calculation accuracy by more than 10$. Is it perfect? Not even close, but it's more accurate than most Manual-Js, even those performed by HVAC professionals. The fuel use calculation already includes standby & distribution losses (that don't appear in a Manual-J) of the heating system, since it's impossible to separate those out, which allows you to size it more tightly without doing all of those calculations for sizing a replacement boiler.

A good Manual-J software package will also report the heating/cooling balance point, if you wanted to run a tighter fuel-use load calculation (base 62 for a balance point instead of 65F or 60F). Manual-J is a linear approximation, offset by the subtracted-off internal heat sources, whereas a fuel use calculation is a linear approximation from the balance point, no offset. The slopes are not identical, so they will diverge a bit, but not by much- the error is less than the error from hot water use and solar gains. The solar gain & water use errors in a fuel use calc are in opposing directions, and largely offset one another during winter periods (low solar gain, higher hot water use), and are small relative to the total heating energy use during the winter, but can be large during the spring & autumn shoulder seasons, which is why wintertime fuel use data is best.

Unless your house is super-insulated to something 2-3x code minimum (something approaching PassiveHouse levels) the error in a wintertime fuel use calculation won't be more than 15% from reality in this climate. So, say the heat load calc for a wintertime period for your house comes in at 37,000 BTU/hr, reality could conceivably be as high a 42,500 or as low as 31,500 using the most sophisticated available modeling. But reality still won't be anywhere near 50,000 BTU/hr, let alone the previously calculated 90K from your Manual-J.

If you want to really want to dig into energy use modeling, download a copy of BeOpt, and run a simulation on your house. It'll be more accurate than Manual-J or fuel use load calculations, but it won't change the size of the boiler you would be installing. The tool is more useful for architects & energy nerds trying to get the best bang/buck out various elements the building design than it is for HVAC design. It might be useful for figuring out some of the details if you opted to heat with mini-split heat pumps though. (Which probably would have been more useful to consider when you were runing the numbers to spec your air conditioning, which I presume has already been installed...) A few years ago when #2 oil was north of $5/gallon right-sized mini-splits paid for themselves in about 3 years in reduced heating costs at MA type electricity pricing, not so much now that oil prices have crashed.

What is the BTU/hr or gallons per hour numbers of your current boiler/burner?

 

[Dana]

The negative effects of oversizing on efficiency are substantial, particularly when there isn't heat purge controls. Take a peek at this research report from the Brookhaven National Labs. Take a look at Table 3 on page 9 (p14 in PDF pagination). Your system is probably most similar to system #2, which at 3x oversizing performs 13-14% below it's steady state combustion efficiency. If your 99% design load is 35K and the output of your boiler is north of 100K your 81% tested combustion efficiency is probably translates no better than 68% as-used AFUE. With heat purging controls the oversizing hit is less substantial (as with system #3.)

The range of direct vented oil boilers tends to be on the ridiculously oversized 100,000 BTU/hr & up side, but the Peerless PO series has a 70K-in/64K out version that would be appropriate for your loads, and it' s north of 90% efficiency too (could be elegible for MassSave subsidies.) The PO series boiler is about twice as expensive as an 87% efficiency triple-pass such as 74K-out direct vented Buderus Logano G115ws/3 (which is probably more than 2x oversized for your load, but is well insulated and comes with heat purging controls.)

The Burnham MPO-IQ84 , omes with heat-purge controls, but needs a chimney in the smaller sizes, though direct vent is available for the larger versions in that series. Cheaper still would be a pretty-good but pretty dumb Biasi B10-3, which would require a chimney, but it's at least more appropriately sized than what you probably have (and a retrofit heat purge control such as the Intellicon HW+ isn't super-expensive).

All of the above worth at least comparing quotes, including the cost of the venting/chimney.

With a better handle on your actual heat load, you may have enough baseboard be able to get there with a Bock or Everhot oil fired hot water heater could be used for both heat & hot water, which would likely come in at cheaper installed price. The high thermal mass of the stored water and the higher insulation levels of a water heater means it never short-cycles, and still delivers reasonable efficiency at 3x+ oversizing, though still not as good as a high efficiency boiler with purge control.

To compare as-used efficiency and likely net savings dowlnoad NORA's FSA Calculator and play around with it a bit. The FSA calculator also estimates your heat load based on your fuel use. You may have to fudge some info to get through the registration process to be able to down load the 2.0 version. Their older dumber FSA calculator has some software bugs and isn't super-easy to use, but is still good enough to be useful, and may still be available on web archive sites.

 

[thomase00]

I used your heat load calculation to figure out my BTU vs degree-hour for a period spanning Jan to Feb for the last 3 winters. I came up with a load of 55K for -15F/68F and 45K for 0F/68F.

I have oil delivery records and assumed that the amount of oil consumed is equivalent to the amount delivered at the end of the period in question.

I initially questioned how I could trust that all 81% of the heat from the combusted oil was actually being delivered to the building because there is always leftover heat in the system after a call, but then it occured to me that being a closed system, with the boiler inside the house, the extra heat in the boiler and delivery system after a heating call winds up in the house regardless. Duh!

Anyway, when I asked my oil/service provider to quote me for a replacement, they chose the. Weil-McLain WGO-4RD which puts out 107K net BTU. Maybe they did this to maximize my water heater recovery time? Without considering hot water tank recovery, it seems that I should instead go with the WGO-3RD or 2RD which put out 85K and 75K respectively. I think I'd rather be a little more oversized (still MUCH less than what I have currently) because I often have 2 LONG showers drawing down the indirect tank simultaneously. In fact, I've noticed the hot water tap dropping in temp occasionally when the double showers go on too long. Unfortunately, I don't think it would be easy to recover the waste heat from my showers very easily. The drain pipe for the master bath shower is behind a finished basement wall and goes pretty much straight to the sewer connection behind the same wall (there is a small access panel). Also, this is at the absolute opposite corner of the basement from the water heater.

 

[thomase00]

No one ever addressed the original question in my post!

Really, I want to know how dirty a power vent or direct vent really is, because it would be directed toward my backyard. If I spend the money on the new metal chimney and delay the boiler upgrade, can I be sure that I can use the new chimney when I finally do upgrade the boiler? On the flip side, I can put that money toward a new power vent or direct vent boiler right now if I don't really need the chimney. There is also the 3rd option of getting a new boiler but spending $$ to move the whole system to the other side of the house where a direct or power vent would just be pointed into the woods.

Regarding your last post, if my combustion efficiency isn't really 81%, than how can I rely on the heat load calculations I just did? If the efficiency is lower, that implies that my load is actually lower. I haven't read the links yet, but I assume the efficiency loss happens when the burner stops and starts (i.e. NOT steady state)?

 

[thomase00]

Another concern is that the required chimney size for my current boiler might be different than for a future boiler

 

[Dana]

No one ever addressed the original question in my post!

Really, I want to know how dirty a power vent or direct vent really is, because it would be directed toward my backyard. If I spend the money on the new metal chimney and delay the boiler upgrade, can I be sure that I can use the new chimney when I finally do upgrade the boiler? On the flip side, I can put that money toward a new power vent or direct vent boiler right now if I don't really need the chimney. There is also the 3rd option of getting a new boiler but spending $$ to move the whole system to the other side of the house where a direct or power vent would just be pointed into the woods.

Regarding your last post, if my combustion efficiency isn't really 81%, than how can I rely on the heat load calculations I just did? If the efficiency is lower, that implies that my load is actually lower. I haven't read the links yet, but I assume the efficiency loss happens when the burner stops and starts (i.e. NOT steady state)?

Huh? Of COURSE the original questions were addressed!

Let's see- the original questions asked...

1) Indicated #1 was a bad idea, since a right-sized higher efficiency boiler probably wouldn't work with a right-sized chimney/liner for the old oversized boiler.

2) Indicated #2 was a bad idea, that investing ANY money in a 33 year old boiler (that was likely ridiculously oversized to boot) is a bad investment

3) Gave multiple model suggestions for direct vented boilers that are more appropriately sized to the load. Can't really comment on your tech's dislike for direct vent, but there are other techs.

4) Didn't really address where you might put a direct vented boiler- there is not enough information to make that call. (So, OK, that part wasn't really touched.) If that's the direct you THINK you're going, we can discuss that. A floor plan with exterior features and window/door locations would be in order to go much further on that.

4') Pointed out that propane, while easier to right size, is a much more expensive fuel, and a micro monopoly, and thus a bad idea (at least most of the time.) A price that's 80-90% the cost per gallon is still quite a bit more expensive heat when propane's energy content is 91,600 BTU/gallon, to oil's ~138,000 BTU/gallon, even if burned at 95% efficiency instead of 85%.

Nowhere in the original post was there a question about how dirty a direct vent really is. (It's cleaner than what you've got, for sure, but keeping any oil burner tuned is the critical part for long term cleanliness.)

If the original boiler isn't operating at 81% AFUE, it's still operating at or near 81% when using WINTER FUEL USE ONLY, due to the higher duty cycle /lower idling losses, when it's actually heating the house. The number derived from a fuel use calculation is an upper bound, since the actual efficiency would only be lower than, not higher than the tested 81% steady state combustion efficiency.

It's an upper bound, true, but it's clear even from running the crude annual fuel use numbers that there's no way your house would be served by anything but the smallest oil-fired boilers.

So it really doesn't much matter what the actual load number is for picking a boiler. The load is already known to be much smaller than the output of the smallest available oil burners, so the true number doesn't really affect your choice. Any 0.5 gph or 0.6 gph burner has it covered, probably by 2x or more, but there's no helping that level of oversizing, since they simply don't make them any smaller than that.

If you want to spring to have a RESNET rater or certified professional engineer to run an aggressive room by room Manual-J on the place there may be mini-split heat pump options worth exploring, that would be cleaner than any oil boiler, at a comparable or cheaper operating cost, and cheaper than or comparable to the upfront cost of the highest-cost oil solutions. Of course the time to do that would have been better if it was before you sprung for the central air. (Did they at least keep the ducts inside the insulation & pressure boundary of the house?)

 

[thomase00]

I did a similar fuel-usage-based load calculation for the previous 2 winters and came up with 52K and 54K BTU/hr at -15F respectively, which is actually a bit less than for the most recent winter. This is a little strange because I had the utility company sponsored air sealing in my attic done AFTER last year's winter (i.e. in April of 2016), which measurably (with a before and after blower door test) reduced my air infiltration. My fuel usage calculations were based on the assumption that the oil tank is always filled up all the way, but if that isn't actually the case, the actual gallons used could be off by a bit. Any other possible explanations?

Below is a rough sketch of the house:

The red dot is the point on the outside wall is roughly adjacent to my current boiler location. This is in the northeast corner of the basement (i.e. can't move any further northeast).

The red outline is my deck with the stairs coming down to the right. The exterior wall adjacent to the boiler is basically right at the bottom of my deck stairs. The basement is partially above grade at this point such that the floor joists for the 1st floor (i.e. rim joist) are about 5' to 6' above the ground.

The black outlined area is a walkway that wraps around from the back of the house leading from the driveway (on the northeast side) to the deck. To get a sense of the scale, note that the back of the house (i.e. northwest facing side) is about 51 feet long. You can see that the walkway around the back of the house is about 2-3 feet from the foundation.

The blue rectangle, is the current chimney, which is going to come down. It is 13ft away from the current boiler. The vent pipe currently goes through the basement wall into the garage where it breaches into the chimney.

The green rectangle on the southwest side is my AC condenser.

https://drive.google.com/file/d/0B-GxnWtJF1ZkZGlkZVhqRVpqMGc/view

Obviously, if a new boiler is in the same location as the old, I can't very well direct vent right at the bottom of my deck stairs. Also, my kitchen window is roughly above the same spot. In order to even consider a direct vent, the vent pipe would need to be run as far away as possible from the deck (and kitchen window). Even then, I'm concerned about the direct vent being only a few feet away from the walkway.

It seems that the best option for direct venting is to move the ENTIRE system to the back-left corner of the house, and vent out of the southwest side. Although, my AC condenser is right outside at that corner of the house on the southwest side, so that would have to be worked around.

The expense of moving the whole system needs to be weighed against the expense of putting up a new metal chimney on the back of the house with a framed chase.

 

[Dana]

Since it's impossible to subtract out the domestic hot water portion, it means your real load at -15F is still going to be less than 50,000 BTU/hr, even if domestic hot water only accounts for 10% of the fuel use. (15-25% of the total would be more typical, but there are many outliers.)

It's a bit silly to design for -15F when the 99th percentile temperature bin is +1F, but even the 0.5 gph Biasi B10-3 covers your ~53K @ -15F fuel use derived load, and the smallest of their lines MPO-IQ84 or Logano G115/3 have it covered with huge margin. Since don't really have any choices for going smaller, so you're covered at -15F whether it's silly to design for that or not!

You can probably get the Logano G115/3 quoted for both the chimney version keeping the boiler in its current position with a new chimney, as well as direct vented, either at/near it's current position or on the other side of the house. As long as it's not venting directly onto the walkway, the main issue with side venting is maintaining the minimum clearances from operable windows, doors & ventilation vents. See figure 2.

 

[thomase00]

If it goes DIRECTLY out the nearest wall, it would be directly on the walkway. If I can get it a few feet away, at least it won't be directly on the walkway. I think the windows are high enough up to meet all of those minimum clearance rules, but still. Do I want this venting essentially into my backyard? I have a friend at work who got the G115ws/4 a few years ago, but his vents out the side of the house instead of into the back yard.

Also, I've been reading more about the G115ws/3.

I don't see anything about it coming with heat purging controls, although the manual does talk about pre-purge and post-purge of the combustion chamber.

Are you talking about the add-on Logamatic 2107?

Edit: Whatever outside temp I'm targeting, I've calculated that the load is 622BTU per degree-hour. I found a closer weather station.

 

[Dana]

My recollection was that the G115ws came standard with a smarter than average control, but that may only be with the Logamatic 2107 add-on.

At 622 BTU/degree-hour, and a 99% design temp of +1F, balance of +65F, that's 64F heating degrees, for an implied load of 39,808 BTU/hr before discounting for hot water use. If that were all heat load (no hot water), upsizing that by the ASHRAE recommended 1.4X gives you 55,730 BTU/hr as an optimal oversizing factor, but whether the chimney vented or somewhat lower output side vented version you'd still be in the ~2x oversize range.

Go over to your friend's house, have him bump up the thermostat, then go out and give the G115ws/4 the sound & sniff test at a relevant distances, see if that would work for you near your walkway. The G115ws/4 is 33% bigger than the G115ws/3 (but only 33% bigger)- you should be able to make the call based on what your nose & ears tell you. The noise will be about the same, the exhaust would be a little less.

 

[thomase00]

FYI, I used 62F as my balance point for calculating HDD. This is a a rough weighted average of the indoor temp over the course of a week according to my programmable thermostat. We keep both zones at 58F during the day when no one is home, both at 68F when we are home in the morning and evening, and 58F downstairs and 64F upstairs when we are sleeping.

 

[thomase00]

Do I need to be concerned if the BTU capacity of my baseboards at 180F is more than the boiler can put out? I'd imagine that the boiler would just reach a steady state temperature somewhere short of 180F if all the zones are calling at once.

Also, I'm still concerned about the recovery time of my MS-40 indirect. I've become accustomed to endless showers ;-) I suppose I could add a thermostatic mixing valve and increase the set point of the indirect, but there will be diminishing returns and reduced efficiency. However, the water here in Andover eats plumbing so I don't know how long a mixing valve would last.

To answer an earlier question, my current boiler has a D.O.E. heating capacity of 173 and a Net I=B=R of 150.4. The input oil is 1.5 GPH. So, REALLY big. I don't know the model number but I looked up the CP# on the Weil-McLain website and it says it was manufactured in '84.

 

[Dana]

A 2gpm shower at a 70F temperature rise (35F in, 105F at the shower head) is 70,000 BTU/hr. The output of the G115/3 side-vented version is 74,000 BTU/hr. Unless yours is a real gusher-shower with six side-sprays or something, you won't run out of hot water, as long as the indirect is given priority by the zone controller.

The net I=B=R output isn't really relevant for sizing the boiler using fuel use calculations, since the fuel use already accounts for the distrubution & jacket losses. The I=B=R is a crude way to estimate the distribution losses when the boiler is outside the conditioned space rather than in your finished basement. The D.O.E. output or steady state combustion efficiency is more relevant.

At 173K BTU/hr the D.O.E output is substantially more heat than your 150' of baseboard + toe-kick heater can even deliver(!), even if you cranked the high-limit up to 220F! (What were they thinking, marrying that much boiler to that radiation?) If the high limit is set to 180F it delivers no more heat to the house than a boiler with ~85,000 BTU/hr of output. Seriously, the beast is already ~2x oversized for the radiation, and even more oversized for the heat load:

With boiler output of 173K and a design load of 35-40K you have a 4-5x oversize factor, which is (unfortunately) all too common, and bigger than typical ~3x oversizing that seems to be the middle range. That means even on design-day your burner duty cycle is no more than 25%, and the winter average is less than 15%, which means you probably won't get more than 70-75% average efficiency even in winter (which means the fuel-use calculated load can be scaled down by about 10%). The boiler room is probably the warmest place in the house when it's cold outside and the duty cycle is high, parasitically overheating the basement, raising the as-used heat load.

With the G115/3's 74KBTU/hr output into 150' of baseboard + another 17' of "baseboard-equivalent" you have a ratio of 74k/167'= 443 BTU/hr per running foot. If ALL zones are calling for heat at the same time, with typical baseboard ratings the output temp will drop to about 165-170F, and the return water temp will be about 145-150F, which is fine. Return water much below 140F risks condensation inside the boiler, which is damaging to most boilers due to the high acidity of the condensate. That can be fixed with near-boiler plumbing tricks when the radiation is oversized for the boiler, which isn't really the case here, though a prudent installer might go ahead with at least a minimal system or boiler bypass branch plumbing (a standard fix with oversized or high-mass radiation) as a matter of course.

With just the 62' baseboard zone calling for heat you have a ratio of 74K/62' =~1193 BTU/foot-hr, so it will cycle on/off a bit even with the temperatures cranked up (which is where heat purging controls can be a benefit, lowering the cycle counts by quite a bit.) With the 72' + toe-kick (17' equivalent) zone you have a ratio of 74/89'= 830 BTU/foot-hr, which will still cycle a bit, but comparatively slowly, and probably not enough to impact efficiency. There's nothing that's going to save the 15' basement zone from short cycling, (other than adding a lot of buffering thermal mass, which isn't cost effective) but when it's cold out the odds of calls for heat from other zones overlapping each other go up, which reduces burner cycling.