What size boiler?

[fredfons]

I bought a 2300 square foot bi-level in Philly suburbs built in 1964 for investment/rehab and am converting from oil heat to gas. I am a carpenter and figured I could handle doing a heat loss calculation to size a new gas boiler using the calculator at www.crownboiler.com/Support/Education/Boiler-sizing. It calculated 56,000 BTU heat loss but the existing oil boiler a New Yorker APU 690U bt4 has a DOE heating capacity 129 MBH. Does my BTU calculation sound too low for this house or is the existing boiler oversized? Was it oversized due to DHW? Or am I misreading the rating numbers? The house is 4 bedroom 2 1/2 bath, 25' x 47', lower level uninsulated brick veneered conc. block and upper level is frame with 2" batts in walls and 5 1/2" loose fill in attic. All new double glazed mid quality windows and doors. ODT= 13. I am looking at Crown mod con BWC070 rated 70,000 BTU. Any thoughts on if this might do the job?

[jadnashua]

Most older boilers were grossly oversized. Many calculators are as much as 100% over the 'real' need. A better solution for this house would be to compare the actual oil used verses the heating degree-day for your location. No idea if that brand and model is good or not...

If a lot of the insulation/window upgrades were made after the last heating season, your new calcs may no longer be accurate.

Generally, there's no need to oversize a boiler to provide DHW unless it is a commercial situation where there's significant hot water useage all day long. It's generally run on a priority zone, and uses all of the boiler's output while heating the DHW, then reverts back to space heating when that's done. The homeowner will rarely notice that short gap.

[Gary in NJ]

A general rule of thumb is 35 BTU's per square foot; 80,500 for your home. However, since you have uninsulated walls I feel that this ROT is useless for you.

[Tom Sawyer]

Gary, there are no rule's of thumb for BTU's/sq' It's that kind of thinking that has screwed up the heating business for generations.

[mage182]

I'm a seasoned DIYer with a Modcon that I'm using for the first time this season. I basically installed it myself after firing 2 plumbers I hired to do the job. I've spent 100s of hours reading and expanding on my plumbing knowledge during the process. Here is what I recommend based on my experiences:

Experiences

The first plumber I hired picked out a unit that would allow for my future expansion plans. This resulted in a unit that is around double the size i need for my house/level of insulation. I'm lucky that with the amount of research I've done and the unit I have (Burnham Alpine) that I can modulate down to my needs and avoid short cycling.

Mounting the unit on the wall looks better but I've had a few people tell me it creates more noise on the first floor when running than their floor mounted models. I may add some insulation to the basement ceiling to see if that helps. The Bimini Buddy looks great though. I would definitely get that if you go wall mount. That would make for a really clean install.

Larger zones are better. I have 4 small zones in my house that I'll be combining into 2 once the heating season is over.

IHW tank is highly recommended. Mine only runs a few times a day and has maintained temp without running for up to 3 days if I was away and didn't use any hot water.


Pretty much everyone else will back me up in saying that the prework for a Modcon is the hardest and most important part. If I had picked the right professional to install it and do the right heat loss and pick the right sized unit, it would have saved me money and I wouldn't have to spend time tweaking the system all the time. Mod cons break almost every norm that has been accepted in the heating world. Everything changes and you need to hire someone who understands that and can do the job right.

[jadnashua]

FWIW, the Buderus wall-hung unit I have is totally inaudible (to me at least) if you are more than 3' away from it or you have the cover off. It is mounted on the concrete basement wall, though, so hard to vibrate anything. the only internal moving part is really the fan, if you discount the gas valve.

[Dana]

Gary, there are no rule's of thumb for BTU's/sq' It's that kind of thinking that has screwed up the heating business for generations.

Are you kidding? There are LOTS of rules of thumb for BTU per square foot...


























...so WHAT if they're all dead wrong? ;-)

If you have fuel use from billing between dates where we can look up the heating degree day data for those dates it's pretty easy to calculate a solid upper bound for the BTU/hr at the outside design temperature. But if this is a rehab with no history you probably don't have sufficient data to work from.

But from experience, a reasonably tight 2300' house with double-panes it would have to have almost no insulation to hit as high 56KBTU/hr at a 13F percentile design temp, and you have at least some. If you were planning to live in it or rent it to a friend it's worth blowing cellulose into the cavities that currently have the econobatts, and blowing over the existing fill to boosting the attic R to something more like code which will bring the peak heat load down considerably.

Even if you're planning to flip it, there are probably subsidies available that can often make it cheaper to hire a pro than insulate it as a DIY. If you can get the heat load under 50K (likely- since it may already be less than that) for cheap you can then go with a smaller cheaper mod-con. (Is the BWC070 even a modulating boiler? The specs only have one input BTU number not a max & min.) Even if it's the same total cash outlay, what you end up with is a tighter more comfortable & efficient place.

I have a similar sized 1-1/2 story house with all framed construction and a 0F design temp, and even before I insulated the walls (it previously had paper-clad half-inch horsehair for wall cavity insulation- maybe R1 on it's best day) the measured heat load (measured by the fuel use & DOE efficiency of the boiler) at 0F was less than 50K. Uninsulated brick is probably similar, and the econobatts slightly better than where I started. The ratio of exterior surface area to floor area may be comparable (or not) and your total glazed area may be comparable (or not), but your design temp is 13F warmer. My gut tells me 56K is the right range, but an upper bound, but that insulating it and air-sealing the attic floor/ceiling interface would likely bring you down to under 40K. Most Manual-J or IBR type heat loss calculators hit ~25% above measured reality, so if you filled in the Crown Boiler spreadsheet model diligently and it says 56K, odds are pretty good it's under 50k already. And $500 in cellulose could easily knock that down another 8-10K putting you in the range of a Munchkin T-50/ Pinnacle T50 or Triangle Tube Solo-60 or something.

What type/how much radiator or baseboard do you have? It makes a difference on how the boiler needs to be set up. Most of these are not great for DIY installation, but if you're determined, see if you can find a manufacturer's installation class/seminar before diving in, or it may end up costing you more than letting a pro do it.

[Runs with bison]

If you have fuel use from billing between dates where we can look up the heating degree day data for those dates it's pretty easy to calculate a solid upper bound for the BTU/hr at the outside design temperature. But if this is a rehab with no history you probably don't have sufficient data to work from.

Yep, and if one is really wanting a precise number that beats any estimate (such as Manual J), record daily fuel use for a season apply the efficiencies, and calculate btu's req. vs. average daily outdoor temp (from a local weather data logger.) That's what I did to size my furnace and it gave a nice linear fit to the plot. With that you can pick any design temp you want to size the unit. In my case it was overkill because the AC air handler frame size limited how low I could go on furnace size. At least I was able to use a two stage furnace, so it almost never runs in high fire mode.

[BadgerBoilerMN]

Jad is right, but, though calculating heat load from fuel history sounds good, it only takes into account one season for a house that was. There is no accounting for improvements in the envelope that are often contracted with the addition of a new boiler. A proper Manual 'J' heat load analysis uses the accepted mean taken over a longer time period and building improvements can be easily and accurately accounted for. We specialize in condensing boilers and use Wrightsoft HVAC software to calculate heating and cooling loads.

I am not a fan of the online load calculators as they are highly simplified (inaccurate) but it is quite obvious from our experience that many "professional" boiler companies are still using "rules of thumb" for sizing residential boilers. We wish they would use ANY heat load calculator. In fact, it has gotten so bad that we write are contracts for boilers "sized to the load" as we find that we are the only heating company that performs a heat load at all (some have their suppliers "knock out" a load to satisfy the Minneapolis/St. Paul building officials). This is most disappointing as the results are evident from double-size boiler described here and the double-size Buderus GB142 that I looked at last week. The practice of over-sizing boilers goes on through mass-ignorance.

Rules of thumb are a lazy man's tool. There are useful benchmarks, to which one might compare the results of a proper heat load, even if you use the marginal HDD method. But having performed hundreds of heat loads and often guessing within 5% the load and design water temperature for a given radiant or radiator system, I still do the math.

In the case at hand, the owner has changed windows and should improve insulation in the attic (I would have chosen the latter first) which, would naturally change the load (throw out the fuel/HDD method) and perhaps the size of the new boiler.

A few facts about modulating condensing boilers:

Some make more noise than others, but all are quieter than any power-vented water heater. If hung on an outside foundation wall, few will be heard by anyone, anywhere in the house. It is more likely that the over-sized pumps will be heard through the distribution piping. We install many brands of ModCon boilers, both wall-hung and floor mounted and choose them for size, low-fire capabilities, control, ease of installation and service. You should too.

The novice should not over-size a boiler for any reason he might make up. This goes for the professional (most only install a few retrofit boilers a year, many only do a few in a lifetime). When sizing a condensing boiler an experienced designer pays careful attention to the rated maximum AND minimum output of the boiler he wants to install. If the boiler is twice too big it may very well bump off the bottom (cycle on the lowest fired setting) giving the owner poor efficiency and service headaches over time. Think of it as highway vs city miles. Never buy a used Taxi.

All ModCons now have weather sensitive controls which, are properly set by an experienced designer, whom has measured the existing or new radiation, selecting the design water temperature according the the Manual 'J' mean for the area in which he is working.

Rule of Thumbers need not apply.

http://www.badgerboilerservice.com/contractor.html

[Dana]

Clearly the fuel use history calc would always be an upper bound (if he even had it on this house), since as-used efficiency on an older and oversized boiler rarely hit's its DOE numbers, and reasonable estimates could also be made for any improvements that may have occurred.

But it's surprising how often Manual-J executed by well-intentioned practitioner using decent software overshoots the mark by more than 50%. I've never seen such a calculation undershoot (or even break-even) relative to a fuel-use/HDD sort of approach, though they can sometimes be within 10%. But my sample size is pretty small- I'm sure there are exceptions to prove the rule.

At the very least, a competently done Manual-J won't oversize by more than 2x the way xxBTU/ft methods do (nearly every time), to the up-front cost and operational efficiency detriment to the owner.

The windows & doors of a house pushing 50 may have needed replacing for reasons well beyond mere energy efficiency. But with double-hungs in reasonable shape it's usually pretty easy to re-weatherstrip them and add better-quality tight storm windows (even low-E storms) for less money and better performance than middle-of-the pack vinyl replacements. Replacement windows rarely break even in an economic analysis unless the heat loads and per-BTU fuel costs are high, but there is often a comfort argument to be made.

I know PLENTY of pros who do as many retrofit boilers as they do new construction, but I may live in an area with a much higher fraction of housing stock pre-dating 1950, and proportionally less new construction. I've seen plenty of 60-70-80+ year old heating systems still operating on the original boilers that should have been retired decades ago- were the market rational and had an economic time horizon longer than 3 years, retrofit boilers could dominate the local situation here for many years to come.

[Runs with bison]

About not having fuel use history: if it is a utility (line natural gas or electric rather than home heating oil tank or LPG tank that the customer owns) then you might be able to get the monthly usage for a year or more back. I did this when I moved into our home. There was about 18 months worth of data from the gas supplier and somewhat less from the electric. It only took a phonecall to each to get it. But try to get it ASAP, they archive it periodically.

Of course, this usage doesn't necessarily tell you what the other person's set points were... The previous owners of our home were electicity/water hogs in the extreme (they used nearly 3x as much electric annually for 2 occupants as we do now for 4.) Their gas use wasn't so out of whack though.

[Dana]

The first line of this thread is:

"I bought a 2300 square foot bi-level in Philly suburbs built in 1964 for investment/rehab and am converting from oil heat to gas. "

Even if you had the number of the last oil delivery out there's no telling if that was the only delivery made. But if it was a late winter delivery and the billing has a K-value that might be a useful bit to use. A heat loss calc is by far the better path in this case.

It's pretty tough to deal with electricity-heated units this way unless the heating was separately metered (rare, but it happens.) I did some spreadsheet modeling working backwards from the previous owner's power use on a small house my mother moved into last summer(I needed to come up with a range for sizing a heating & cooling mini-split), but verified the estimates against a Manual-J using liberal assumptions on air leakage, and they weren't far apart, but maybe that was just dumb-luck. With a mini-split it's better to estimate on the high side anyway, since they run more efficiently in the bottom compressor speed than at higher speeds, and even if 2x oversizing it won't short-cycle, being a modulating system with a significant turn-down ratio. If we missed the mark and it doesn't keep up, the resistance electric hot-air system was pretty new, and would cover any shortfalls. I might have shot for higher precision were it the one & only heat source for the house.

[Runs with bison]

Dana, we were past the scope of the thread already. That's why I mentioned getting the monthly numbers from the utility for doing evaluations, not really for the OP.

You can extract pretty good values for the unit's utility consumption even for electric heating/cooling monthly data, as long as the heating/cooling season isn't unreasonably mild. It's not too hard to determine the baseline months and subtract them from the other seasons. However, you have to be able to screen out oddball seasons (luck of the draw with mother nature.) Three summers ago we had a wierdly cool summer and used 1/4 as much electric for AC as the two summers bracketing it. The biggest difference from the norm was the low humidity and low temps we were having at night. This let us open up each evening, and close off all day. Normal summers here we go several months this is not possible: the relative humidity is so high that if we opened the windows we would feel hotter even if the temp was several degrees cooler.

The difficulty with AC/heat pumps is then converting that back into a heat load since the operating efficiency range can be wide. Gas furnaces are easy because the efficiency is not likely to be too far off the general ranges for the three types: 90-95%, 80%, or 60-65%. And that is another thing I've noticed about sizing, the input Btu rating is often used going from one to the other...rather than the output. So a vendor is likely to suggest a further oversized unit to replace an already 2x oversized one.

[Dana]

With resistance electric heating the error bars can be unusually large if the habits of the previous occupant aren't known. For frequent bathers the incoming water temp on the HW heater can masquerade as significant space-heating load too, even if you THOUGHT you had a good handle on the background load for other uses. An undocumented winter vacation can also introduce a significant error. In my mother's situation the HW heater was cranked to ~150F and outside of fully conditioned space, and most of the lighting was incandescent (duty-cycle unknown), so working from billing data supplied by the utility, even analyzing it with very good HDD data for the billing periods, parsing background assumptions several ways did not converge on a consistent result. It was possible to come up with a range, but it was a large range from a percentage point of view (between ~17K-26K load @ design temp.)

I'd never work backwards from a heat pump power use, it could never be as consistent or reliable as a Manual-J.

Oversizing on gas furnaces has to be truly grotesque to impact efficiency, but right-sizing is definitely better from a comfort point of view.

In many places in the US even if peak AC loads are dominated by the sensible load, it's not uncommon for the latent load to dominate the seasonal average load. (This is definitely true at my house, where the central AC runs fewer than 20 hours/year, but the dehumidifier becomes measurable chunk of the July/August power bill.) I've never tried to determine AC sizing via power use, too many moving variables in the model with big error bars.

[Runs with bison]

I can look back on utility data from 5 different homes and see fairly consistent delta's in both electric and gas that are tied to the season. The first two homes were incandescent and had fairly high shower/tub/washer usage. This included homes with extremely low, mid and high SEER AC, as well 65%, 80%, and 95% furnaces. And in this last home I was able to determine the previous owner's inefficient practices from the data by comparison with our results for the same seasons.

Perhaps it is because I am accustomed to doing plant energy and mass balances and getting accountability close to 100%, but hot water is not hard to account for, including the seasonal factor. (I calculated the seasonal adjustment based on typical water supply temp diff summer and winter, using 105 F shower temp that is typical.) Separate metering of the water heater isn't really necessary. It's a rather small portion in peak season. Water is just another utility so it is a matter of getting the data and looking for non-watering seasons for a baseline. Combine that with off season energy use and some reasonable assumptions about the percentage hot water and you get good closure.

I would be highly reluctant to trust to an estimation (that is known to miss by a mile) what I can more easily measure. And even AC electric use vs. load is not too hard to work out. With baseline electric use in hand and temp. data available, all that is left is to measure power use at the meter during mid cycle on a representative day or two vs. when it is down. The tonnage is already known within an acceptable margin, and unless something is seriously amiss with the unit, this will give a decent result. Realisitically, with wear and tear near the end of its life, this nominal cooling load will be on the high end--which is good way of sizing for the full life cycle of the replacement machine as well. And the best way to check the sizing is to see if the machine is keeping up on a day that is hitting the design temp or close to it (particularly the 2nd or later day of several in a row--for here that was around 105 F) If it is falling behind you know you don't want to go smaller. If it is keeping up and still cycling off you can keep track of the down time vs up time for a few hours to determine excess capacity. The same is true for heating season for a gas furnace.

I putzed around with Manual-J briefly but the apparent error bars associated with its inputs were larger than the unknowns in the measured data. There are too many elements of the structure that I can't be certain of, too many cavities I've discovered that lacked insulation where it was to be expected for example, unknown "tightness" of the envelope. I wouldn't have confidence using it on a structure that I hadn't watched through construction. Now, if I had some experience using Manual J and getting high accountability, I might have some confidence in it. But when I have higher confidence data that I've gathered and determined multiple ways vs. a black box with many unknowns...the black box is the last one I would trust.