Conflicting Contractors...

[MarkeyHall]

I'm in the same situation as most people around here (replacing an old oil boiler to a new natural gas system). My house, about 1600 square feet, was built back in the 1950's (Massachusetts). I'm in the process of converting my old oil boiler (115k BTU with a coil/tankless water heater) over to a high efficiency gas boiler/indirect water heater. I recently replaced all of the windows in my house with high efficiency windows but the insulation is pretty lousy in the house. I have 2 full bathrooms and there are 4 of us living in the house so showers/baths go every night. 1 last thing, I have 2 zones in my house and the house is heated with cast iron baseboards (which I've been told are very good) in most rooms.

Now to the question, I've had a number of different contractors in here over the past few weeks but only 2 have done any type of 'math'. 1 did a full heat-loss calculation and the other just measured the baseboards. The one that did the heat-loss calculation told me that I could get away with an an 80K BTU boiler (which he later up'd to an HTP EliteFT 110k BTU) with a 40 gallon indirect hot water heater (Plug N Go PRO41ZPG) while the guy who just measured all of the baseboards told me that I needed a 140k BTU boiler and recommended a Buderus GB142/45 (160K BTU) with a 40 gallon Superstor.

Based on the great feedback I've seen on this site, I'm confused. In one aspect, I trust the company that did the full heat-loss calculation, but on the other hand, the 2nd contractor (who measured the baseboards and supposedly got the boiler sizing requirements from the local Buderus sales rep) told me that I needed the higher BTU's to support the indirect water heater as well as the heat.

I don't want to oversize my boiler but I also don't want my family to be cold in the winter - any suggestions on who I should believe?

Thanks for your help!

 

[Jadnashua]

Personally, I doubt either of those are correct! Scan some of the older threads for a fairly reliable method to calculate what you need. If you have your NG bills from the last year, and you can estimate the portion that was for heating verses cooking or other things (like maybe a dryer), and downloading the heating degree day info for your zipcode, you can get a very good indication of your real, measured, heat requirement.

My guess is that your need is closer to 30K BTU or so, but you'll never know until you do a good assessment. A properly sized boiler would run 100% on the coldest day of the year. Cycles cost money and shorten life. That's why matching the device to the load is so important, and it ends up being more comfortable, too.

 

[Sponsor]

 

[MarkeyHall]

Personally, I doubt either of those are correct! Scan some of the older threads for a fairly reliable method to calculate what you need. If you have your NG bills from the last year, and you can estimate the portion that was for heating verses cooking or other things (like maybe a dryer), and downloading the heating degree day info for your zipcode, you can get a very good indication of your real, measured, heat requirement.

I'm actually going from oil to natural gas so I have no record of previous usage. I'll continue to look through the old posts for guidance on doing the calculations myself.

 

[Dana]

What Jim (jadnashua) said. They're both wrong.

But the idiot that measured the baseboard is the MOST wrong, since his basic methodology is the most fundmentally wrong. The baseboards have absolutely nothing to do with the heat load of the house, even if they determine the water temp at which the actual heat load can be met. While it's an important part of the heating system design, it has no bearing on the SIZE of the boiler required. And up-sizing for domestic hot water capacity is almost always wrong too. I find it astounding that he'd have the lunacy to recommend a 140K or 160K condensing boiler to replace a wheezing 115K oil boiler that was probably running at no better than 75% as-used AFUE.

Oversizing a mod-con on a 2-zone system will almost certainly lead to short-cycling and excess boiler wear at part load, even if the min-mod output of the condensing boiler is below the design condition load. The smallest boiler that actually meets the design condition load will have the best overall performance, since it will run fewer & longer cycles, spending most (or all, if there's enough baseboard) in condensing mode. Cast iron baseboards have reliable output characteristics at low-low-condensing temps (unlike fin-tube), so don't blow it by oversizing the boiler!

If you have some oil bills from this past winter kicking around that have a "k-factor" stamped on them, let's have that number, as that will put a stake in the ground for an upper-bound on the boiler size.

For reference, my ~2400' house (+ 1500' of semi conditioned basement) in Worcester has pretty marginal insulation too, and still has the circa 1923 double-hung windows everywhere (with storm windows somebody added in the 1980s) but even before I started upgrading the place the measured heat load based on fuel use was under 50K, and is now well-under 40K @ 5F (the 99% outside design temp for Worcester.) If your windows are U0.34 or lower and the place isn't super-drafty, odds are you're already under 40K too, and getting it down to 30K or less shouldn't take a huge effort.

Bottom line, even the 80K unit the first contractor suggested you could "get away with" is probably 2x oversized for your true heat load. I'd be good down to about -70F with an 80K boiler, a temp not seen in Worcester since the last ice age. (And that's how silly it is!)

Paying an energy nerd to do an independent room-by-room Manual-J type calc with realistic inside & outside design temps that also notes the baseboard length in each room would be worth it. Given the recommendations these two contractors came up with, paying an independent hydronic heating designer to come up with the basic design based on Manual-J and the amount & type of baseboards you'd likely come up with a better-cheaper-more comfortable solution than letting either of them design it, even if you hired one of them to do the installation.

If you have enough baseboard to hit design-day numbers with 130F water you might be able to set it up to run with a condensing hot water heater instead of a mod-con boiler. I heat my place with 125F heating system water, but have a combination of radiant-floors & hydro-air to be able to do it all at that low temp. It keeps up even when it's -10F out, despite being radiation-limited to about 43KBTU/hr at 125F water temp, but it would take quite a lot of baseboard to do that. It wouldn't surprise me if you could do it with 140F water out of the boiler at 0F outdoor conditions though.

BTW: If you have R9 or R11 batts in those walls, blowing cellulose in over the batts is a cheap and easy way to tighten it up considerably- you'll usually see energy-use and comfort improvements well beyond the very modest incremental change in R-value. Sealing up the old flue and foam-sealing/insulating the foundation sill & band joist helps a lot too, as does insulating the basement walls. (Basement walls can be done at reasonably low cost without creating moisture problems if you do it "right".) Even if your heat load happens to be 40K now, doesn't mean it will be after a few rounds of insulating and tightening. Replacement windows are usually most expensive/least effective part of a building-envelope efficiency upgrade- the rest can be cheaper and easier.

 

[MarkeyHall]

So you think an 80k is 2x? Wow - even with an indirect hanging off of it? I want to make sure I have good hot water in the winter time (a problem I have with my old system now).

I'll definitely look into hiring a 'nerd' to do an assessment on my house. Since you live in Worcester, know of any you'd recommend?

 

[Jadnashua]

A typical stand-alone gas WH has around a 40K burner. If you have an 80K boiler, it usually sets the WH up as a priority zone, so all 80K is available to heat the water in the tank. Once that is satisfied, it reverts back to home heating. In most situations, you'll never notice it's not heating the house. You do NOT upsize the boiler for the tank unless you run something like a commercial spa where you need huge amounts of hot water continuously. If you need lots of water in a short period of time, you get a larger tank, not make the boiler bigger. Typical use is maybe high in the morning, then none or only a little most of the rest of the day (maybe a peak again at night). So, the boiler just merrily keeps the house warm in the meantime.

 

[Tom Sawyer]

Converting to gas may not save you anything. The cost of high efficiency condensing gas equipment will most likely way surpass any savings that could be realized by going with high efficiency oil instead. There are 140,000 BTU's in a gallon of oil and the equivalent in natural gas is only around 86,000 BTU's Modern, high efficiency oil units like the Buderus and the System 2000 and other will outperform gas equipment all day long. Go to www.nora.com and download their comparison software and run the numbers. You will be quite surprised.

 

[Dana]

Tom, get a grip! The world has changed! Oil hasn't been cheaper than gas in New England in a decade, and it's flipped quite strongly the other direction- gas has hit record lows, and oil hasn't even come close for more than 5 years. Looking forward, #2 distillates are likely to keep going up in price, but there's so much gas being produced from the nearby Marcellus & Utica shale formations NOW they're having to cap wells for lack of places to store it- a very strong moderator of regional gas prices. You'd have to be NUTS to install a new oil boiler if you're on a gas main in MA!

Buck a therm gas like this year's rate (even $1.25/therm, if it goes up 25%) at 90% efficiency looks pretty damned good next to three-buck oil at 87% efficiency (not to mention $4 oil, like it was this year) any way you slice it, and that's BEFORE getting into the fact that even the smallest oil-burners are going to be 2x oversized for this place.

The simple math goes like this:

At $1.25/therm in a 90% burner you get 90,000BTU into the house for $1.25, or ($1.25 x 1,000,000/90,000=) $13.89/MMBTU

At (lower than the 5 year average and a huge discount from this year's pricing) $3.00 oil in an 87% burner you get (0.87 x 138,000=) 120,060BTU for $3.00, which is ($3.00 x 1,000,000/120,060=) $24.99/MMBTU

Yeah, I think I'll heat with oil, seeing as how how if the price of gas goes up 25% next year rates heating with oil will cost only 80% more to heat the house if price of oil DROPS by 25% next year.

At this year's (painfully real) rates it was nearly 3x as expensive to heat with oil- $33/MMBTU with high-efficiency oil to $12/MMBTU for condensing gas.

I'm quite impressed, NORA notwithstanding!

People off the gas-grid locally have been retrofitting mini-split heat pumps for lower cost heat, even if when undersized for, and even with 15-18cent electricity and even at a lousy COP of 2.5 (many will do better than that in southern New England) it's less than half the cost of heating those zones with oil, paying for themselves in under 4 years just on the offset in oil use.

MarkeyHall: I'm my own nerd, and do my own heat load calcs but I don't do it for a business. Old-schoolers in my neighborhood will eyeball a place and if it has single-pane glass call it 35BTU/foot (which would be 56KBTU/hr for 1600'), but if it has new windows and feels reasonably tight they'll call it 25 BTU/foot (which would be 40KBTU/hr for a 1600' house), and both are usually well over the true heat load. Any decently tight house with double-panes or storms will come in at 20BTU/foot or lower at a design temp of 0F (as in say Springfield.) No place in MA has a design temp lower than -5F, which would add less than 10% to the 0F load. So odds are you really ARE in the 30-40K range even if it's somewhat drafty, but it's sounds like there's room to cut signficantly from there, which would provide higher comfort as well as fuel savings.

Since even the very smallest mod-cons all put out more than 40KBTU/hr at high fire you don't really need a heat loss calc to pick a boiler- it's the smallest of the line, whatever manufacturer's line you use.

What Jim said about upsizing for an indirect- run it at a priority zone and even a tiny 50K Peerless Pinnacle T-50 or 60K Triangle TubePrestige Solo-60 would apply more burner to the hot water than a typical 40 gallon tank. Size the indirect for the biggest tub you have to fill.

The reason your current boiler can't always keep up with the hot water loads is due to the necessarily small heat exchanger size of embedded coils, plus the fact that the higher temp of an oil boiler causes them to lime-up with even a hint of water harness, leading to ever lower heat transfer rates over the years. The burner may keep up, but the heat exchanger can't. With an indirect neither the burner nor the heat exchanger has to keep up with the instantaneous flow rate the way a tankless coil does- the water is already hot. As long as you can fill a tub with the volume of water stored, you're golden. Even tied to the smallest mod-con it will have recovery rates faster than a standard gas-fired tank, and unless you need to be able to serve up 60 minutes of serial-showering at a time a 40 gallon indirect will usually handle the load just fine. There is no need to bump the boiler size or hot water in nearly all cases. If you had 3 showers and a spa to fill, and no space for a 100 gallon tank you might need more burner, but that doesn't sound like your house.

 

[Samantha Nangle]

Hey,
Thank you so much for informative information.