Advice on converting oil boiler to gas, adding indirect water heater

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jefferson17

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Added: fyi on hot water needs. We have 4 showers. There is 1 bathtub upstairs (4th floor apt) but is used only for showers. Each upstairs apt has 1 shower. We have 2 showers in our unit (floors 1 and 2). It's just Amy and I in our unit. All shower heads (including ours) are Delta h20 kinetic - using 1.85 gpm.

>> I'd have to run the real numbers on the real construction to come up with a sort of real heat load number. The type of brick matters when it's 18-24" thick- something difficult assign a U-factor to, or even a range, but it's probably at least R5 (U0.20) from a thermal mass dynamic modeling point of view, and could be much higher. I assume this is a town house, and the other side of the common wall with the "twin" is a heated space (U-factor = 0.)

It's a victorian twin, not townhouse. So one wall is shared with the adjoining structure (except rear addition, which is open on 3 sides). The upper 3 floors are brick and about 8" thick. But that brick is pretty leaky, for sure. The lowest living floor has MUCH more thick masonry - about 18", sometimes closer to 24" We've had to drill through it so we really do know.

>> Any single pane windows with leaky or absent storm windows can cost-effectively be upgraded with tight-fitting low-E storm windows. Even though they are more expensive than clear glass windows, the payback is 5 years, not 10 due to the higher performance. The as-is clear glass storms+ single panes have a U-factor in the U0.5-U0.6 range. Tight low-E storms over those antiques would deliver U0.31-U0.34. Harvey makes the tightest storm windows in the biz, and have a low-E glazing option. The Larson low-E storms sold through the box store chains don't suck if you spring for at least the "Silver" version (the low end "Bronze" series leak a lot more air.)

Unfortunately, it's just not a good option for us. Replacement windows get quite expensive for our larger sizes. I've shopped around. Our house is also historical so more PITA. The payback would be at least 10 years - probably more like 15. Even more if we went with wood not vinyl. The money we spent on air sealing and insulation was well worth it though.

>> In the basement it's far better to put the foam on the exterior foundation walls that to cut'n'cobble between joists. This is for several reasons: Putting it on the foundation walls brings the joist edges and boiler completely inside the conditoned space, which means the wood stays warmer (= drier), and the standby losses of the boiler accrue to the conditioned space. Furthermore, it's damned near impossible (even with copioius amounts of spray foam) to really air seal at the basement-ceiling, but fairly straightforward at the foundation wall.

>> Unless you're using fire-rated Thermax you'll be required to put in a thermal barrier like half-inch gypsum over any wall -foam. An inch of foil faced iso and a 2x4 studwall with unfaced R13s might be cheaper, and would deliver ~R16 whole-wall (U0.06) after factoring in the thermal bridging of the studs. Keep the bottom edge of any polyiso off the slab, as well as the bottom plate of the studwall by putting it on an inch of EPS as a thermal & capillary break. (Polyiso can wick ground moisture, as does wood. EPS won't.)

Outside isn't an option but I appreciate the info. Hmmmm ... ok so maybe not worth doing right now. I thought I could just use double-foil 2" foam boards (I've got leftover of this stuff), but I guess it's not an option without installing a bunch of drywall too (PITA right now). We can get around to that when we get more drywall done in our rear addition next year.


>> 200 gallons on 3.5-4 weeks of "...really cold month..." isn't enough to go by, unless you have the EXACT fill up dates and a complete fill-up volume, so we can look up the heating degree-days for your zip code. But let's play the game anyway. The binned hourly mean temp for January in Bristol is ~32F according to weatherspark.com data, so lets assume a really cold month averages about 30F. That means that your heating degree day averaged (base 65F) 65F-30F= 35 HDD/day. You're burning something like 200gallons/25 days or 4 gallons per day.

I assure you that we burned about 6.5 gallons per day (averaged) for the 3 coldest months last year. We filled up some 200 gallons for 3 months straight and the interval was always 3.5-4 weeks. We certainly used about 1000 gallons last year, perhaps a bit more. That boiler is just a monster.

Of course - all of that is BEFORE we sealed the attic penetrations and filled it with as much blown fiberglass as would fit and before spray foam in rear addition (heat pump area), etc.

>> Are you still thinking 75K is "low" heat load number?

Ok ok - I'm believing you! LOL. So maybe 75K is high ... but after reading everything from you, it seems like 60-65K is at least pretty close and not "overly high". I'm going to look into getting a professional heat loss calculation done. That would be a worthy use of $100 I think.


>> If you can, add up the total fuel use in the past year. If it's under 800 gallons, any 50K boiler (condensing or otherwise) will cover your actual loads, assuming you didn't spend January in Belize with the thermostat turned down to 45F.

We definitely have used 1000-1200 gallons per year. But ... we also didn't add air sealing and insulation in the attic until march 2013 and spray foam in rear addition until may 2013.

Gosh I WISH we spent Jan in Belize :). We have 2 apts upstairs, so even if we WERE away we'd still need to keep the thermostats set to 68-70. They are fully programmable, and I've got them set for wake, away, back and night - with separate settings for sat and sun. We need to keep us (and the tenants) comfortable. All thermostats are in our areas on the 1st and 2nd floors. The tenants can't change anything (but they can open windows if they get too warm in the winter).


Do you have any opinions on any specific high eff mod/cons? Here's one Buderus example, for discussions sake: http://www.pexsupply.com/Buderus-GB...odulating-Condensing-Gas-Boiler-Nat-Gas-or-LP.

I'm just thinking ... if 90% mod/con is $3300 and a Burnham ESC4 (78K) is $2264 that HOW MUCH GAS in $ would 90% "high efficiency" really save us vs an 84% ESC or similar? My wild guess is maybe $100 per year? So that's 10 years for an ROI. If the mod/con needs ANY professional yearly touch - that would be wiped out and why even consider it? Any thoughts?

Thanks!

Jeff
 
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Dana

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I'm not talking about replacement windows, but rather single pane low-E storm windows, with a indium tin-oxide hard-coat low-E coating on the inner surface. The difference between code-min replacment windows and low-I storms on installed-cost is huge, but a tight low-E storm and an antique wood double hung can sometimes outperform a code-min replacement window, whereas clear-glass storms can't.

You can still use double-foil on the interior side of the basement you can still get there by strapping them to the wall with 1x furring through screwed to the foundation with 3.5" TapCons 24" o.c. and mount half-inch wallboard to the gypsum on the furring. (This is how I insulated the foundation on my 1920s bungalow. In my case I used reclaimed 3" fiber faced 2lb roofing iso at about R18.) With 2" 1.5lb density foil faced you'd be at about R13, which is fine. Just keep the cut edge off the floor to avoid ground moisture wicking. You don't have to paint or fully finish the wallboard if you don't want to- its sole function is to provide a code-approved thermal barrier for the foam. As a DIY using your leftovers or reclaimed foam from de-construction vendors its VERY cost effective, and improves both the thermal & moisture conditoins in the basement by quite a bit. Use a thin shot of spray foam seal & insulate the foundation sill & band joist to the wall-foam, and fatten up the R with batts on the band joist. (You can use the same cut-up foam board on the band joist if you like, just seal it with can-foam at the seams.)

We'd need to know the average daily temperature for that 6.5 gallons/day usage to turn that into a heat load number. If you plug in 1200 gallons in to the FSA calculator using Philly as the location and 1200 gallons/year usage, even with a highest-efficiency 105KBTU/hr 88% AFUE heat purge boiler it's coming up with about ~57K for a heat load @ +10F.

If I plug in 1000 gallons/year it comes up with ~47K @ +10F with the highest-efficiency boiler.

Using the model for the 83.7% AFUE + typical indirect with 150KBTU/hr of output (more similar to your setup, but probably still more efficient) at 1200gallons/year it's coming up with a heat load of 50KBTU/hr @ +10F, and at 1000 gallons/year it's 39,400 BTU/hr @ +10F.

Your current boiler is nowhere as efficient as the heat-purged boiler less than half the size, and the odds of it being more efficient than the 150K boiler with the indirect are between slim & none which means your real heat load is under the 60KBTU/hr DOE output of the ESC3 , and about half the DOE 90K output the ESC4. The FSA calculator models for the boilers are based on real test data on real boilers, but the heating degree-day data are for the 25 year average season, not last season's data, so if last year was milder than the 25 year average there is a bias toward undersizing. But it would have to have been the warmest winter of the past century for your heat load to actually be 60K, perfectly matching the output of the 3-plater. It' highly unlikely that your heat load was higher as 55K even in the less-sealed less insulated state it was in, and after improvements already under way it is likely to come in quite a bit under 50K.

With the boiler installed in the basement it is rightly sized in this type of installation by it's DOE output, not the I=B=R output. Only if the boiler is out in garage or otherwise outside the thermal envelope of the house would the I=B=R numbers be relevant. (That's one of the many reasons why you want to insulate the basement walls, not the floor, if the boiler is in the basement.) The ESC3's DOE output is 60,000 BTU/hr, and the ESC4's is 90,000 BTU/hr. With your heat load eventually settling below 50K (it's really already there), the ESC3 is by far the better choice in that product line.

The efficiency you can get out of mod-con is a function of how much radiator you have, which determines the return water temp. The lower the average water temp, the higher your combustion efficiency. The AFUE testing is done with 120F return water, and I suspect you have sufficient radiation that most of the time you would have lower return water temps than that (if you dial in the outdoor reset curve), which means you would beat the published AFUE. Buderus makes nice boilers, and the minimum-fire of the GB142 (which tests at 95% efficiency in an AFUE test, not 90% BTW) is about half your design condition load, which means with the system and reset curves dialed in you'd be getting BETTER than 95% efficiency, compared to 85% with the 3- plate Burnham ESC. So after further building upgrades if you would be using 1200 therms with the ESC, you'd be using less than (1200 x 85%/95% =) 1074 therms with the mod con, probably more like 1050 therms, a savings of 150 therms/year. At a buck a therm it takes awhile to make up the difference in installed cost, but gas prices are currently at historical lows- they simply can't go lower and still support the production cost of getting the gas out of tight-shale reserves. (It takes an order of magnitued more drilling to get gas out of shale or coal than from traditional basin formation gas.) The price may stay reasonably stable near current pricing, but I'd expect them to rise over the lifecycle of the boiler. Designing & setting up a mod-con is more complicated than with simple cast iron too- not a DIY project by any means (unless you can convince the manufacturer to let you enroll in their installer training.) The difference in installed cost is guaranteed to be more than the sticker price of the boiler itself.

At your heat loads the Triangle Tube Prestige Solo-60 or the Peerless PF-50 might be more appropriate, since their minimum-modulation is considerably lower, which means it will run in a modulating mode MOST of the time rather than just half the time with the GB142. But installer competence and local distributor support are more important than the which boiler vendor with mod-cons. Mod cons with aluminum heat exchangers are more sensitive to the system's water chemistry than those with stainless or cupro-nickel.

If you're doing this DIY, go with the 3-plate cast-iron. Even though the ESC series tolerates 110F return water, with your high mass probably-oversized radiation you'd likely be seeing temps below that during periods of low load, so it's worth plumbing in a radiation-bypass loop with a ball-valve to be able to tweak the return water temps so that it's up to at least 110F by the time it's 5 minutes into a cold-start of the system.

A flow of 4x 1.85gpm at a 65F mid-winter temperature rise is 240,000 BTU/hr, which your beastie boiler can pretty much handle. But with any boiler reasonably sized for your space heating load you'll absolutely need an indirect tank, and a fairly big one. For showering performance you can buy a lot of capacity with a drainwater heat exchanger downstream of the shower drain, feeding both the cold feed to the water heater AND the cold feed to the shower, but with multilple showers it's usually easier to let it feed the cold distribution for the whole house. At eastern PA incoming water temps the output of the DWHX will never exceed room temp by more than a few degrees, so it's not a real issue for other cold-water users during showers. At 2.5gpm a 3" x 60" or 4" x 48" returns more than 50% of the heat from the drain back into the system, roughly doubling the apparent capacity (and apparent efficiency) of the water heater in a shower type draw where the drain & hot water are flowing simultaneously It does nothing for tub-fill efficiencies though. If you really think you'll ever be running 4 showers simultaneously this technology would reduce the size of the indirect required from the 100-120 gallons to something in the 60-70 gallon range. From an energy savings point of view it takes awhile to pay off at a buck a therm, but being able to down-size the indirect it's a net savings up front, as WELL as the energy savings.

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