Can I do this to my modcon?

[ToddinMaine]

I'm a homeowner trying to get a handle on my newly installed Utica SSC-100 modcon boiler. It cycles incessantly with short burn cycles.
I don't know how to post the diagram, but if anyone cares to look, my system is quite well represented in fig 5.5 on page 14 of the Utica IOM manual here: http://www.ecrinternational.com/secu...ument/2563.pdf.
Referring to fig 5.5, I tried closing the "heat exchanger ball valve", thus adding the entire volume of the CH loop to the heat exchanger loop, allowing for extended burns, in contrast to the constant heating and cooling of the small internal loop. In fact, in the system of fig 5.7A on the next page, the ball valve is shown to be shut, as noted.
My question is this: Can I simply leave the heat exchanger ball valve closed on my system and enjoy the benefits of longer burn cycles?

Thank you,

Todd in Maine

[jadnashua]

It depends on how your system is piped...it is designed for a primary/secondary loop. That heat exchanger loop is built-in as a primary loop. Unless the flow to the external load is not plumbed properly, that local (internal?) primary loop should work. There are some very specific requirements for the distance between the takeoffs on the primary loop that must be followed or it doesn't work properly. WHen there's a call for heat, most of the heated water should travel through the heating loop, and not just return to the boiler. If yours does, it is not plumbed properly.

The thing would probably tell you why it is shutting down. Is there an error code? If your external heat loops are not done well, the flow may short circuit back to the return, and it could reach max temp and shut off quickly. Otherwise, most of the heat will exit that primary loop and not reach the limit which would shut off the boiler.

[ToddinMaine]

The thing would probably tell you why it is shutting down. Is there an error code?

No, it's not faulting, but simply reaching temp. setpoint, sometimes within 30 seconds and shutting down.

I can't judge whether or not the piping is correct (though the house IS warm today - a 12 deg. day), but I do know when I closed that valve, essentially making the entire house the primary loop, the boiler brought the zone up to temp then throttled back and cruised for fifteen minutes until the demand was satisfied. I didn't observe any ill effects, and I wonder why I shouldn't be able to leave it set up that way.

Thanks - Todd

[jadnashua]

That boiler is designed to have a primary/secondary loop. THey provide a primary loop internally that is enabled when the valve is open. When you close it, it will work properly IF there's a primary loop installed external to the unit. All of the heating loops come off that primary loop and are called secondary loops. A primary loop allows the boiler to operate with less stress - bypassing it could shorten its life. If you study the diagrams in the installation manual you provided, you'll see that all of the alternative configurations provide that primary loop externally to the boiler. It may be that your boiler is way oversized for your house. This is a common (unfortunate) situation if a proper heat load analysis was not done prior to its selection. Replacing a boiler with the same size as was originally there is rarely the right size - often 2-4x oversized which affects efficiency, costs more to buy, and makes short-cycling more common. A mod-con can help since it can throttle down from max, but even 20% of what you have may be too much most of the year.

[BadgerBoilerMN]

A condensing boiler wants to be loaded while maintaining nominal flow through the heat exchanger. The distribution piping and radiation determine flow if the by-pass is closed.

[ToddinMaine]

Got it. It's all about insuring the heat exchanger has adaquate flow through it regardless of what's going on in the secondary loop. So then, is this a classic example of the need for a buffer tank in the primary loop?

[Dana]

Got it. It's all about insuring the heat exchanger has adaquate flow through it regardless of what's going on in the secondary loop. So then, is this a classic example of the need for a buffer tank in the primary loop?

How much radiation (and type) do you have? What is the total plumbing length & diameter (including that embedded in the radiation), approximate number of ells, and the pump used for the CH loop?

What is your 99% outside design temp and heat load at that temp?


If yours is a single-zone system (as in figure 5-5) using low-mass heat emitters (like fin-tube baseboard), as long as you have sufficient flow and a sufficient amount of baseboard it should be tweakable to a balancing point where it modulates with long burns. But if you don't have enough baseboard to get the heat out (at any flow rate) at the output temperature being called by the outdoor reset, it'll short cycle. I didn't see a programmable hysteresis in a quick glance through the manual. Increasing the output temperature/curve increases the BTU output of the radiation, which may help, but increasing the hysteresis (the difference between the low & high temp at any ODR setpoint) would also help, if that's an adjustable number.

If the CH pump is undersized creating a low-flow condition in the radiation it may be contributing to the problem too, and when you close the primary bypass valve completely you essentially have both pumps driving the radiation flow, which may "kinda work", but it's not exactly legit (it would be OK in some systems, not so much in others).

If your heat load at a design temp of -10F or so is less than 40KBTU/hr and the system has only 50-60' of fin-tube you will likely have cycling issues with a boiler that big unless you add either mass or radiation, (or run it fixed-temp at 160-180F, losing all condensing efficiency.) At 130-135F average water temp (the condensing threshold on most systems) 50' of fin tube can only deliver ~13,000-14000 BTU/hr, which is below the 17-19,000BTU/hr minimum-fire output of the boiler. But if you have 150' of fin-tube on a single zone you can probably run in condensing mode 100% of the time and it would never short-cycle if the flows are set up properly.

Whether a buffer tank or a buffering hydraulic separator is needed depends on the system particulars, so let's have 'em! In most single-zoned residential systems in central ME the 20KBTU/hr min-mod input for this boiler wouldn't have a short cycling issue when it's +12F outside, so rather than throwing a thermal mass band-aid at it, lets run at least a napkin-math analysis on the system to see if there's a simpler/better way to address the issue.

[ToddinMaine]

Whether a buffer tank or a buffering hydraulic separator is needed depends on the system particulars, so let's have 'em!

I think I see the problem - I've got a 100000 BTU boiler heating a 27000 BTU house!


House:

2 story saltbox 1900 sq.ft.
built last summer to Energy-Star standards
6" walls, R-50 in attic
Unheated full basement.
A basic heat load calc. for -2 deg. F design temp comes out to 27000 BTU.

Heating system:

First floor heat is radiant floor - Viega Fostapex- 1/2" tubing suspended between floor stringers with aluminum fins clipped in place. R-11 fiberglass insulation on underside.
Second floor has 40' total of fin-tube baseboard.
4 Zones - 2 up, 2 down.
CH pump - Grundfos UPS 15-58 FC
Indirect HW pp - same
Internal pri.loop pp - Grundfos UP 15-42 F

Thanks for your help,
Todd

[Dana]

The minimum-fire output is well under 27,000BTU/hr, and if your whole house heat load at -2F is 27K, the heat load at +12F is going to be over 20,000BTU/hr, so if it were a single zone it shouldn't be short cycling at +12F outdoor temps.

The problem is you have it chopped up into 4 zones, each of which is a fraction of the total heat load, and each of the lowest-mass zones on the upper floor have PITIFULLY low output relative to the min-fire of the boiler. Even if you dropped in a 50K mod-con with a min-fire output of 12K, a 20' stick of fin-tube baseboard at condensing temps is delivering less than half that to the room. Combining the upstairs into a single zone would help some, but probably not enough unless there's some way to increase the hysteresis on the boiler.

The total volume on the radiant zones give it some mass, but assuming it's about 700-800' of radiant floor you're looking at maybe 1000' of half-inch PEX tubing which adds up to about 10 gallons. Split into 2 zones the smallest radiant zone has at most 5 gallons. With a generous assumption of 3 gallons for the boiler + distribution + manifold volumes the smallest radiant zone has maybe 8 gallons or ~65lbs of water. If the boilers base-line hysteresis is as low as 5F, at min-fire output of ~18,000BTU/hr (300BTU/minute) the burn cycle serving that zone during low-load periods would be about (65lbs x 5F/300=) 65 seconds. If the hysteresis could be bumped to 20F you'd be OK with 4+ minute minimum burn times on an 8 gallon zone, but if your stuck with whatever hysteresis the engineers at Utica deemed appropriate.

The total volume on either of your fin-tube zones is at most half that, which would explain 30 second short cycles at whatever the hysteresis actually is (probably more than 5F, but probably way less than 20F.)

The indirect has substantial thermal mass and are typically set up with a few 10s of degrees hysteresis- it will never short-cycle the boiler.

If the baseboard & radiant floors are designed to run at the same temp (a dubious proposition, if you have but 40' of fin-tube total for the two upstairs zones), you can add mass to the heating side by employing a cheap 40 gallon electric HW tank (not wired up) plumbed with a tee at the HW output port and another at the drain port to use as a massive hydraulic separator:



The indirect zone would tap off inside the boiler loop, not off the radiation side of the hydraulic separator.

To get the most efficiency out of the boiler you may want to consider replacing the fin-tube with low temp panel radiators sized to be able to deliver heat at the same water temp of the radiant floor, which would allow you to lower the outdoor reset curve so that it ALWAYS runs in condensing mode.

You actively DON'T want to plumb the buffer in series with the primary loop, since that would add a considerable response time delay for the indirect, and deliver higher than needed water temps to the radiation after the indirect's call for heat was satisfied. Keep the radiation's buffering mass separate from the water heating loop.

[ToddinMaine]

Thank you, Dana. That's a good deal of information to ponder.

Above is my system. The supply and return are 1 1/4" dia. before they drop to 3/4", then the 1/2" pex. With pex, of course, the number of elbows is minimal. If I run my circ pumps on the fastest speed, would you feel good about the flow rate through the boiler if I closed that primary loop ball valve?

With a HW tank/buffer as you described, why wouldn't you want to just pipe it in series with the CH supply piping?

Todd

[Dana]

Series plumbing with the CH loop at the boiler output lowers the response time on the system a bit. Putting the mass at the point of hydraulic separation the radiation sees nearly undiluted boiler output when the boiler is firing.

If you put it in series with the return leg to guarantee that the radiation sees full boiler output, it raises the modulation level of the boiler when it fires up, reducing both the efficiency and length of the burn slightly. (There's no intuitive simple-math explanation that's easy to lay out in a forum posting, but the real math has been worked out by hydronic enginerds doing more than first-order estimates or WAGs like the rest of us.)

They're generally more expensive, but full-on already ported massive hydraulic separators are out there (BoilerBuddy, Ergomax BT26 or BT48 ) with fatter pipe connections, but at the flows you're running the added impedance of the short section of 1-1/4 x 3/4" reducing-tees into a water tank isn't going to change the flow performance appreciably. The impedance of the tank itself is very low if you hook up to the HW output port and the drain port. The cold water inlet has a length of 3/4" dip-tube, which would add a bit more restriction to the flow (still probably not a show-stopper, but why bother when the other options are available?)

I wouldn't close the valve on the bypass completely without running the numbers on the actual flow under different scenarios, which isn't exactly a great design-by-web-forum type of exercise. Empirically you can throttle it back some and keep checking with your ear to the boiler listening for sizzling micro-boil gone macro on the heat exchangers, but it's likely to vary with ODR temp and firing rate- you don't want any noisy unpleasant (or warranty-voiding) surprises, eh? Even full flow isn't going to magically fix the short cycling when the only zone calling for heat is one of the low-mass low-output fin tube zones. With the massive hydraulic separator architecture you can close the bypass valve though- the tank wpuld then be taking the place of that short section of pipe with the bypass valve. In the meantime throttle back the bypass flow somewhat gingerly- don't close it completely.

I just noticed on your prior description that you're using the clipped-fins as opposed to heat exchanger plates on the radiant. That sort of system raises the operating temp of the radiant a few 10s of degrees compared to a plated system, which results in lower combustion efficiency at the boiler, but probably brings the overall operating temp up closer to that of the fin-tube.

[BadgerBoilerMN]

You know how much I love buffer tanks, but I would get one or sell your modcon on Craig's and buy the right size boiler for the job. If you called me, it would be a new boiler sized to the load and likely plugged right in to the piping provided. As Dana states, someone has to do the math.

[ToddinMaine]

... sell your modcon on Craig's...

I'm considering doing just that. If I've read the Utica literature correctly, the 50 and 100K BTU models are dimensionally identical, so the swap would be easy. The cost of the boiler wasn't broken out of the contract, so I don't have a clue what a modcon with 8000 ignitions on it would be worth. Any suggestions?
Or, perhaps I can work a trade arrangement with the systems' installer.

Todd

[BadgerBoilerMN]

Now you're thinking like a wet-head!

[Dana]

Even if you swap out the boiler for the -050 you may still have cycling issues on the very low mass sticks of fin-tube when the outdoor reset is calling for a low temp. Combining the fin-tube zones into a single zone would help: The min-fire input of the -050 is 10K, so at condensing temps the min-fire output would be at least 9K. With an AWT of 130F a 20' stick of fin-tube only delivers ~5-5.5KBTU/hr, it'll still short cycle. Combined into a single 40' zone it would have a bit more mass, and it could deliver more than 10K @ 130F AWT. You'd probably be still be getting decent burn times at 115F-120F AWT, well into the condensing zone.

If the fin-tube was swapped out for lo-temp panel radiators capable of delivering 10KBTU/hr on any zone at an AWT of 130F you'd probably be OK, since it would be even more thermal mass, and it would have much better low-temperature output characteristics than fin-tube (which has very non-linear output at temps under 120F.)

If the original installer was also the system designer (or even if they weren't), they should have done at least the napkin math on this and not installed such an oversized boiler for both the whole house heat load and the smallest zone heat emitters. The short cycling was an easily foreseeable issue, and there's a good argument that they should correct the situation. Typical decent ~100K mod-cons run typically $3000-4500 at internet retail pricing, ~50K mod-cons run ~$2500-3500. I'm not sure what the distributor's list-price on the Utica boilers are, but if you down-size by half you probably have some cash to negotiate too. A boiler with 8000 cycles on it is still very early in it's service life, but it would be tough to do better half the distributor's list on it in trade. A craistlist customer might do you a bit better than half the list price if your installer won't work with you.