Need advice on converting from oil to natural gas.....combis?

[Edgeman]

With all the incentives and rebates out there, it is time to switch to natural gas! I spent the last few week reading a ton of info on the internet. I live in a coastal CT town in a 1960 sq. ft wood shingled raised ranch with a timberline asphalt roof. This week I had an energy audit done. My oil boiler is a 25 year old Utica 122K BTUs which runs at 79% efficiency as per the energy audit with 3-zones but one is slaved into another so I have 2 thermostats. With the blower door test the infiltration was reduced after caulking, seals, etc. from 4470 to 3830 CFM. The SlantFin heat loss calculated at 41,200 BTUs with a 9 degree design temperature. There are 4 people living in the house and the coil in the oil boiler is inadequate and has been a pita.

What size gas boiler should I look for? Do I have to size up the boiler for hot water? A couple of contractors have suggested a Navien CH-210 Combi with the tankless HW. After reading comments I am not sure about the reliability and maintenance cost of the Navien. I am thinking of a high efficiency gas boiler AFUE 90%+ for the rebates and operating cost mated with an indirect-fired water heater. What size gas boiler and indirect tank should I get?

Does the 41,200 BTUs heat loss sound correct for my size house? Do I have to increase it for indirect hot water heating? One contractor suggested a 96K BTU ballpark whereas another proposed a 175K BTU Navien CH-210asme Combi. Am I underestimating my heat loss?

Another contractor said that the inherent problem with all combis is that the water adapter has to swivel from heat to a priority demand for tankless hot water which already has a delay to feel the HW. So on cold days the heat shuts off to accommodate the HW demand anytime someone turns on the faucet. Or if someone takes a shower, the heat shuts off and the house temps drop. He suggests the Triangle Tube Excellence over the Challenger in this regards.

Any suggestions and comments are greatly appreciated and I will be speaking to contractors this week. Thanks.

[jadnashua]

Most all boilers with indirects are wired to provide priority zone for it - this means when the tank's aquastat calls for heat on the tank (by no means every time you use some), all of its heat goes into reheating the water in the tank. Normally, this doesn't take too long, and most people never notice that the house is not getting heated for those (usually) 10-15 minutes or so. You do NOT upsize the boiler when using an indirect unless you are running something like a spa or a Laundromat where there's a call for lots of hot water constantly (and then, you'd probably supply one dedicated to that use).

That heat loss actually sounds a bit high, but might be right for your house right now if they did the calcs properly. Additional insulation might get that figure down as might new windows, so something like a 60K mod-con would still have a margin for your use. In boilers, bigger is not better.

[Edgeman]

Most all boilers with indirects are wired to provide priority zone for it - this means when the tank's aquastat calls for heat on the tank (by no means every time you use some), all of its heat goes into reheating the water in the tank. Normally, this doesn't take too long, and most people never notice that the house is not getting heated for those (usually) 10-15 minutes or so. You do NOT upsize the boiler when using an indirect unless you are running something like a spa or a Laundromat where there's a call for lots of hot water constantly (and then, you'd probably supply one dedicated to that use).

That heat loss actually sounds a bit high, but might be right for your house right now if they did the calcs properly. Additional insulation might get that figure down as might new windows, so something like a 60K mod-con would still have a margin for your use. In boilers, bigger is not better.

I was referring to the time lag for combis. He likes indirect water tanks although they would be more expensive. I counted that I have around 160 ft of baseboard in my house. A contractor said that if I multiply that 160 x 560 = 89,600 BTUs that would be the maximum BTUs that can be outputed. Is that true? Another contractor who seems very knowledgeable is suggesting the TT Prestige Excellence PE110 boiler with internal indirect hot water heater. He likes it better than the TT Challenger Combi for hot water generation and thinks it would be more reliable than the combi.

[jadnashua]

The spec sheets of the radiators will give the heat output with various water temperature inputs. Feeding them cooler water does drop off their output, but the advantage of a mod-con is that it can adjust to the need (within reasons). It's unlikely that you ever needed a maximum output from the radiators. Having the ability to feed the house more can help if you use a setback since it can reattain the set point faster, but the efficiency of a boiler decreases when you fire it at max. Plus, you get less expansion/contraction noises if you run them at a lower temp longer and are going to be more comfortable.

Personally, I don't like combi's. They can limit your maximum hot water use and it also means that your boiler needs to be hot 24/365 to provide it...not a particularly efficient thing in the summer. And, to get a reasonable flow of hot water out of one, you may need a bigger boiler than you would normally need for space heating - this is a constant efficiency hit. Plus, the heat exchanger may need to be delimed periodically, increasing maintenance costs, or the heat transfer ability decreases.

A good indirect can have as little as 1/4-degree per hour drop in temp - i.e., they generally have quite good insulation, and might not need to be rewarmed over a long period unless you are using it regularly.

You do not want to buy a bigger boiler than you need, and the only way to tell is to perform a good analysis...a WAG, which is taking square footage or the old boiler size, or length of radiators is not good enough. BIgger boilers cost more, tend to short cycle, are less efficient, and wear out faster because of the many cycles. A 'proper' sized boiler has fewer cycles (the shutdown is where you lose the most efficiency) and provides a much more consistent heat output, is less expensive, and will last longer.

The last thing a contractor wants is to have someone call and say the new boiler can't keep the house warm. Unless they do the calculations, the tendency is to sell you one bigger than needed for their comfort, not yours. The house doesn't automatically become a refrigerator if it's too small. For example, say it can keep the house warm at -10 Degrees, and it gets to -11. Since that doesn't usually stay there for long periods of time, at worst, it would cool off one degree, but it would depend on the insulation and air infiltration. If you had a calc that said 40K BTU, it's probably optimistic...a 60K BTU boiler running at 90%+ (at least 54K out) would be more than enough.

Find someone that can do a good manual J. Then decide on what you need. Oversizing is a big waste.

You can get a fairly good estimate of your actual needs by some calculations based on your real oil use and the heating degree-day info available for your area. How to do this has been discussed lots of time, and a search will find it for you. Basically, each gallon of oil has a certain amount of energy in it. Your boiler converts that into heated water at a fairly fixed efficiency (say 80%, but your boiler may differ). THen, compute how much heat (from the degree day charts available to anyone for free) was actually used to keep the house warm...that will give you the amount of heat required to keep your house warm. This takes into account more than some manual-J calculations, since they may not know of how well the insulation was actually installed, or the real quality of the windows.

[Edgeman]

How do I calculate heat lost base on oil consumption? My energy audit says in 2012 I used 775 gallons of #2 diesel at 79% efficiency. The gov data says that my region had 4753 heating degree days in 2012. I keep the house at 68 degrees. How do I estimate the hourly BTU heat loss from this data?

[Edgeman]

From http://www.heatinghelp.com/forum-thr...ss-Calculation

Applying Brad's formula:
BTHU = (Annual Fuel usage/0.6)*(house temp-design temp)*efficiency*fuel heat value)/HDDx24

Where:

68 house temp that we set to
9 design temp from gov data
0.79 efficiency from energy audit
138690 fuel heat value for #2 diesel
775 fuel usage from energy audit/oil company logs
4753 Heating Degree DaysHDD from gov

yields = 73,197 BTU loss per hour

[jadnashua]

Now, take a summer month and subtract that amount. When you run a boiler with a hot water coil, you run the thing constantly so you can have hot water. Some of that heat is going to heating water, and being hot all the time has a fairly big energy hit. Using an indirect would make it much more efficient, as it can only run when needed.

I think I'd run the calc for say one or two winter months, then a couple in the summer when the heat is off, say July-August. I think you're wasting a lot just for the hot water, and the house itself doesn't need that much.

[Edgeman]

Now, take a summer month and subtract that amount. When you run a boiler with a hot water coil, you run the thing constantly so you can have hot water. Some of that heat is going to heating water, and being hot all the time has a fairly big energy hit. Using an indirect would make it much more efficient, as it can only run when needed.

I think I'd run the calc for say one or two winter months, then a couple in the summer when the heat is off, say July-August. I think you're wasting a lot just for the hot water, and the house itself doesn't need that much.

Good point...thanks.

[Edgeman]

Looks like I'm looking at the TT CC125s (aluminum heat exchanger) with the TT Smart 40 Indirect Hot Water Heater vs. Weil McLain GV90+4 with the Weil McLain AquaPlus 45 Indirect hot water tank which is 5% cheaper. The GV90+$ has a cast iron heat exchanger. Will this have a longer life? Which one would you choose. The contractors are suggesting the the TT Prestige 60 is too borderline with the indirect tanks. Thoughts?

[jadnashua]

Unless you are using hot water all day in large quantities, it is not necessary to upsize any residential boiler to accommodate an indirect. Yes, it normally is wired as a priority zone, and yes, the house will not be getting heated when the WH is calling for heat, but unless you have no insulation and a drafty house, you'll probably never notice that fact since it was warm before your water use, and will only be 'off' for a fairly short time while the tank is being reheated. THere's also normally some residual heat in the pipes and radiators that will continue to warm the house while the indirect is being heated. 30-45 minute showers are not uncommon in my house, and I've never noticed the house getting colder during that time.

[Edgeman]

I currently have a propane stovetop which uses maybe 30 gals a year @ $4.33 currently.or $130 per year. I also have an electric clothes drier. Should i bother piping natural gas to change the stovetop and perhaps have a gas drier if ever my electric drier dies? It cost $500 to run the gas lines on the outside of the house to hook up the stovetop and have the ability for the gas drier later. Is it worth it? Not sure what the equivalent cost are but if natural gas is 1/6th of the price of propane then the payoff is 5 years. Should I bother or keep the propane? My Maytag electric drier never dies.

[Dana]

41KBTU/hr is a very reasonable heat load estimate at a +9F design temp for a ~2000' house since 1950 if it has storm windows or double-panes, and ANY amount of insulation, but has NO foundation insulation. (It's in fact a bit on the high side. at over 20BTU/ft.- you probably have some low hanging fruit to pluck on weatherization.)

Using 73,000 BTU/hr (>35 BTU/foot) would be INSANE heat load at 9F even with single-pane glass, unless you have several panes missing or sleep with the bedroom windows open!

Steady-state efficiency is one thing, but with oversized boilers you have to properly account for the cycling losses, distribution losses, and standby losses, which in your case are probably considerable since the boiler is probably at LEAST 3x oversized, with no "smart" controls purging heat from the boiler, and it's probably close to 10x oversized for any one zone, leading to WAY more burn cycles that a single-zone system that's 2-3x oversized.

If you have mid or late winter oil bill with "K-factor" stamped on it, that number is the heating degree-days per gallon, and you can use that ratio and your measured combustion efficiency. If the K-factor is say, 4.4 that's 4.4 HDD per gallon, or 4.4 x 24 degree-hours per 0.79 x 138,000, BTU, or:

105.6 degree-hrs/109,020 BTU

Flipping the ratio and doing the division to come up with BTU per degree-hour that's

(109,020.105.6=) 1032 BTU/degree-hour.

Assuming 65F as your approximate heating/cooling balance point (that was base-65 heating degree-days, right?), with a +9F design temp that works out to (65F - 9F= ) 56F heating degrees.

So at 9F the heat load would be 56 x 1032 = 57,792 heating degrees as an absolute upper bound.

Discounting at least 10% for hot water heating (4 people, right), more if you have tub-bathers or folks who take endless showers and it would be on the order of 51K.

Do the math on the real K-factor, if you have it, and know that it's really an upper bound.

As a sanity check you can also use the NORA FSA calculator (scroll down about mid-page for the link to download the tool), which is based on boiler modeling work done at the Brookhaven National Labs to come up with something, which will also model the as-used efficiency at your oversizing factor. It's pretty good- WAY better than WAG, but not quite as good as a mid-winter K-factor and a tested steady-state combustion efficiency. The tool can take either annual gallons or K-factor as an input but doesn't let you choose a design temp (and it assigns a 99.5% design temp or cooler, much cooler than you really need to use.)

It's not a perfect tool, crashes a lot and sometimes has conversion-error issues, but plugging in 775 gallons/year, 79% efficiency, and 122KBTU/hr out, using Bridgeport CT as the location the FSA tool estimates a design heat load of 25,556 BTU/hr and an as-used efficiency of 53.1% due to idling losses (most of which occur over the shoulder seasons and summer.)

The 25.5K number (13BTU/ft) number actually NOT insane for a house that size if the basement is insulated or mostly below-grade, and the house is reasonably tight. By comparison, the heat load at my house in Worcester MA (2400' of fully conditioned space + 1500' of insulated basement that stays about 65F all winter) comes in the neighborhood of ~35,000 KBTU/hr, @ +5F based on gas usage and efficiency of this (modulating) burner. And this is a 1920s 2x4 framed house with known gaps in the insulation, original single-pane double-hungs with clear-glass storms, and 2x6 rafters for about R20 average attic insulation. MOST houses built since 1970 have better U-factors than this place on both windows and wall/ceiling. While raised-ranches are prone to air leaks and poor insulation on the cantilevers, they tend to have reasonable exterior surface area to floor area ratio.

I'd be truly shocked if the 41K estimate was an undershoot. Most Manual-J type heat loss calcs come in about 15-25% over measured-reality, so that would put you in the low to mid-30K range, or HALF the 73K calculation.

The length of the baseboard, while relevant for heating water temp purposes, is in no way related to your actual heat load. To keep the boiler from short-cycling on zones, the length in the smallest zone needs to be able to emit as much heat as the boiler is delivering, and that will change with output temperature. The WORST thing you could do for a modulating condensing boiler is to oversize the thing, since the lower-bound temperature before it starts short-cycling is determined by the minimum-fire output of the boiler and the amount of radiation on the smallest zone. The lower the water temperature, the higher the condensing efficiency, but if it's short-cycling on zones it'll be throwing away much of the efficiency in cycling losses, and burn it's way into an early grave. The absolute SMALLEST boiler that actually meets the heat load at the 99% outside design temp is going to be the right choice, since that will be the boiler that can reap the most condensing efficiency. The domestic hot water heating can be done as a "priority zone" with no impact on heating-comfort.

My bet is that your house (like almost all 2000' houses in the US) can be heated with the very smallest 50-60K boilers from any of the big players. (You couldn't PAY me to install an oversized combi like the Navien or Rinnai in my house.) The TT Solo 60 woould probably be a good fit, or the ~50K Peerless mod cons (PI-T50, or PF-50). The combis are kludges, with WAY more burner than can run efficiently for your low heating load chopped up into three zones, sized to be able to deliver reasonable hot-water heating. An indirect sized for your biggest tub fill and mod-con sized-optimized for your heating loads is a much more satisfactory system, and will run far more efficiently.

The min-mod fire of the CC125 is about 30KBTU/hr, which would (barely) be OK if your heating system ran as a single zone, but it's g'dawful oversized for 50' of baseboard, and your smallest zone is probably smaller than that. To get 30KBTU/hr out of 50' of baseboard to keep it the boiler from cycling at min-mod takes about ~180F for average water temperature, which is WAY over condensing temps. Running 180F water is fine if you were running an 83% AFUE ast iron pig of a boiler but a total waste of condensing burner. The mini-mod output of a typical 50K mod-con is about 13KBTU/hr, so in a 50' stick of baseboard or 260BTU/ft it balances with ~130F AWT, and you're condensing! And even with 110-120F water, with hysteresis in the boiler's outdoor setpoint control it would have reasonably long burns, and you'd be getting 95% efficiency out of it rather than 85% with the CC125.

But going with any combi smaller than the CC125 would leave you a bit starved on the hot water end in mid-winter, which is exactly why combis SUCK, in most applications. (Even the CC125 doesn't support much more than one high-flow shower this time of year- try starting the laundry when your spouse is in the shower, you'll see! :-) )

The GV90+4 doesn't have sufficient internal thermal mass to keep from short-cycling on zone calls with your chopped up 160' of baseboard either. Even as a single zoned with it's thermal mass you'd be looking at 150F+ water and ZERO condensing efficiency to keep it from short cycling into an early grave, and at 3 zones it's hopeless without adding a lot of thermal mass. You might get about 85% as-used AFUE out of it, but not more.

Seriously, try to narrow in on the true heat load, and measure the lengths of the baseboard on each zone. You need to be able to match the boiler's output at it's lowest (or only) firing rate to the smallest zone radiation reasonably, or any condensing efficiency will be eaten up in cycling losses and boiler maintenance.

My house is multi-zoned, and radiation-limited to about 44K out at the temperature I'm running it, and it cruieses through -10F weather without losing ground. It's highly unlikely that your house is lossier than mine, and if it is, it's cost effective to fix it so that it isn't.

What jadnashua said is dead right: "Oversizing is a big waste", but worse than a waste, it's a design problem that has to be worked around. (And there are probably dozens if not 100s of threads on this site describing the necessary workarounds when you step into the oversizing trap.)

[Edgeman]

Nice analysis guys, appreciate everyone's input. I'm going with the TT Prestige Solo 60 with the TT Smart 40 Indirect Hot Water. I just got approval for a 2.99% 10 year loan with no money down for the boiler replacement under a CT energy saving incentive program, booyeah! Plus a $750 rebate and $150 federal tax credit for AFUE at 95%. Looking to break ground for the gas lines on April 15th.

Now what about the stovetop/drier gas piping question, should i stay with propane? I'm 59, who knows how long i will be in this house. Is the payback too long to bother?

[jadnashua]

A gas dryer has typically twice the BTU's of an electric, so it can dry your clothes faster than an electric, often keeping up with the washing machine so that load and the dryer run about the same...depends on what you put in it and the temperature you select. I'd run gas at least to the location of the dryer. For the stove, you'll have to also change the gas regulator and probably the orifice(s). NG isn't as energy dense as propane, so you need a bigger orifice and different regulator. But, if you have a new stove planned, I'd go with plumbing NG to it.

[Edgeman]

What jadnashua said is dead right: "Oversizing is a big waste", but worse than a waste, it's a design problem that has to be worked around. (And there are probably dozens if not 100s of threads on this site describing the necessary workarounds when you step into the oversizing trap.)

One contractor said that with the TTPrestige 60, "you don't have enough BTUs to meet minimum flow rates. You're going to need at least 110k." On the spec sheet of the Smart 40 indirect, it says "Boiler Output BTU/Hr = 112K BTU. Does that mean that I need 112K BTUs from the boiler to get the spec 1 hr recovery of 180 gals, continuous flow of 150 gals, and peak flow of 5 gal per min? Will the TT60 impair the recovery heating rate of the Smart 40?