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Comparing combi boilers - minimum input, maximum input, and gas savings

Jeff_in_MA
Jeff_in_MA Member Posts: 12
edited September 20 in Gas Heating
Hi all,
We need to replace our propane heat/indirect HW system (14-y.o. Buderus has been discontinued, leaks, needs resets, no parts, no service, #$&%!). Going to a combi setup. Existing system is 75K BTU, has been fine for our 1400 s.f., tight, highly insulated house. Contractor says a 90K Rinnae system "would be great for us." My first reaction was, too big! Obviously 75K is plenty. But after trying to educate myself on all this, I have a basic question. Which is going to save us more gas: (a) a 90K BTU system with a minimum input of 15K BTU (Rinnae I090CN), or (b) an 80K BTU system (Bosch Singular) with a minimum input of 20K BTU? We tend to keep the heat off till Thanksgiving if we can, and turn it off as early as we can in the Spring, so we run at the minimum for say half the year. Over 6 months, the 15K minimum system should save like 240 gallons of propane versus the 20K, or close to $1800 in savings at the current MA price (which will certainly go up). But is the 15K/90K system going to cost us MORE to run during the heating season, versus the 20K/80K? Or will the two systems be just about as cost-efficient when actually heating the house? (If it's a stupid question, well, I was an English major and a biologist, never an engineer.) Thanks in advance for your help! And of course any comments about the two systems in terms of reliability or whatever are welcome, too, or if you have another system to suggest.

Comments

  • EBEBRATT-Ed
    EBEBRATT-Ed Member Posts: 12,362
    I would be careful with both brands. Bosch uses aluminum heat exchangers which can be problematic. Rani I am not familiar with. Most use Viessmann (top of the line) HTP has some good products and we seldom here about serious Lochinvar problems. I would keep looking and stay clear of Navien
    ZmanJeff_in_MASuperTech
  • kcopp
    kcopp Member Posts: 4,010
    If your indirect water heater is still in decent shape why not pair it w/ a similar output boiler? I personally line the IBC line. But it is what the installer knows best, can service best and get parts for asap.
    Jeff_in_MASuperTech
  • hot_rod
    hot_rod Member Posts: 16,875
    14 years on the indirect also? Heavy usage? Hard water?  It may be time to upgrade both.

    Or a 120 combi. I have had excellent service from the Lochinvar. The Knight turns down to 11,000 btu/hr and you can step fire it also
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
    Jeff_in_MA
  • Hot_water_fan
    Hot_water_fan Member Posts: 730
    edited September 20
    You’re a little sideways here but that’s okay! You’re in the right place. Those systems are the same efficiency. However - before you had a 75kbtu boiler with an indirectAn indirect (or a standalone tank heater) operates with a buffer to even out demand across a long time. A tankless heater (combi or domestic hot water only) is a just-in-time operation. As such, it usually has to be much bigger. For example - say you have a shower going that needs 2 GPM of 120 degree hot water and you have an incoming water temp of 40 degrees. That means you need 80kBtu output for just that one shower. If there’s a washer, sink, etc also going, that shower will be cold with those suggestions. However, an indirect wouldn’t be because while the output might be smaller, it has a head start. For example, a standalone tank water heater might have an output of 12 to 40kbtu and that’s what the vast majority of Americans use. 

     The combi has to be sized for the high DHW needs and the home’s heating needs. Often this results in a mismatch - the combi is much larger than the heat load, which is an issue. The easiest suggestion would be to get a replacement boiler that’s properly sized for the heat loss (which will likely be significantly smaller than the existing boiler) and keep the indirect. Or you’ll likely need a larger combi than what is suggested and live with some short cycling. 

    Minimum output is helpful for comfort and some efficiency improvements but your heat loss is your heat loss. If you need 25kbtu for a particular hour, that’s what you’ll get from the boiler regardless of minimum output, assuming the thermostat is being satisfied. Minimum output helps only when the heat loss is low - it can keep the boiler going when others might be turning off. So you won’t save 240 gallons just because one turns down lower. 

    how many annual gallons do you use? What does your winter months usage look like? 

    As a clarification, is the Rinnai a boiler or a tankless water heater? You want a tankless boiler, not a tankless water heater being used as a central boiler. 
    ZmanJeff_in_MA
  • hot_rod
    hot_rod Member Posts: 16,875
    You’re a little sideways here but that’s okay! You’re in the right place. Those systems are the same efficiency. However - before you had a 75kbtu boiler with an indirectAn indirect (or a standalone tank heater) operates with a buffer to even out demand across a long time. A tankless heater (combi or domestic hot water only) is a just-in-time operation. As such, it usually has to be much bigger. For example - say you have a shower going that needs 2 GPM of 120 degree hot water and you have an incoming water temp of 40 degrees. That means you need 80kBtu output for just that one shower. If there’s a washer, sink, etc also going, that shower will be cold with those suggestions. However, an indirect wouldn’t be because while the output might be smaller, it has a head start. For example, a standalone tank water heater might have an output of 12 to 40kbtu and that’s what the vast majority of Americans use. 

     The combi has to be sized for the high DHW needs and the home’s heating needs. Often this results in a mismatch - the combi is much larger than the heat load, which is an issue. The easiest suggestion would be to get a replacement boiler that’s properly sized for the heat loss (which will likely be significantly smaller than the existing boiler) and keep the indirect. Or you’ll likely need a larger combi than what is suggested and live with some short cycling. 

    Minimum output is helpful for comfort and some efficiency improvements but your heat loss is your heat loss. If you need 25kbtu for a particular hour, that’s what you’ll get from the boiler regardless of minimum output, assuming the thermostat is being satisfied. Minimum output helps only when the heat loss is low - it can keep the boiler going when others might be turning off. So you won’t save 240 gallons just because one turns down lower. 

    how many annual gallons do you use? What does your winter months usage look like? 

    As a clarification, is the Rinnai a boiler or a tankless water heater? You want a tankless boiler, not a tankless water heater being used as a central boiler. 
    I guess I’m not seeing the mismatch on a combi if it has a 10-1 turndown and the heating output can be locked anywhere between the lowest and highest output, plus the ramp delay?

    So a 120 combi, able to supply 3 gpm, could also be a 12,000 heating boiler, an efficient one at that as you have a large HX exposed to a small flame so lots of condensation. In condensing mode, anyways. Not many homes with a heating load at or below 12,000 btu/ hr
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
    GGross
  • Hot_water_fan
    Hot_water_fan Member Posts: 730
     I guess I’m not seeing the mismatch on a combi if it has a 10-1 turndown and the heating output can be locked anywhere between the lowest and highest output, plus the ramp delay?

    So a 120 combi, able to supply 3 gpm, could also be a 12,000 heating boiler, an efficient one at that as you have a large HX exposed to a small flame so lots of condensation. In condensing mode, anyways. Not many homes with a heating load at or below 12,000 btu/ hr
    If 3 gpm is all that’s needed, I totally agree! With an existing indirect though, why? Floor space is the main advantage I can identify. 
  • Zman
    Zman Member Posts: 7,375
    You are getting great advice above. I should point out that the Rinnae is just another 14-year boiler. Buderus is owned by Bosch so you can look forward to more of the same.

    As noted, your math on savings does not make sense. The BTU's burned is related to the heat loss of the building, not the boiler rating.

    No one has asked about your hot water needs. I combi boiler does not have any storage, going from an indirect to a combi will leave you with much less available hot water.
    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein
    GGrossJeff_in_MA
  • Jeff_in_MA
    Jeff_in_MA Member Posts: 12
    Zman said:

    You are getting great advice above. I should point out that the Rinnae is just another 14-year boiler. Buderus is owned by Bosch so you can look forward to more of the same.

    As noted, your math on savings does not make sense. The BTU's burned is related to the heat loss of the building, not the boiler rating.

    No one has asked about your hot water needs. I combi boiler does not have any storage, going from an indirect to a combi will leave you with much less available hot water.

    Thanks, Zman, for helping to "unstick" my thinking about the minimum input/fuel savings issue. I now see that although the top of my brain knows that pilot lights are no longer a thing, further down the neurons were still treating "minimum input" as a pilot light kind of situation, i.e. gas being burned all the time, even when nothing is happening. Any system I ever had before this house had pilot lights, so it was just an unexamined thought pattern. But OK, past that now! So my original question made no sense, but don't worry, I have lots more! :-) -- As for hot water needs, there are two of us, and sometimes we're using both showers, or a shower plus the dishwasher or whatever. But the systems I've been looking at (the Rinnae I090CN and the Bosch Singular) each puts out 4 GPM at 67-70 degrees rise in temp. I purposely avoided the systems that would only give 2.6 GPM. It seems that we could still both shower on 4 GPM, even if we might be better off avoiding laundry or dishwashing at the same time. 2008 house, so the showers are low-flow. But if we needed to, we could easily shower one at a time -- I'm retired and not on a schedule, and anyway Susan is normally up and running before I even twitch. Aside from burning less gas for its own (i.e. the planet's) sake, and the fuel cost savings, the impetus for the combi is that the state (MA) will give us a $1600 rebate if we change to a combi, as an efficiency upgrade from our existing setup. Just replacing one high-efficiency boiler with another while keeping the indirect DHW would torpedo the rebate. MIght also torpedo the 0% financing, I haven't checked, but that would be an even bigger issue. -- Thanks also for the comments on manufacturers. I will check out others suggested above, although I'm not sure there are local installers for all of them.

  • Jeff_in_MA
    Jeff_in_MA Member Posts: 12

    I would be careful with both brands. Bosch uses aluminum heat exchangers which can be problematic. Rani I am not familiar with. Most use Viessmann (top of the line) HTP has some good products and we seldom here about serious Lochinvar problems. I would keep looking and stay clear of Navien

    There are a lot of boiler manufacturers out there I never heard of! I will certainly look into all three you mention. Partly it will matter who installs and services in SE Massachusetts. But I would note that the Bosch Singular brochure states that both primary and secondary heat exchangers are stainless.
  • Jeff_in_MA
    Jeff_in_MA Member Posts: 12
    kcopp said:

    If your indirect water heater is still in decent shape why not pair it w/ a similar output boiler? I personally line the IBC line. But it is what the installer knows best, can service best and get parts for asap.

    The indirect is just as old as the boiler. But in any case MassSave (the state energy efficiency program) will give us a $1600 rebate for switching to a combi, which they won't for just swapping one HE boiler for another one. It might or might not also affect whether they'll give us the 0% loan, I'm not clear on that. But yes, I definitely agree we want something for which there's a good local installer who is familiar with the system and can take care of it. That was an issue with the Buderus even before they discontinued it, some local companies were like "Wha??"
  • Jeff_in_MA
    Jeff_in_MA Member Posts: 12
    hot_rod said:

    14 years on the indirect also? Heavy usage? Hard water?  It may be time to upgrade both.

    Or a 120 combi. I have had excellent service from the Lochinvar. The Knight turns down to 11,000 btu/hr and you can step fire it also

    Yes, the indirect and the boiler were both installed when the house was built in 2008, so I'm sure something will go wrong with it before long (just because)! In any case, the state will give us a $1600 rebate for changing to a combi, but not for just replacing one HE boiler with a different one. -- OK. Now I need your help in explaining this whole "turn down" thing. And I also have no idea what "step firing" is. (I did mention that I was an English major, right?) Are you saying that, say, a 100K BTU system can be set to operate as if it was a smaller system, running at a lower maximum output? Because if that's true, I don't understand why I read all over the place that having too large a system leads to short cycling and extra wear and tear and whatever else it's supposed to do. Or is that just if nobody set up/adjust the system to suit the house, or if it's an older type of system that can't be adjusted? Understanding this would really help in figuring out how large a system we should be looking at. Thanks for your help. (Oh and what's step firing?) -- I hadn't heard of Lochinvar before this discussion. They look pretty great (except the DHW output on the smallest system is low), but I'm not sure if anyone is installing or servicing them South of Boston. Will be looking into it.
  • Jeff_in_MA
    Jeff_in_MA Member Posts: 12

    You’re a little sideways here but that’s okay! You’re in the right place. Those systems are the same efficiency. However - before you had a 75kbtu boiler with an indirectAn indirect (or a standalone tank heater) operates with a buffer to even out demand across a long time. A tankless heater (combi or domestic hot water only) is a just-in-time operation. As such, it usually has to be much bigger. For example - say you have a shower going that needs 2 GPM of 120 degree hot water and you have an incoming water temp of 40 degrees. That means you need 80kBtu output for just that one shower. If there’s a washer, sink, etc also going, that shower will be cold with those suggestions. However, an indirect wouldn’t be because while the output might be smaller, it has a head start. For example, a standalone tank water heater might have an output of 12 to 40kbtu and that’s what the vast majority of Americans use. 


     The combi has to be sized for the high DHW needs and the home’s heating needs. Often this results in a mismatch - the combi is much larger than the heat load, which is an issue. The easiest suggestion would be to get a replacement boiler that’s properly sized for the heat loss (which will likely be significantly smaller than the existing boiler) and keep the indirect. Or you’ll likely need a larger combi than what is suggested and live with some short cycling. 

    Minimum output is helpful for comfort and some efficiency improvements but your heat loss is your heat loss. If you need 25kbtu for a particular hour, that’s what you’ll get from the boiler regardless of minimum output, assuming the thermostat is being satisfied. Minimum output helps only when the heat loss is low - it can keep the boiler going when others might be turning off. So you won’t save 240 gallons just because one turns down lower. 

    how many annuaa l gallons do you use? What does your winter months usage look like? 

    As a clarification, is the Rinnai a boiler or a tankless water heater? You want a tankless boiler, not a tankless water heater being used as a central boiler. 
    Hot_water_fan, "a little sideways" was a very kind way of describing my confusion over minimum input and fuel savings. Part of my brain was still subconsciously stuck in the age of pilot lights. (See my comment to Zman above.) Thanks for helping to un-confuse me (at least that far)! -- I am aware of the issue of DHW output from a combi system, and both of the systems mentioned above will do 4 GPM, which I hope should handle two low-flow showers at a time, even if we need to stay away from showering + laundry or dishwashing. (And yes, the Rinnae is a combi boiler not a water heater.) -- I asked a related question of hot_rod above, but could you say more about short cycling, its evils, and when/why it occurs, and also about "Minimum output... can keep the boiler going when others might be turning off"? If these boilers can be "turned down", I don't understand why short cycling would be an issue. And I don't understand why "keeping the boiler going..." would be a good thing. I'm sure both of those statements just show that I don't understand how any of this fits together, so thanks for your patience.

  • Hot_water_fan
    Hot_water_fan Member Posts: 730

    But the systems I've been looking at (the Rinnae I090CN and the Bosch Singular) each puts out 4 GPM at 67-70 degrees rise in temp.
    In MA, is 4gpm at 70 degrees enough for you? That’s saying incoming water is 50 degrees if you’re trying to reach 120. I think it’s probably fine with low flow but it’s definitely close. 

    In regards to short cycling, it’s when your minimum output is more than your heat loss. All boilers will cycle at times, it’s just something to limit if you can. Some boilers can turn down to 10% of max, some can’t get that low. For a tight, highly insulated house like yours, you might have a heat loss of 20-30kbtu, which means the Bosch would be cycling nearly the entire winter. There’s a limit though - you probably can’t do much better than 8-10kbtu at the low end. How much gas do you typically use per year? 
  • Jeff_in_MA
    Jeff_in_MA Member Posts: 12


    But the systems I've been looking at (the Rinnae I090CN and the Bosch Singular) each puts out 4 GPM at 67-70 degrees rise in temp.
    In MA, is 4gpm at 70 degrees enough for you? That’s saying incoming water is 50 degrees if you’re trying to reach 120. I think it’s probably fine with low flow but it’s definitely close. 

    In regards to short cycling, it’s when your minimum output is more than your heat loss. All boilers will cycle at times, it’s just something to limit if you can. Some boilers can turn down to 10% of max, some can’t get that low. For a tight, highly insulated house like yours, you might have a heat loss of 20-30kbtu, which means the Bosch would be cycling nearly the entire winter. There’s a limit though - you probably can’t do much better than 8-10kbtu at the low end. How much gas do you typically use per year? 
    Thanks, Hot_water_fan. Sorry, I was away for the weekend (delayed 50th HS reunion, how'd we get that old?). As far as the showers, we 99 per cent shower at different times anyway, and if the 4 gpm proved to be an issue, we could easily make that 100 per cent. (And anyway, Susan likes hers cooler than I do.) Does this mean if we just decided to go with one-at-a-time showers, a system that only puts out 2.6 GPM would be ok, or would we always be having borderline-comfortable showers?

    Looking at our last 7 propane deliveries (all that are showing up on our account), and averaging each sequence of 4 deliveries, we have recently averaged 516 gals/12 months. Our existing Buderus system has a low end input of 25.7KBTU, and I have no idea if it's been adjusted at all to take advantage of that, since I haven't started learning about any of this until we started looking at replacing it. No doubt we've wasted a bunch of gas versus what we could have done.

    But if I'm understanding the turn-down business correctly, then a properly adjusted Rinnae system (min 15KBTU) could save gas versus the Bosch (min 20KBTU), and (for example) a Lochinvar NKC110 (min 11KBTU) could save even more, despite having a higher top end (110KBTU) than the other two? (Assuming we can find anyone South of Boston to install and service the Lochinvar.) In other words, it's the minimum input you worry about to reduce short-cycling, not the maximum?

    I'm assuming in the above that the house heat and DHW can be turned up/down separately. The Lochinvar only puts out 2.6-2.9 GPM (77 or 70 deg temp rise). Assuming we'd want the DHW cranked up.

    I'll stop there for now, so you can respond to whatever idiocies I've spouted before I add more. Thanks for your help!

  • Jeff_in_MA
    Jeff_in_MA Member Posts: 12
    Hot_water_fan, I will add, though, that it doesn't seem like the current system runs a crazy amount, so it might be adjusted just fine.
  • Jeff_in_MA
    Jeff_in_MA Member Posts: 12
    Hot_water_fan, looking at what I wrote above, comparing the three systems and trying to rank them for gas savings, I think I'm just making the same mistake I started with! I take it that the real point is, as long as I can turn the system down far enough to match the needs of the house, short cycling won't be an issue. But I should then compare the systems on the basis of other factors, e.g. reliability, DHW output, etc.
    Hot_water_fan
  • Hot_water_fan
    Hot_water_fan Member Posts: 730
    @Jeff_in_MA you’re on the right path. You have a low heat loss by that usage, so a boiler that fits your DHW needs without being too huge is fine. All three of those seem reasonable. 
    Jeff_in_MA
  • pedmec
    pedmec Member Posts: 290
    Lochinvars can be rate limited on the heating side. So don't be afraid of going with a larger combi just because of short cycling as it will really won't short cycle. Lochinvar nobles have 10-1 downturn ratio and can limit the maximum output to closely match your heating load on the coldest day of the year.

    Also they are represented very well in Massachusetts by FIA of woburn. They stock a lot of lochinvar parts in case you will need repair which is rare outside of the standard maintenance. I have been installing lochinvar for years and rarely get any complaints.

    also if you don't like using the combi you could add an indirect water heater down the road.
    Jeff_in_MA
  • hot_rod
    hot_rod Member Posts: 16,875
    Most shower heads are 1.5 gpm, have been for years. If you have 50 psi or more you can get some great 1.5 heads these days.

    Remember also that you probably don’t shower with hot water above 103- 105 f So with the typical .77 degree rise your incoming water could drop to 33f😉 and you still get a hot shower, at the rated gpm.

    Your lifestyle is similar to ours and you sound like a perfect candidate for a combi

    plenty of good brands available, Lochinvar, Viessmann, Ideal, US Boiler, Laars, Baxi, etc

    Your current boiler is a 75K,  I’m guessing it cannot be turned down to 8K like some of the small combis?

    Look for a brand and model that has the rate limit, step firing, and boost. That covers all the firing options and may give you a boiler that modulates without shutting down for 70% or more of the heating season
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
    Jeff_in_MA
  • Jeff_in_MA
    Jeff_in_MA Member Posts: 12
    hot_rod said:

    Most shower heads are 1.5 gpm, have been for years. If you have 50 psi or more you can get some great 1.5 heads these days.

    Remember also that you probably don’t shower with hot water above 103- 105 f So with the typical .77 degree rise your incoming water could drop to 33f😉 and you still get a hot shower, at the rated gpm.

    Your lifestyle is similar to ours and you sound like a perfect candidate for a combi

    plenty of good brands available, Lochinvar, Viessmann, Ideal, US Boiler, Laars, Baxi, etc

    Your current boiler is a 75K,  I’m guessing it cannot be turned down to 8K like some of the small combis?

    Look for a brand and model that has the rate limit, step firing, and boost. That covers all the firing options and may give you a boiler that modulates without shutting down for 70% or more of the heating season

    Thanks (again!), hot_rod. Appreciate the encouragement on the combi. And oh good, MORE brands of boiler to compare! :-) -- No, that's right, the current Buderus only turns down to 25.7KBTU. -- I found something about "boost" online, saying that it applies only to systems with "multiple stages" of heating. Not sure if that's what we're talking about with turning the system up/down, or something else. But it may not matter. If I was a "boost" type of person I wouldn't have been driving my little Prius C entirely on "Eco" mode since I bought it. However, I still can't find a thing about "step firing" online, and haven't seen it mentioned (at least in those words) in any of the manufacturers' literature. Can you give me the English major explanation? Thanks!

  • Jeff_in_MA
    Jeff_in_MA Member Posts: 12
    pedmec said:

    Lochinvars can be rate limited on the heating side. So don't be afraid of going with a larger combi just because of short cycling as it will really won't short cycle. Lochinvar nobles have 10-1 downturn ratio and can limit the maximum output to closely match your heating load on the coldest day of the year.

    Also they are represented very well in Massachusetts by FIA of woburn. They stock a lot of lochinvar parts in case you will need repair which is rare outside of the standard maintenance. I have been installing lochinvar for years and rarely get any complaints.

    also if you don't like using the combi you could add an indirect water heater down the road.

    Thanks for the review on Lochinvar, and the suggestion re FIA. I will check them out!

  • Jeff_in_MA
    Jeff_in_MA Member Posts: 12

    @Jeff_in_MA you’re on the right path. You have a low heat loss by that usage, so a boiler that fits your DHW needs without being too huge is fine. All three of those seem reasonable. 

    Thanks, Hot_water_fan. Appreciate all your help!