Is it bad if a modulating boiler never runs deep into the turn down ratio, or high in its capacity?
My second post here (first one was about a leaking radiator pipe, fun). My goal here is to properly size a new, more efficient boiler for my hydronic system, saving some $ on fuel and adding better comfort to our home. But my specific question is about modulation ratios and whether it is "BAD" for a boiler to not get "deep" down into the lowest levels of its capacity, and for it to also rarely/never run into the highest levels, either.
I found a boiler that I think will work for me (PDF link), it's a Rinnai with 112,000 BTU heating capacity, and a turndown ratio of 8:1. So it is able to slow down to as low as 14,000 BTU.
However, there are two things I am wondering about this and I am not sure if they are a problem.
#1 issue: after I make some insulation modifications to my house over several years (down the road, but hopefully not too long from now), the highest long-term load that this boiler will "see" is probably only 67,500 BTU (based on the 99% temperature of 7 degrees here in SW Ohio and indoor temp of 68 degrees). Even if we had a bad run of days at, say, -15 (this does happen), it would only need to run at 91,800 BTU to keep up- which is only 82% of the boiler capacity. This feels to me like I might be oversizing the boiler…? Also even at my CURRENT, less insulated heat load of 92,500 BTU, it will be able to handle a delta T of about 74 degrees when going full blast. That won't happen very often and like I said, I intend to insulate more.
#2 issue: the total EDR for my radiators is 764 (it's a huge, old gravity system!) and so the slowest pace my radiators can possibly run at only 110 degrees is about 22,900 BTU. So that's only about 20% of the boiler capacity. Anything less than that and the system would short cycle. Happily I figure that at 110 degrees my system should be able to deliver a delta T of 15 degrees when running at that 22,900 BTU… so my radiators aren't so oversized that it can't be dealt with by adjusting the water temperature.
BOTTOM LINE QUESTION: is it OK if a modulating boiler is sized so that the actual loads it will ever see over the long term are only between 20% and about 80% of its range?
MORE BACKGROUND INFO (since I know you people love background info)
I've been doing a lot of calculations and research, and have a decent level of confidence that my current 99% design temperature heating load for my house is roughly 92,500 BTU.
I won't go through all the calculations, but to sum up: I made heavy use of this article at GBA, and I did calculations using previous heating bills based on HDD at different balance points, and I also did a Slant/Fin app load calculation, being very very careful to not be too conservative on all the U-value assumptions (which I understand can be a big problem when doing that kind of load calculation). It turned out that I was able to get these two methods to roughly agree, based on the assumption that my current decades-old boiler is only running at about 70% efficiency (originally 80% on the label) at a 65 deg balance point for the house, both of which seem reasonable. So that's where the 92,500 BTU/hr is coming from. BTW the current boiler is way way oversized.
That 92,500 calculation includes about 25,000 from the basement (94 linear feet of 7 foot tall, 16" thick stone foundation walls, so delta T of 48 x 94 x 7 / (0.08 x 16) = 24,600; my band joists are already insulated with closed cell foam). I intend to someday insulate the basement walls and bring this portion down to about 2,500. So the future total load someday should be more like 70,000 BTU/hr.
I am also going to be improving attic insulation down the road (adding an occupied/conditioned 3rd floor with its own heating system) too, so that will further reduce the load for the boiler system. It will go from about R20 to R40. This will remove another 2,500 BTU or so from the load, making it 67,500 BTU.
Hopefully that's enough info.
Comments
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I doubt that never reaching full power — or never reaching its minimum firing rate — is particularly a problem for a modulating boiler. What is of significance, however, if one is to obtain the advantages of a condensing boiler (the "con" half of "mod/con") is that the return temperatures need to be in the condensing range — or well into the condensing range. That is more a control issue.
Br. Jamie, osb
Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England1 -
I see. So if I understand, that has to do with making sure the reset curve is properly determined. Like in my example, assuming my calcs are right, I would want the water temp to be more like 110 degrees with outdoor temps in the 50s/high 40s, and ramp up from there but not get too high since my radiator system is so freaking huge. Assuming my 67,500 BTU number is right, for my 764 EDR system, that would be a max water temperature of about 140 degrees.
And you can manage the return temp by pumping the water correctly…. right? This is something I haven't learned about yet.
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Actually thinking about it: I guess the other thing that would have a big impact on the return temp is the sizes of the zones. Hmmm. I plan to have 3 zones right now. Maybe 4. And this is something I ignored in all the discussion above. I suppose it is possible to get into the low end of the range after all, if I have a smaller zone calling for heat by itself some of the time.
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Oh yes. And if the zone is small enough, the boiler will have to short cycle…
Br. Jamie, osb
Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England1 -
Based on the EDR, even if you chop it into 3 it is still a giant lump of iron filled with water. There no way a 14000BTU output will short cycle.
The efficiency of a boiler doesn't change all that much based on output but it does greatly change based on RWT.
Make sure to set your reset curve as aggressive as you can, ensure your flow rates are correct to get the most delta T. Some modcons allow for higher delta T, you can usually squeeze a couple more percent efficiency by running the output a bit hotter but with higher delta T which brings the RWT down a bit. For example:
-supply at 130F return 110F, so mean temp of 120F.
-supply of 140f and return of 100F, same mean temp of 120F
Looking at the curve bellow (random curve off Google, but they all look similar), that is around an extra 3% to 4% fuel savings for free.
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#2 issue: the total EDR for my radiators is 764 (it's a huge, old gravity system!) and so the slowest pace my radiators can possibly run at only 110 degrees is about 22,900 BTU.
Why have you concluded that the lowest temperature at rads is 110F? You could run the rads at 100F when the OAT is 60F and they would deliver something around 17,000 BTU. If the boiler can get down to 14,000, it would run constantly without cycling unless the building reaches the setpoint on the stat with 17,000 BTU.
The goal is to send the coldest water possible and still maintain temperature in the space. Ideally, you set the 'stat at 72F and the building never quite reaches the setpoint (assuming your comfort level is 70-71).
Many folks get all perturbed when the space remains slightly colder than the setting on the 'stat………………….."MY BOILER WON'T HEAT MY BUILDING"…………….. That is the perfect way to operate it. It is all related to the heating curve that YOU program into it.
I do believe the Rinnai is a bit large compared to your HL calculations but not excessively so.
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On any given 50° outside temperature day, the reset curve may only allow 120° water temperature to leave the boiler (that is a guess and it is based on the curve you select based on how much radiation mass you have) that said, if the returning water is at 110°F and you are maintaining comfort in the home, the burner only needs to supply the amount of BTUs required to reheat that water to 120°. Now if that happens to be 15,023 BTUh then your boiler will be operating at the minimum of 14,000 when the outdoor temperature climbs to say 52°. Any higher than that and your burner will cycle off for some short time then back on for some short time maintaining the even cooler boiler water temperature. So I would not worry about it never operating near the minimum. Outdoor reset snd mild temperatures will take care of that.
As far as operating at the maximum rated temperature. If you were to add a indirect water heater to your system, then the DHW demand would force the burner to the MAX Output on ever call for DHW. So there areboth of your fears handled.
And BTW…. I don't think that your suggested problem is a real thing. Whenever the power goes out, the boiler start up goes to full MAX flame for a few seconds on the startup sequence anyway.
Edward Young Retired
After you make that expensive repair and you still have the same problem, What will you check next?
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I thought I read somewhere that 120 deg is about as low as you want to go because of Legionella…? And I was just pushing it down a little bit more to make my math work better. So we can go lower without any problem?
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Depends on whether you plan to drink the water or shower in it — or just keep it in your heating loop.
For domestic hot water, yes 120 F is the lowest you should go, and 140 with a tempering valve on the output is even better. That is for any tank storage, of course. An instant hot water heater will be different.
For the heating side, you can go as low as you want, and as the folks above have pointed out the lower you can go the better your efficiency overall will be.
Br. Jamie, osb
Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England1 -
The boiler you specified in your link is not a DHW heater
It does not say Combi or Water Heater. It states "Heat Only". That indicates a closed system central heating system of some design.
I personally don't think that Legionella is going to happen in a closed system. and even if it does, the next heating cycle over 140° will certainly kill it off. Furthermore if you do not have any reason to open the closed system, then anything nasty in there won't be getting out any time soon.
Of course you can always connect an indirect water heater to that boiler. then you can use the DHW in the tank. but that tank is separated by a heat exchange from the closed system boiler water.
Edward Young Retired
After you make that expensive repair and you still have the same problem, What will you check next?
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Correct. As stated above, you can go as low as you desire in a CLOSED system, which your setup will certainly be because you're not heating water for home use.
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many of the mod cons allow you to step fire them My Lochinvar with a 10-1 turndown has 6 adjustable steps.
I run 2 minutes at 20%
1 minute at 40, 60, 80 and 90%.
It also has anti cycling, where you block the next heat call for a programmed time. So you can program out most any potential short cycles. You can also limit the high fire btu outputThe lower the firing rate, ie a 119k boiler limited to 67k, the boiler is more efficient as you have a large HX exposed to a small flame, so more condensation
See what control options are in that boiler
Bob "hot rod" Rohr
trainer for Caleffi NA
Living the hydronic dream2 -
You’re doing great. The lowest you can go is 8kbtu on the low end, so even if you got the smallest modcon, you’d only gain 6kbtu.
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Great to know that 100 degrees or even 90 degrees are options in a closed system for my gargantuan radiators!!! Yay!
Thanks fellas.
Yes I understood all along this was a closed system but I had read so much about Legionella I guess it just scared me. Maybe I was imagining it somehow migrating past the backflow prevention valve and into my potable water system, I dunno. Probably crazy.
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How can I find out what "control options" are available for it?
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I would read the I/O manuals for each of the heaters that you are interested in using.
OR
If you already selected the one you previously posted then read that manual.
Also look for service updates on that model of heater.
I remember when Honeywell did not tell Weil McLain all the capabilities of the control they sold them for the ULTRA Gas boilers. It turns out that after they were in the field being used by customers, they were getting questions about stuff they did't know about. A year later they publisher a "Revised" set of instructions for that boiler.
Edward Young Retired
After you make that expensive repair and you still have the same problem, What will you check next?
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Not the last time honeywell pulled that move! Another manufacturer went through an issue with LP gas conversions they thought it had something to do with the difference in their LP from Europe to America, turned out Honeywell had changed the parameters in their controller without telling anyone.
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After doing some more research, I think something like a Lochinvar is probably a better choice even though it costs significantly more. The 10 yr warranty is a big deal to me, as well as the 10-1 turndown ratio, and many other features as well.
Are there other higher end brands similar to Lochinvar that I should also consider?
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Lochinvar, triangle tube, Viessmann, IBC would be my go to brands that I know support their products well, they should all have models with 10:1 or better TDR. Most important thing imo is which brand has local support (parts distribution etc) equally important is the install. Not sure about IBC but all the other brands should have parts accounts set up with supplyhouse so if there are not local parts, or local suppliers are not stocking what they should, parts are pretty easily accessible. If you have an installer you trust I would defer to their preferred brand generally. Good installers will usually prefer specific brands because they are comfortable with them, and have positive experience with the local support network and warranty processing
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