Can I do this to my modcon?

[ToddinMaine]

Man. This thread has been a real crash course in home heating systems, and I truly appreciate everyone's contributions. I've even been credited with thinking like a wet-head, which I think is a good thing.
Ironically we jettisoned panel radiators from our plans in favor of the more budget-friendly fin-tubes. You see how that worked out for me. For the same cost I could have had an appropriate sized boiler and panels, perhaps even saving the cost of a buffer tank. 20-20 hind sight and all that.
So I think I'll negotiate a Boiler-Buddy and maybe a smaller boiler with the original installer, and then watch the performance of the fin-tubes for a while. Our upstairs comprises two bedrooms which are kept cool anyway, and a hall/sitting area and bath which pretty much heat themselves from heat rising in the stairwell, so it all may be okay.
But to anyone new to the science of modcons and hydronics, my advice is to do your homework before calling a heating guy, not after as I did.

[Tom Sawyer]

Adding a buffer tank will do little to help the condition. It too will come up to temperature fairly quickly and from then on, just add to the cycling problem as now there is more mass to dissipate. There is no solution other than to dump heat somewhere or get a smaller boiler.

[Dana]

Show your math Tom, 'cuz not all your words are making sense.

What means "dissipate mass" in the vernacular you speak?

The burn cycle time is a function of both the heat emitted but also the hysteresis and thermal mass of the system. Adding mass NEVER shortens the burn cycle, and ALWAYS increases it. At the same firing rate it takes 10 times as long to raise the temp of 30 gallons of water over the same hysteresis as it takes to raise 3 gallons of water, and can take a boiler-killing 30 second short-cycle out to a fairly efficient 5 minute burn, without changing ANY of the radiation.

The net effect of the standby losses of the buffer tank on system efficiency is miniscule compared to short-cycling losses. (If you cared about standby & distribution losses you'd insulate all of the system plumbing, which is still a good idea in many cases.)

He's talking about both going smaller, AND adding mass. Whether adding mass it "worth it" depends- doing the napkin math is always worth it to sanity check any proposals. But a cheap electric hot water heater is a lot less money than a BoilerBuddy or ErgoMax, and at the likely here the 3/4" ports would not present a problem. (Adding more heat emitter in the lowest-mass zones would likely be cheaper than BoilerBuddy too.)

[Tom Sawyer]

Once everything comes up to temp you are right back where you started if the boiler is that over fired and the radiation and heat loss are minimal.

[ToddinMaine]

Posted In Error

[ToddinMaine]

Once everything comes up to temp you are right back where you started if the boiler is that over fired and the radiation and heat loss are minimal.

Won't the circ. pump just draw water from the buffer tank until its' water temp drops enough to fire the boiler? Then the boiler's got to bring 30-40 gal. back up to the hi limit? Or am I missing the whole point.

As an interim measure, I've tied the two downstairs zones together by running them off one t-stat ... At no real sacrifice in comfort, I might add. Given the minor demand on the second floor fin tubes, I'm essentially presenting the whole house load at each burn. Additionally, I've set the max boiler output at 60%, so it doesn't rage up to setpoint temp (180 deg) and overshoot. Now, it will typically take 5-10 min. to make setpoint then fall back to 10-20% power until the T-stat is happy, which can be a half hour or so, given the slow response of the radient floor system. That's okay for now, but when the outdoor reset starts bringing the supply water temp down from 180 deg. the long burns will probably disappear without the added mass of a buffer.
I assume a number like 20% of max power represents 20% of the boilers' modulating range, in this case 80KBTU. So 20% power is 16KBTU plus the 20KBTU minimum rate, which = 36KBTU. Correct?
Is setting the output limit at something less than 100% a valid way to bring an oversized boiler into line? Why else might that adjustment be provided?

Todd

[Dana]

Once everything comes up to temp you are right back where you started if the boiler is that over fired and the radiation and heat loss are minimal.

That is simply not so!

Mod con boilers operate in a hysteresis high/low limit band around the outdoor reset (or fixed-temp, if so operated) setpoint. On some that hysteresis is programmable, others not. On the versions that aren't programmable the hysteresis band is typically between 5-10F wide. If you have 3 gallons in the system/loop it's about 25lbs of water and a 5F change in temp takes (25 x 5=) 125 BTU.

At the SSC-100s' min-fire of ~20,000BTU/hr-in or ~18,000BTU/hr-out, it's dumping heat into the system at (18,000/3600=) 5 BTU per second, so it'll shut down in about (125 BTU/5 BTU per second=) 25 seconds, then fire up again when the heat emitters pull enough heat out of the loop that the system temp has fallen that 5F.

With a 27 gallon buffer tank plus the original 3 gallons you now have 250 lbs of water, and the minimum firing time is now 250 seconds (over 4 minutes) rather than 25 seconds, which is a HUGE difference in both efficiency and wear & tear on the boiler.

Swapping in the SSC-50 it would have half the min-fire output, and twice the minimum burn time. The 25 second short cycles now become 50 seconds, which is better, but it's still quite a hit in performance. But even 15 gallons of buffer would be enough.

Putting enough fin-tube in the micro-zone to deliver 9000BTU/hr (the min-fire output of the SSC-50) into the small zones at 120F AWT probably isn't going to be possible (that's about 40 feet!). Combining the fin-tube zones into a single zone would work fine with the smaller boiler, but it would still short-cycle with the existing boiler.

In the mean time, setting up the boiler to run at a fixed temp output of 160F+ would kick the burn times up to over a minute. PEX is only rated for 180F, so don't push it too far. At 170F out of the boiler you'll likely see an AWT of about 160F, and 20' of fin tube will deliver ~9000BTU/hr (about half the min-fire output of the boiler) at an AWT of 160F, which should roughly double the lengths of your short-cycles.

[Tom Sawyer]

Sounds good in theory but having fought this battle countless times I'm here to tell you that a buffer tank will not solve the problem. The boiler is 3x oversized for the radiation. You need to get away from the calculator and into the field.

[Dana]

I'm SO glad you're looking out for my needs, Tom!

Math works pretty good in the field too, just like gravity- it's more than just a theory. Buffers may be a band-aid, but properly applied they can work. Simply throwing a buffer at it without doing the math on the whole system out doesn't cut it though. I too prefer increasing the radiation size of the smallest zones and reducing the boiler size for a better match, but limitations of first-order simple-math models aside, add mass can (and does) work, if you do it right.

[SteveW]

I know nothing about boilers but noticed what looks like an unsupported expansion tank on the right side of the photo - is this true?

I would personally want to see this supported so that when the diaphragm eventually fails and the entire tank fills with water, it doesn't go to ground and turn your basement into a swimming pool.

[Tom Sawyer]

I'm SO glad you're looking out for my needs, Tom!

Math works pretty good in the field too, just like gravity- it's more than just a theory. Buffers may be a band-aid, but properly applied they can work. Simply throwing a buffer at it without doing the math on the whole system out doesn't cut it though. I too prefer increasing the radiation size of the smallest zones and reducing the boiler size for a better match, but limitations of first-order simple-math models aside, add mass can (and does) work, if you do it right.

Agreed but from what I'm reading, I just don't think a buffer tank by itself will increase cycle times sufficiently. And I know nobody likes the thought of buying a new boiler but it's really the only way out.

[ToddinMaine]

Aside from a min. fire rate of 20KBTU rather than 10, does setting the max boiler output at 50% equal a 50BTU unit? I seem to be doing okay right now with the two 1st floor radiant zones tied together and max output set at 60%. It will throttle back to 10-20% and go continuously until the t-stat is satisfied.

[ToddinMaine]

... looks like an unsupported expansion tank on the right side of the photo...

Hmmmm. Good call. My only defense is that for some reason it looks a lot bigger in the photo than it really is. On the other hand, it wouldn't take much to brace it off the HW tank. Thanks for pointing that out.

Todd

[BadgerBoilerMN]

Dana is right as usual but the real rub here is the common mistake made by professional and novice alike, which is over-sizing condensing boiler. The most important thing is not making sure the boiler is big enough...it is making sure the minimum fire is low enough to serve the smallest zone load without out short-cycling (about a minute run time on modern condensing boilers). The exception to this is your typical sun room sporting the highest heat load per square foot at with the lowest load (amount of radiation). This combination inefficient in any hydronic design but deadly in an over-sized modcon.

The thing to take away from this, is the root cause. DIY design.

We often use condensing boilers to replace cast iron serving fin-tube baseboard but never specify or install baseboard on new construction. If you can't afford anything but fin-tube you should be looking at forced air.

[Tom Sawyer]

After the mistake is made is when everyone scrambles to find a cure and they invariably mess with the boiler when the problem can't be solved there. Even adding radiation won't cure the issue because the heated envelope can't shed heat fast enough.