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

[jadnashua]

The spec sheet often has a chart showing the output at various input heat levels. The better thing to decide is how much do you need, and if a 40-gallon tank isn't big enough, buy a bigger tank, not a bigger boiler. You'll pay extra every use for a bigger boiler in operating and efficiency...you'll only pay once for a bigger tank, if you need one. Most gas WH are in the range of 40-50K BTU and aren't as efficient (75% is not bad, so 30-38K BTU output). The boiler is (slightly) bigger input, but lots more efficient output. Another way to make a tank 'look' bigger is to run it at a higher temp (often 140-degrees). This needs to be fitted with a tempering valve, but where I live, they require one on all tanks, however heated, to drop the level down some. An indirect is lots more efficient and better insulated verses most any gas fired one. Some of them have a lifetime warranty as well.

[Edgeman]

My hot water demands are not extravagant. We have 4 people in the house and 2 baths but one only has a shower. 3 people will shower in the morning but not at the same time. I shower at night. The shower heads are High Sierra 1.5 GPM High Efficiency Low Flow Shower Head (highly recommended by the way). Wifey said that the new shower heads were stronger and better than the 2.5 gal heads I replaced. The Gaggenau dishwasher will run at night. Then there's the standard Maytag washer, 4 loads per week. My view is colored by the crappy coil in the oil boiler we currently are living with. It can't feed one shower so I will catch all kinds of hell if we upgraded and run out of DHW.

[Edgeman]

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.)

The Slant Fin Heat Loss calculation for my house came out to around 42K BTU loss per house. While my upstairs zone is 31,600 BTU heat loss, the downstairs is only 11,085 BTU heat loss. The upstairs fin baseboard is 100 ft 2 zones slaved into one thermostat while the downstairs fin baseboard is only 50 ft. I am concerned about inefficient short cycling. My current oil boiler short cycles frequently for the downstairs zone. I think the TT Prestige Solo 60 matches well for space heating as you fellows suggested. Now should I get the Smart 40 or opt for the Smart 50?

Dana, you mention that the W-M GV90+4 in not appropriate. Does the W-M GV90+3 with 70K BTU work better as it is cheaper than the PTS60?

[Tom Sawyer]

I recommend a Buderus Logano 315 with the logamatic 4000 reset controller and Buderus's SST series indirect.

[BadgerBoilerMN]

We install and service the Triangle Tube Prestige 60 and use the TT40 in most residential applications. Stick to the plan.

A proper heat load the first order of business. If I had to guess (thank goodness I don't) I would put my money on Dana.

Combi boilers and water heaters typically use a stainless steel flat plate heat exchanger to make domestic hot water on-demand and switch to space heating mode by programmed design. With DHW the boiler is typically at full fire and modulates with water draw like a tankless, but in space heating mode goes back to outdoor reset and the programmed design temperature the installer has set--if he reads and understands the installation manual.

If you are filling a big tub the combi boiler is rarely the right appliance, but if your tub if over 50 US gallons, you would be fine with a condensing combi water heater.

Now for fin-tube baseboard an extra caution must be taken as some of the European condensing boilers will not fire to 200°F like the old cast-iron monsters of old, nor would you want them to unless you were short of fin. It appears you are not and the Triangle is the right choice.

By the way, you do not want to mix up the latest and greatest technology e.g. Bosch GreenStar combi or Triangle Tube Excellence with the ancient "tankless coil" that sits in an always hot cast iron fuel hog and waits for someone to open a tap (faucet for all us in the lower 48). The new combi boiler is a work of art with computers almost always smarter than they need to be only firing up when the tap is open and off when the flow stops.

"Endless" hot water if sized, installed and programmed correctly.

[Edgeman]

We install and service the Triangle Tube Prestige 60 and use the TT40 in most residential applications. Stick to the plan.

Thanks everyone for your help. Sounds like a good plan that I will be going with. The contractors all want to talk you up in BTUs and Indirect size. One wanted 110 with a 60 gal indirect. I think they make more money the larger the installed system.

[Edgeman]

Upon oil to gas conversion, getting rid of the 275 heating oil tank?

The 5 year old 275 gal heating oil tank is above ground and in a shed next to the driveway. The gas contractors are quoting me anywhere from $500 to $750 to remove it from my property.

Being that it is above ground, is it ok to put on on craigslist list to get rid of it? The previous owner had to remove the in ground tank when I bought the place 5 years ago. He had the new one installed. I figure perhaps i can sell it for $100 or at least give it away to avoid the onerous removal fees. The town only has restrictions and laws for removing in ground oil tanks. The tank looks new and i expect i to be near empty in mid April when I do the conversion. There is currently 140 gals to get me through 6 weeks in coastal CT. Thoughts?

[Edgeman]

Most of my contractors are insisting on the TT110 rather that the TT60.

"The 110k will not Short Cycle it is designed to modulate, these boilers run on low fire 80% of the time. You will need 110k BTU with your hot water demand setting the tank high will causing scaling and other problems. We have been designing systems for years and are good at it, we also see a lot of poor designs."

"We don't set up the boiler with priority, as you may now, when the hot water demand is calling the boiler will not produce heat for the home. I also seen in the past in the event the hot water heater malfunctions for example, if the pump fails the demand for hot water will never be satisfied therefore the boiler will not switch over to heat. The same goes true if the aqua stat fails on the water heater the boiler will never switch over to producing heat. Having siad that if you insist and you want the TT60 I will quote it for you. I feel very comfortable and confident with the 110, it will not short cycle due to the turn down ration, but I want to make sure you are happy. I will email you over the quote for the 60 on Monday."

[jadnashua]

Using a bigger boiler to accommodate an indirect as a 'normal' zone is a big mistake. His observations are correct, though, but they occur so rarely, that it's not worth the long-term continued expenses. Those boilers have a modulation range...the bigger one's smallest fire is near your worst case coldest day. Those occur so seldom, it would rarely be in condensing mode.

[BadgerBoilerMN]

Any boiler that is over-sized will short-cycle.

Many of the contractors that never did a proper Manual 'J' heat load in the past, feel even more comfortable not doing their job since the advent of the modulating gas boiler about 10 years ago. If the total load, or a high-load zone, that the ModCon serves, can't "unload" the Triangle Tube Prestige110 at its lowest output (28mBtuh) it will shut down the boiler and cycle on and off, "bumping off the bottom". We find this is the case more often than not in the ModCon systems we have not designed. Of course this will happen under full-load, i.e. the coldest few days, so that in warmer weather (the other 360 days of the year) when the boiler should be running cooler, condensing more and producing more comfort with less gas, it is actually cycling on and off losing its full potential and giving the ModCon an ill-deserved reputation for under-performing and expensive maintenance. All engines want to run. Like stop and go traffic an over-sized boiler starts and stops, using more fuel and wearing out faster than it should.

Domestic hot water is almost always prioritized since it allows the use of a boiler properly sized to the heat load and almost never over-sized for the 20 minute peak load of the typical domestic hot water heater. Whereas, it may be true, that the space heating will be "locked out" if the DHW is not satisfied for some reason, it is irrelevant to the homeowner who will not go without either for long.

If you don't, or won't, do heat loads and will not prioritize DHW (necessitating, by definition, an over-sized boiler, running too hot for anything but fin-tube baseboard during design conditions) the only boiler for you is the IBC 15-150. In fact just reading the IBC installation manual will give most "experienced" boiler installers a new education. But I digress.

How sweet it was in the old days, when boilers were made out of cast iron, could be double-sized, burn twice the fuel, any pump would do, as many zones as you want, run to 200°F all-the-time, with a mind-less-tank-less coil making DHW as fast as you could turn the faucets on, and nary a thought to the how or why, just do, do, do and be done.

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

[Dana]

Listen to the Badger- oversizing the boiler is paying too much to end up with lower efficiency and a shorter boiler life!

The simple-math model goes something like this:

The Solo 110's min-mod output even a a higher non-condensing temperature mode is about 25,000 BTU hr or higher, (considerably higher than min-mod on the Solo-60). On a low mass stick of heat emitter like the 50' of fin-tube baseboard it would only balance at 500BTU/foot. Take a peek at the spec sheet for a random popular baseboard model. Note that it takes 160-170F average water temp (AWT) for the baseboard to get that 500BTU/foot into the room. When outdoor reset control (ODR) is calling for a lower temperature, it will definitely cycle- the only question is "how much?". You can push the temps lower a bit but not too much. As long as the burn lengths are at least a couple or three minutes long, and only a handful of burns per hour and it won't trash the boiler. But can you get it low enough to condense? Not even:

You need an AWT of under 125F to break 88% efficiency on a mod-con, and even if you could program the hysteresis on the ODR to 20F (letting it overshoot the curve's setpoint to lengthen the burn) there is nowhere near enough thermal mass on the 50' baseboard zone to yield burns that are long enough to not eat up all the condensing efficiency in ignition & flue-purge losses, (and add a lot of wear & tear on the boiler to boot.) At an AWT of 125F you're looking at about 12K being emitted by the fin-tube, and about 28K being delivered by the boiler- it'll short-cycle, guaranteed.

BTW: If the ~41K heat load calc was done with SlantFin Hydronic Explorer, that tool is known for having some of the highest oversizing margins out there- 25-35% over measured heat loads. Assuming that the 41-42K number is at least 1.25x reality (a pretty good assumption, 42,000/1.25= 33,600BTU/hr, which is also a credible number. With 180' of baseboard in the whole house that's under 200BTU/foot, so it looks like you could even run the thing with a fixed-output temp of 125-130F and be in condensing mode all the time, and never be cold, as long as you don't short-cycle it to an early death on zone calls. With fin-tube the output of the heat emitters has non-linearities that make output somewhat unpredictable if you tried to run a AWT of 100F, which would make tweaking in the ODR curve difficult & paradoxical, so you'd probably want to set the floor of the curve at 120F anyway. But since you'd still get design day heat into the house with 130F output it'll be a pretty short curve- still worth doing. With 50' of fin-tube and 120F out/100F return for a 110F AWT the fin tube would deliver on the order of 8000BTU/hr, or about half the min-mod output of the Solo 60. With bigger programmed hysteresis you can probably keep it from short cycling, but you may have to experiment a bit when dialing it in. Ideally you'd get at least 3 minutes of min-burn when serving the single zone, but as long as it's only of handful of burns per hour it won't much matter. If the burns are under a minute it's something of an efficiency disaster, and you'd do better to bump the temp up 5F or even 10F if that's what it takes to get at least 100-150 seconds of burn out of it at a time, even at low-low duty cycle.

Bottom line- the TT-60 or similar will be able to work reasonably with your zoned baseboard, whereas the -110 would not- it's too big to heat your house at condensing efficiency. Any fixed-output boiler with even 40K of output would never run in condensing mode without either adding a lot of thermal mass or more radiation to the system to stop the short-cycling (buffer tanks, or more massive radiators like radiant slab floors.) Higher-mass boilers would ideally see 10 minutes of min-burn since they incur higher standby and cycling losses than low-mass mod-cons.

[BadgerBoilerMN]

Minimum flow rates? The lack of basic hydronic field knowledge in the hydronic field is astonishing.

First, a Manual 'J' heat load to properly size the boiler to the "block" load, which does not change. Once the size of the boiler is determined you may measure the baseboard to confirm that you have enough radiation to unload the selected boiler. This is also an excellent time to measure the fin each zone. The smallest,shortest zone will determine how often the boiler may cycle an if you go for the 100mBtu units you will see some serious "bumping off the bottom".

In most homes a condensing boiler properly sized to the heating load, coupled with a indirect, properly sized to the DHW load will yield the most efficient, reliable and comfortable system available today. If you can't find a local contractor to perform this critical first step, find a hydronic designer that can specify the equipment for the local contractor to install using the skills they should have. Ask to see a picture of one of their condensing boiler installations and a sample of a heat load they have performed.

No heat load, no job.

[DonL]

Upon oil to gas conversion, getting rid of the 275 heating oil tank?

The 5 year old 275 gal heating oil tank is above ground and in a shed next to the driveway. The gas contractors are quoting me anywhere from $500 to $750 to remove it from my property.

Being that it is above ground, is it ok to put on on craigslist list to get rid of it?

There are plenty of scrap metal places that will pick it up for free.

You may be able to have someone take it for scrap and split the money they get for it. Or take it yourself.


If it was me, I would keep it. You never know when it may become handy. Extra Fuel is nice to have in a emergency.

[Edgeman]

Thanks for your input, guys. Looks like the TT60 with a Smart 50 is the way to go. For the extra $200 I get a 10 gallon margin over the Smart 40 to cover the DHW.

[Dana]

DO take Morgan's advice about finding a competent designer/installer, if you can. The more idiot-proof systems & choices get, the more creative the idiots seem to get. I'm positive there are plenty of fools capable of screwing this one up, even with all of the right components in hand.

Anybody advising a 110K boiler for a ~2000' raised-ranch in CT moves to the "creative idiot" column in my book, unless they can show their math and it actually adds up. For the real heat load to actually be higher the non-condensing output of the TT-60 would take multiple code violations on window type & window/floor area, etc. Even the lipstick-on-mirror type math I've laid out about cycling on zone-call issues shows how bad a choice that would be, and I'm not a professional hydronic designer (though I've de-bugged, dialed in, and designed more than a handful systems at this point- everybody has a hobby, right? ) There are code-min 2000' houses in Fairbanks AK that wouldn't need the -110 (but with a design temp of about 40 below zero you can bet there are some that might.)

Sure, if you cranked the temp way up the -110 wouldn't short cycle in your house- you'd always be warm and never run out of hot water, but it'll never hit it's AFUE numbers. With the -60 there's a very good chance that it will, and the heat will be more even.

Unless you have a bigger than standard tub you probably don't need the 50 gallon HW heater either. A 40 gallon tank with over 50K of boiler output recovers VERY quickly (a typical standalone 40 gallon tank has about 30K of burner-output behind it, and that's probably the most common size & type of HW heater in the US), but the "extra" 10 gallons never hurts. Rather than spending an extra $200 in hardware for maybe 20-25% more showering capacity, a $400-600 drainwater heat recovery heat exchanger would more than double it (!), and even the -60 would be able to deliver literally endless showers (especially with your low-flow shower head.) I have a 4" x 48" version on my system and a ~3gpm shower head and even with all zones calling for heat with someone in the shower (not priority-zoned), the output never modulates over 60K. It does nothing for tub fills, but it's like having another 25-40K (depending on flow) of burner behind it in shower mode, but it's "burner" that uses no fuel. From a fuel cost savings point of view it would take forever (at least a decade for a 2-person family) to pay off at current natural gas pricing, but for oil-burners it pays off pretty quickly.