BOSCH GREENSTAR COMBI 151 -PRO- vs GREENSTAR COMBI 151

[Ordie]

Can someone explain in detail why someone would install the Bosch Combi PRO 151 over the non PRO 151?
They our the same boiler except the PRO doesn't have the accessories.
I plan on using a single intake/exhaust pipe with 5 zones and external expansion tank.

 

[Dana]

Why would anybody install EITHER of them on a system with 5 zones?

5 zones of low mass radiation like fin-tube you're almost guarnteed to short-cycle either Greenstar 151 into low efficiency and an early grave. According to the manual (see page 12) he MINIMUM-fire output of the thing in condensing mode at 122F (where it'll hit it's 95% AFUE) is 35,000 BTU/hr (enough to heat my whole house at +5F outdoors). It takes at least 175' of typical fin tube baseboard to emit that much heat- the thing can't modulate to a lower fire than that, so it's going to be doing a lot of cycling on zone calls unless you have at least 175' of baseboard PER ZONE.

With modulating boilers it's better if minimum firing rate is less than half your design heat load, and every zone can emit at least 3/4 of that minimum firing rate at a condensing temperature. There is no point to hooking up a modulating boiler to a system where it can't really modulate, and it's terrible for efficiency and longevity to hook it up to zone radiation where it's going to short cycle. ( For a rough rule of thumb for baseboard use 200 BTU/hr per foot. Divide the min-fire output by 200, that's how much baseboard it takes to balance reasonably.)

More on sizing modulating condensing boilers here.

The Greenstar 151 (like most low-mass combis) is only a good fit for houses with quite LARGE heat loads, and modest domestic hot water needs.

 

[Sponsor]

 

[Ordie]

Thanks Dana,

Here's the specs on my house:

House is a 1- 1/2 story in central Maine, with 6 zones 1 for DHW, 2 on 1st floor, and 3 in basement.
1st floor 1672sf, 2nd floor 400sf, and basement is 1078sf. WE RARELY USE BASEMENT or TURN UP HEAT DOWN THERE.

First floor with open floor plan LR-DR-Bath-Kit on 1 zone ,
-Living room 21x21 with 15 foot ceilings with 150 sf of double pane windows and glass doors {lots of glass on one wall}, BB heat with 18 feet of new HE3 baseboard {https://smithsep.com/products/he3-silent-fin-heating-edge-baseboard/}.
-Kitchen is 20x10 with 7ft ceiling, and 1 kick space KS2004 quiet-one (equals 7ft of BB ?) for heat.
-Dinning room is 21x14 with 7ft ceiling and 10ft BB heat.
-Bathroom is 17x11 7ft ceiling with 6 ft of BB
-MBR 25x22 with 7ft ceilings heated with 22ft BB zone 2.

Second floor has 2 bedrooms, not heated yet, BR 1 is 17x14 and BR 2 is 18x9. No heat yet

Basement is poured concrete and brick walls heated but rarely used, thermostats are set at 50 degrees and never come on because oil boiler is down there and usually keeps it warm enough not to freeze.
-Game room is 24x19 has 26ft of BB zone 3,
-Room 2 is 25x20 has 14ft of BB zone 4,
-Bathroom is 7x6 and adjacent room 10x8 both heated with 10ft BB zone 5.

BoilerMate DHW zone 6.

Contractor #1 install bb 2nd floor, and Bosch greenstar 151 boiler propane (input range 36-151) using existing BoilerMate for DHW.
Contractor #2 install bb 2nd floor, and HTP EFT-110pu Boiler propane (max110 - min22) with PurePro PRO41ZPG-P for DHW.
Contractor #3 install bb heat 2nd floor, and Vitessmann Vitoden 100-35 propane (input range 37-118) boiler, and use exiting BoilerMate for DHW.
Contractor #4 install bb 2nd floor and Viessmann Viotocrossal boiler propane (model unknown), use existing BoilerMate for DHW.
Contractor #5 install bb 2nd floor, and Bosch Combi 151 boiler propane (input range 36-151) for heat and DHW.

 

[Dana]

The "high output" HE3 baseboard puts out about 350 BTU/hr per running foot at an AWT of 120F (the specs live here),which is where it needs to be to hit 95% condensing efficiency. So the 18' of HE3 is still only emitting 6300 BTU/hr of the necessary 35,000 BTU/hr per zone.

The KS2004 is putting out something like 1800 BTU/hr @ AWT of 120F (a WAG based on the specified 2095-2240 BTU/hr at an EWT of 130F.)

Assuming the other 38' of baseboard on the first floor is all standard output baseboard it's putting out another 7600 BTU/hr at condensing temps.

Add it all up and you only have enough radiation on the first floor zone to emit 15,700 BTU/hr in condensing mode on that zone. That's less than half the minimum fire output of the Bosch 151s, and only about 3/4 of the min-fire output of the EFT 110. The all propane versions of the Vitoden 100 only throttle back to 31,000 BTU/hr-in, 29,500 BTU/hr out in condensing mode, so you'd have about half the necessary radiation to operate it in condensing mode. The very smallest Vitocrossal 100 can throttle back to about 8500 BTU/hr in condensing mode, but doesn't appear to be marketed in the US. It's not clear of the propane version can go that low. The Vitocrossal 200's or bigger are HUMONGOUS output commercial boilers.

Even running the boiler at 180F you only have enough radiation to emit 45-50,000 BTU/hr. None of the proposals can actual run anywhere near their max firing rate (except when serving the Boilermate)- they would at best modulate in their mid firing range even at non-condensing termperatures

Don't quite know what to say other than installing any of the above (unless it's actually a Vitocrossal 100, and it can modulate to under 15,000 BTU/hr) would be a mistake, since you don't have enough radiation on even your main zone to run in condensing mode without cycling, and would put ridiculously high constraints on the minimum radiation for the upstairs that it would take to keep from short-cycling the boiler into low efficiency and an early grave. These contractors all flunk the 5th grade arithmetic test on matching the boiler modulation range to the radiation (let alone the space heating load.)

A better bet than the EFT-110 would be HTP's UFT-080w + existing Boilermate. The UFT-080W has a min-fire output of about 7600 BTU/hr, but can still deliver ~70,000 BTU/hr at high fire at non-condensing temperatures, and twice the BTU output of a typical standalone propane water heater. With as little as 40' of standard baseboard per zone it wouldn't short-cycle at condensing temperatures. So even though you won't need 40' of baseboard to heat the upstairs, the UFT=080W you would install 40' (or 22' of HE3), which is probably at least do-able without relocating doors.
There are other boilers out there that can work too. The Bosch Greenstar 57 can throttle back to ~12,000 BTU/hr- out in condensing mode, yet still puts out ~50,000 BTU/hr at non-condensing temps at high fire. Marrying that to your Boilermate wouldn't be a disaster, but it would take about 60' of baseboard on the upstairs zone to suppress excessive cycling at condensing temps.

A reasonably tight reasonably insulated 2100' house with an 1100' tight but uninsulated basement would usually have a heat load of 45-50,000 BTU/hr @ -10F outside, 70F inside, maybe even less than 40,000 BTU/hr if the basement walls are insulated to IRC 2015 code min (= R15 continuous insulation). If the existing radiation has been keeping the house warm enough, you don't need anything with a high-fire bigger than the Greenstar 57. But the smaller the min-fire the better, to allow operating at condensing efficiency. There are MANY 50-80K-in boilers out there that can serve up less than the 15-16K of condensing radiation you have on the first floor, but find something that modulates to under 10K if you can, or you'll have to ridiculously oversize the radiation on the second floor to keep the boiler happy & healthy.

 

[Ordie]

I really appreciate all the info Dana, can you recommend a reliable boiler in the range I need?

We won't be using heat on the 2nd floor bedrooms much, but sounds like it needs to be on the same zone as the main floor to help put the boiler in condensing mode?

 

[NY_Rob]

The HTP UFT-80W would be a good fit for you- and it's got a self cleaning Stainless Steel heat exchanger vs. the Greenstar's aluminum heat exchanger. It's a simple robust boiler that should work well w/your radiation as it modulates down to 7.6K BTU's so you have a good chance to be in condensing mode most of the heating season.

http://www.htproducts.com/UFT-Boiler.html

 

[Dana]

What NY_Rob said.

The UFT-080W comes pre-plumbed with a port and internal controls for supporting the BoilerMate- it's a dead-easy installation compared to most. Since you have a proposal from an HTP installer they might be willing to submit quote on this boiler. I somewhat doubt you're within their usual service territory, but Rob Brown at Rockland Mechanical in Rockport ME is very familiar with the UFT series boilers.

Others that can work:

Navien NHB 80.....................................8,000 BTU/hr min 80,000 BTU/hr max

IBC HC 13-50 .........................................13,000 BTU/hr min 50,000 BTU/hr max

Weil McLain ECO-70............................14,000 BTU/hr min 70,000 BTU/hr max

NTI Trinity TX51...................................7,100 BTU/hr min 57,000 BTU/hr max

Lochinvar KH-055 (or WHN056) ...............8300 BTU/hr min 55,000 BTU/hr max
.................KH-085 (or WHN086)..............8500BTU/hr min 85,000 BTU/hr max

 

[Ordie]

How would the zone piping be put together to create one zone?
Looking at combining the 3 zones in the basement to 1 zone, then combine the 1st and 2nd floor together, or at least put the main floor on 1 zone instead of 2.
Currently all 5 zones plus 1 for the DHW tank all come to the supply and return manifolds at the existing oil boiler.
Could just connect the taco zone valves together, then disconnect the unused thermostats ? Then later on if someone wanted to seperate the zones they could wire them back?

 

[NY_Rob]

You can combine the zones in a series configuration if they don't get too long, or a parallel configuration.
Both designs have benefits and drawbacks.

In series config, the rads at the end of the line get significantly less BTU's than the rads at the beginning of the loop.
In parallel config you would have less variation because the loops would be shorter... but you would probably need balancing valves on some (or all) of the zones to ensure they all have adequate flow.

 

[Dana]

You're calling them "zones", but are they in fact 5 separate zones, each with an individual zone thermostat and a zone valve?

If yes, that's a recipe for severe short cycling issues- the Greenstar Combi 151 couldn't possibly serve 5 micro-zones efficiently without a large buffer tank, and most of the others would still short-cycle (if not as badly.)

If it's truly micro-zoned the ratio of the zone loads to the zone radiation could be all over the place, which could make balancing it all pretty tough when combined into a single zone.

If not Manual-J, at the every least run an I=B=R type room-by-room, zone-by-zone load calculation in a spreadsheet, along with the radiation dedicated to each zone. The load/radiation ratios per existing zone would tell you which zones might be able to be combined, and which can't.

Basements have very different heat loss characteristics than above-grade floors, and almost never work well when combined with above-grade zones, though running the basement as a single zone on it's own usually works. The heat loads of insulated basements are pretty small, and even if the room-to-room balances are pretty far off from a radiation point of view that rarely matters, unlike above grade floors.

 

[Ordie]

Will be installing one of the boilers Dana listed above, can't seem to find a HTP UFT-080W yet, but have found the Navien NHB 80.

Trying to combine the first and second floor zones to keep the boiler in condensing mode (quote from Dana "Add it all up and you only have enough radiation on the first floor zone to emit 15,700 BTU/hr in condensing mode on that zone".)
Looks like I will need to use balancing valves as the previous oil furnace used?

The way its hooked up now, zone 1 which is the first floor goes in 4 directions after the valve opens, one branch goes to the living room, second branch goes to the kick space heater in the kitchen, third branch goes to bathroom, forth branch goes to dining room, all have balancing valves to keep flow even to all locations? If the MBR is added as the fifth branch this wood complete the first (main) floor all on one zone?

 

[Dana]

Since both the Navien NHB 80 and HTP UFT-080W can throttle back to about 7,600 BTU/hr the first floor already has twice as much radiation as the minimum needed for running at condensing temperatures on that floor with those boilers and doesn't need to be combined. It would do fine even with the Weil McLain ECO-70, which can only throttle back to 14,000 BTU/hr, but would be a disaster with the Greenstar Combi 151.

The basement has 50' of baseboard if combined into one zone, which can deliver up to 10,000 BTU/hr in condensing mode without short-cycling, so it's fine too.

It sounds like your first floor has just one thermostat, but there are home-runs to the room radiation from manifolds, with the room to room balance tweaked with balancing valves. That's perfect- that should continue to work just fine. If it's done similarly in the basement that too should continue to work just fine with the NHB 809 and UFT-o80W and most of the others, but would be cycling a bit with the ECO-70.

Is there already radiation installed on the second floor, or do we have the option to specify that?

 

[Ordie]

No heat yet on 2nd floor, first floor is on 2 zones, one is the MBR and the rest on another zone with home runs to each room with balancing valves ( see post #3).
I was concerned the MBR would short cycle if left on its own zone?
Second floor could be put on its own zone as long as it won't short cycle?

 

[Dana]

As long as there is enough radiation to emit the minimum fire output of the boiler (or at least the major fraction thereof) the zone won't short cycle. With 40' of baseboard the NHB-80 or UFT-080W won't need to cycle at all if the boiler output is 125F or so. With 30' it will cycle a bit but probably not badly. With only 20' it could be edging into the short cycling disaster zone.

 

[Dana]

If you are going to add the MBR to the first floor zone, run an IBR load calculation on that room as well as on the the living room. For the radiation in the MBR, shoot for about the same heat load/radiation ratio as the living room or slightly higher. To be able to easily fine-tune it and, for max comfort use a low temp panel radiator with it's own thermostatic radiator valve. Most bedrooms in reasonably insulated houses have a heat load less than 2500 BTU/hr, often less than 2000 BTU/hr, but it could be 3500 BTU/hr, which is why you have to do the math.

On the value-priced end, Myson's CV21 & CV22 low height (baseboard height) panel rads have options that would probably fill the bill. As just one example, the CV22- 2000 is about 8" tall x 80" long ( 83" long including the thermostatic radiator valve), and delivers about 6000 BTU/hr at an average water temp (AWT) of 180F, and about 1700 BTU/hr @ 120F, for about $200-225, thermostatic radiator valve and other hardward included. In that series they come as short as 2 feet, and as long as 10 feet. A 10 footer would deliver about 2500 BTU/hr @ 120F AWT. But there are taller & narrower versions of similar output- sometimes it's nice to have the radiator extend to 3-4' off the floor to get more direct radiation. Sometimes a skinnier taller version placed between windows helps average out the mean radiant temperature of the room, making it more comfortable at a lower temp, etc. There are plenty of options, from multiple vendors. When you have the load numbers and load/radiation target ratio figured out I can help sort out the options, if you like.

 

[Dana]

Just curious, but why propane rather than an appropriately sized oil boiler? In most of New England the price per million BTU (MMBTU) delivered is far higher for 95% efficiency propane than for 85% efficiency #2 oil. Most new oil boilers come with heat purge controls to minimize standby & distribution losses, and minimize short cycling. If it's about the modulation comfort of outdoor reset controls, there are outdoor reset mixing valves that would be appropriate. Oil burners are higher maintenance and put out more localized air pollution than propane, but the operational cost difference is substantial in my area (MA).

This year in ME propane prices ran between $2.50-$3.00 /gallon for 91,600 BTU/gallon of source fuel energy. Burned at 95% efficiency that delivers about 87,000 BTU/gallon. So it takes 1,000,000 /87,000 = 11.5 gallons/MMBTU. Assuming an average price/gallon of $2.75, that's $31.63/MMBTU.

Oil in ME also averaged between $2.50-3.00/gallon this year, but that's for 138,000 BTU/hr source fuel energy. Burned at 85% efficiency that delivers about 117,000 BTU/gallon. So it takes 1,000,000/117,000 = 8.5 gallons/MMBTU. Assuming an average$2.75/gallon price, costs about $23.38/MMBTU.

That's about 25% lower heating cost.

It may work differently in your area, but most propane delivery folks in my part of New England own and maintain the tank, and have the exclusive rights to filling it, with a fairly steep fee for removing it. That makes them something of a micro-monopoly, which is often abused. A pretty common marketing ploy is to offer a very low per gallon price on the first fill-up or first season, then higher than state-average pricing forever after. Heating oil is by contrast a very competitive business, and much harder to abuse.

On a second floor bedroom there is usually a very real cooling load, even in ME. It's a bigger chunk of cash up front, but it's not unreasonable to use cold climate ductless (or ducted) mini-split heat pumps for the second floor rooms, depending on where the load numbers turn out, and that can be even cheaper heating than using oil. (That's true in my area, and I have 21 cent/kwh electricity, some of the highest in the lower 48.) Mitsubishi has a nice half-tonner that can deliver ~6000 BTU/hr @ -13F, yet can throttle way back to 1600 BTU/hr @ +47F . It might be appropriate if it turns out your room load is 3500 BTU/hr or higher. If it's less than that, Fujitsu has a 3/4 ton slim-duct unit that could probably do the whole upstairs. It doesn't have a capacity spec at temps below -5F, but it's "nominal" heating capacity at +17F is 12,000 BTU/hr, and it would likely be able to deliver more than half that at -15F. Most of Maine has 99% outside design temps warmer than -10F (which means it's only colder than that 1% of the time), and these things really DO work in your area.

 

[Ordie]

House was built in 1980's, previous owner built it to be heated by wood only, then decided to heat with oil later on and installed a oil boiler. The boiler was installed on the floor in what now is the garage under the kitchen.
We are wanting to remove the large brick fireplace (10'L x 6'W x 15' H) that is taking up alot of space in the center of the house.
Removing it would take out all 3 chimneys , so by installing propane I wouldn't need a chimney, and would be able to get the boiler off the floor in the garage and mount it up on the wall.

Link to photos of house, current oil boiler, and in work progress of propane boiler
https://drive.google.com/file/d/1Fh0zw2Zt0Kv383Igqg_hUgxeYHYVqzhX/view?usp=sharing
https://drive.google.com/file/d/1pfQGZFLGwxcxYofKwznWnY4JKSMD1KBx/view?usp=sharing
https://drive.google.com/file/d/1Fh0zw2Zt0Kv383Igqg_hUgxeYHYVqzhX/view?usp=sharing
https://drive.google.com/file/d/11OFjwnSHaXvYL0bPu6TdMbVCj42icesS/view?usp=sharing

https://drive.google.com/file/d/1jIxRmLgjHuIgYTaO4JDkb4orwsLp8T-R/view?usp=sharing

https://drive.google.com/file/d/1vggzeWQldD93HE4whkMTVlwhsIwbR_Fw/view?usp=sharing
https://drive.google.com/file/d/1wvRsLg7nxpBJejWbY_avnOq1YnY43fOe/view?usp=sharing

https://drive.google.com/file/d/1jIxRmLgjHuIgYTaO4JDkb4orwsLp8T-R/view?usp=sharing

 

[Dana]

A couple of comments...

Buderus G115 series oil boilers all have direct vent options, where both the combustion air and exhaust venting are out the side, just like a condensing propane burner. The Burnham MPO-IQ oil boilers also have direct venting options for burners between 1.05-1.65 gpm, but even the 1.05gpm (= ~123,000 BTU/hr out) is probably more burner than you need. There are probably others. The smallest of the line Buderus G115WS/3 is probably going to be the best fit. To know for sure, run a fuel use heat load calculation. If you have been on a regular oil fill-up services that stamps a "K-factor" on the billing slips, a couple of mid to late winter fill-up K-factors and your ZIP code would be sufficient for establishing an upper bound on the actual heat load.

The manifold under construction looks like you are planning on micro-zoning, which can be problematic even for oil boilers. The MPO-IQ comes with a heat-purging economizer to help limit the short-cycling, whereas smart controls are an optional upgrade with the Buderus, but no matter what some amount of analysis should be done on the zone by zone radiation relative to the boiler you run with. With either condensing propane or higher temp oil there has to be some matching of the zone radiation to the output of the boiler. To balance perfectly with the 74,000 BTU/hr output of the G115WS/3 at an output temp of 180F takes about 150' of baseboard. There is some thermal mass in the oil boiler to work with (a lot more thermal mass than a propane mod-con) and the smart controls will help, but if micro-zoned it will likely need a buffering thermal mass of water in a tank. Micro-zoning with five separate thermostats and zone valves without a buffer tank is almost always a mistake.

It looks like the foundation is completely uninsulated(?), with a substantial portion that is above-grade. If uninsulated the basement losses could easily add up to 25% or more of the heating bill, even if not actively heated, even if there is insulation between the floor joists for the first floor. Insulating it to the IRC 2015 code minimum of R15 continuous insulation or better performance is going to be worth it, especially with expensive fuels like propane & oil. To insulate the foundation without risking a mold problem needs to have at least some continuous air-impermeable insulation such as rigid foam against the foundation. A non structural studwall with a vapor barrier would trap ground moisture in the studwall, but without the interior side vapor barrier the summertime ventilation air would condense on the cool side of the edges. For a Maine climate installing 2" of EPS or 1.5" of polyisocynurate against the foundation wall, with unfaced or kraft faced batts in a 2x4 studwall works, both winter & summer. The thermal performance would exceed IRC code minimum by a bit, and the moisture risks would be low. There are a lot of details to get right, but it's not rocket science.

It looks like the boiler is currently in the garage(?). As rule garage doors leak a lot of air and garages runs colder than the rest of the basement. Insulating the garage walls would help and is worth doing, as well as installing in insulated garage door, especially if the boiler is going to continue to be located there. It would be better if the boiler were located completely inside the thermal and air-pressure boundary of the house, even if it's in unheated (but insulated) basement.

 

[Ordie]

Going to put the 3 zones in basement on 1 zone, 1st floor on 1 zone, and once installed, 2nd floor on 1 zone,
3 zones total. Basement is almost off most of winter 99% of the time.
Also the DHW boilermate will be on zone 4
So currently there will be 2 zones in use, 1st floor and Boilermate DHW.

Like to get rid of oil boiler and fuel tank to make room in garage, and also concerned about new oil boiler with direct vent so close to back deck and smell? Still researching direct vent boilers. Really leaning toward wall mount propane boiler as long as its not really overboard with fuel cost?

 

[Dana]

The propane fuel cost really is the question, isn't it?

If you insulate the basement and it lowers the heat load by 25%, the operating cost of the propane burner would be about the same or less than what you have been paying.

If you install a decent performance mini-split solution for the upstairs zone instead of hydronic heat, the total operating cost would be lower than oil or propane. It's definitely more money up front, but it also provides high efficiency ultra-quiet air conditioning too. Back when oil hit $4 /gallon and up the simple payback for a mini-split on heating cost savings alone was typically less than 5 years, sometimes (rarely) less than 3.

Without intending to be scare-mongering here, oil pricing is highly volatile and subject to all sorts of international politics influences. Heating oil pricing has always tracked crude oil pricing, and traditionally propane prices have always tracked almost in lock step with heating oil. The somewhat lower oil pricing of the past couple of years has been largely a combination of slower than expected demand growth in the developing world, and the return of Iranian oil onto the world markets. World crude prices dropped from about $100/bbl in the summer of 2014 to about $50/bbl by January 2014 (for a complex set of reasons), but the agreement with Iran and the west in 2015 put further downward pressure on oil pricing as western investment in their oil infrastructure returned.

The recent rejection of the Iran nuclear accord by the US has already inserted a big hike in the crude pricing, along with a surge of pent up demand in the developing world. If the current world crude prices are sustained over the summer, #2 oil and propane will take a big hike in pricing next winter over what they have been for the past 2-3 winters. If things continue to fall together and apart in Syria & Iraq there will be more bumps to come, but the return to $25/bbl oil doesn't seem very likely. Even sustaining the ~$50/bbl average pricing of the last 4 years may not be likely, despite innovations in recovering fracked oil.

By contrast, electricity pricing in New England has tended to track natural gas pricing, which (due to innovations in fracking and lack of export capacity) has a lot of downward price pressure keeping the lid on, and is nearly immune from international politics. A mini-split or two, though expensive up front, can be considered something of a hedge against oil & propane price volatility. As the regional offshore wind industry picks up, that will also put downward pressure on the wholesale price of electricity. Long term it seems cold-climate heat pumps are going to be the best bet for low heating costs. In areas with lower than the regional average retail electricity rates it's even competitive with natural gas right now.