Near boiler piping, Hydrolink and CI protection

PGor

Hello,

I'm in the process of installing an indirect DHW tank and want to take this opportunity to clean up the boiler plumbing a tad. The current near-boiler piping has some mistakes (air scoop off an elbow, circulators pumping into the expansion tank, no hydraulic separation, etc), and I am exploring a couple plumbing options.

The house is 1100 square feet, climate zone 5, two stories and two zones of fintube on circulators. Burnham SCG4 boiler derated to 91,000 btu/h in, 78,000 BTU/h out. Down the road I may want to add a small zone of radiant floor to the bathroom (70sf, ~2000 btu heat-loss).

The house is quite small and the utility room is very cramped, but I'm looking for "the right way" to do this, and am willing to pay a little extra. Since I am ripping apart the near-boiler piping, I wonder if I should
a) plan to build in CI boiler protection, and
b) include P/S piping in the way of Caleffi Hydrolink.

I realize that the CI boiler does not need P/S, but I've been impressed with Caleffi's product line (and Idronics!); the Hydrolink appeals to me because it's a very tidy installation for my small space, and it can be easily expanded if I ever add radiant floor in the bathroom. The downside is that it's pricey and possibly makes my system more complicated than it needs to be; I'd need 4 circulators (System, DHW, 2 zones) and one more if I do the bathroom floor down the road. The indirect would go on the system side, not off the Hydrolink.

Question 1: Am I overthinking the design for such a small space? Should I be looking at simpler alternatives?

Question 2: Do I need boiler protection? I think for the current system (fintube only) the answer is probably no. But is it good to have anyway, say, to prevent cold shock after a set back on one zone, and especially if I add that radiant zone in the future? If I end up doing it, would it be better to plumb in a system bypass with the Caleffi ThermoMix now, or should I worry about it when I decide on the radiant zone, and regulate the return water temperature from that zone alone e.g. with an i-series mixing valve?

Thanks for all your time. I've learned a lot reading these forums.

Paul G.

hot_rod

Maybe start with a heat load calculation, it sounds like way too much boiler for 1100 sq. ft?

How many feet of fin tube connected?

Doubtful you would need boiler protection with that boiler if it is grossly oversized.

PGor

Thanks Hot Rod. The boiler is definitely oversized. Previous owners had tenants, and I guess they were complaining of not being warm enough, so the owners put in this boiler about five years ago. I'll measure the fin tube length when I get home.

Firecontrol933

The indirect water heater is going to be the only load on that boiler that will ever come close to making it operate correctly (long enough run times) and then only if you choose the right circulator and plumb it correctly.

Your baseboard loops can only take away at best about 40,000 btu's/hr and that's if there is enough length in them, which I doubt because of the size of the home. I am assuming 3/4" piping, 20°F temperature drop and a maximum flow of 4 gallons per minute through the loops.

So, you have 2 zones of baseboard and the possibility of adding a radiant zone. Your boiler is going to short cycle constantly (except possibly when making hot water) and that is going to cost you money, comfort and maintenance issues. Short cycling can cause comfort issues because if your limit is set at say 180°F the boiler "normally" won't come back on until the water temperature drops to 160°F. This constant swing in temperature being supplied to the baseboard can cause heating issues if the length of the baseboard wasn't sized for the lower temperature. A boiler that short cycles all the time wastes some energy, but worse than that is the wear the constant on and off does to the unit as a whole.

If you are going to keep the current boiler you need somewhere for it's capacity to go, because it can't go fast enough to the heating elements. A buffering tank will "help", but in your case the excess capacity will still be a problem.

The best solution would be to replace the boiler with one sized as close as you can get to the load it's going to see.

You mentioned the boiler was de-rated. I'm curious as to how that was accomplished.

hot_rod

As FireC mentioned, the boiler will short cycle it's brains out. most locations are at design conditions less than 10% of the year, so even with the correct size you could get excessive cycling.

Short cycles are very fuel and component inefficient and depending on the flue, may cause condensation and rotting away of the materials as the boiler will not run a long enough cycle to warm and dry the flue. Especially with masonry flues.

PGor

I measured the fin tube, it is in fact 3/4", and I have 43' on one zone and 50' on the other. Assuming 500 BTU per foot, that works out to about 47,000 BTU/hr.

The boiler does short cycle. I am lucky if it runs for more than 7 minutes, and frequently it runs for less than a minute. It helps a little if both zones are calling, but even then it never runs longer than 10 minutes per fire.

The boiler is sitting right against an exterior wall (the house is on a slab), and is direct vented. There is only about 3 feet of 3" Z-vent, and it gets hot to the touch, so I think it's managing to burn off the condensation by the end of the run.

FireC,
I hear your points and agree with you. Unfortunately a buffer tank is out of the question. There is no place to put it; the utility closet is barely big enough for the boiler and the water tank, and there is no basement. Replacing the boiler would be a stretch too. This one is only 5 years old and was put in by the previous owners, so I am stuck with it until it breaks. The boiler had a factory recall and I had a plumber come out to field modify it. As part of the recall he replaced the gas orifices that brought the BTU rating down.

Can you elaborate on plumbing the indirect correctly? I have a 50 gal Bradford White that's ready to go in. That tank has a 1" coil inside and I was planning to connect it with 1" copper to 1" black iron at the boiler. I believe the headloss is about 1.5' through the coil at 14 gpm. I haven't picked a circulator yet, but I think the pump curve for the 007 with IFC looks reasonable. Am I missing anything else?

Do you guys have any other thoughts on how to attempt to mitigate this overcapacity? If only I could use the DHW tank as a storage tank by letting it fluctuate between, say, 160F and 130F with a mixing valve on the output.

Also, any thoughts on using the Hydrolink, or is it just an extravagance for a system that doesn't need it?

Many thanks, and sorry for the long post.

Paul

hot_rod

Some suggest a reverse indirect, like a TurboMax is helpful to provide buffing and DHW production.

No need to pump 14 gpm, from a 78K output boiler, for that indirect.

I don't think you application cries out for a HydroLink, to be honest.

Downsizing would be first on my list, second adding buffer, or a combiboiler. Or enlarge the home

One rule of thumb is a minimum on cycle of 10 minutes for a boiler like that.

Zman

If your DHW needs are predictable and you do some careful math, a reverse indirect might be the ticket. It adds a significant amount of mass which will buffer the system nicely.
I don't share FireC's concerns (I do respect his knowledge) about widening the on/off delta t on the boiler control. I would be surprised if the heat loss of the home is more than 33k so there should be plenty of baseboard capacity even with lower water temps. I some cases the temp swings will cause noise issues in the piping, this just depends on the installer. A 155-185 range would help with the short cycling.

Firecontrol933

I've been in a lot of small boiler rooms in houses that required thinking outside of the box to get everything in there that's required. Look up, just because there isn't floor space doesn't mean there isn't space.

SWEI

PGor said:

I measured the fin tube, it is in fact 3/4", and I have 43' on one zone and 50' on the other.

Are those overall lengths or element lengths? Elements are usually 5-6" shorter than the enclosure. An AWT of 170°F on Fine/Line 30 should put out 444 BTU/ft (after removing the 15% marketing effect.)

PGor

I like the idea of linking the two zones to increase the load on the boiler. I am going to try that down the road, but I think I'll still keep two circulators, because I think changing out the electrical is easier than changing the plumbing. I might put an ECM variable speed circulator on one "zone" to have a convenient way to regulate the flow. Would that be alright or do you still recommend a globe valve instead?

Unfortunately my house really benefits from having two separate zones. The bedrooms and bath are on the first floor and the living space and kitchen (with cathedral ceilings) are on the second floor. We let the first floor get pretty cold when we're away, but the upstairs has to stay warm for the dog. We do like to keep the bedrooms cool while sleeping, but it would be nice to warm up the bathroom (hence the original question about radiant heat) and bedrooms occasionally as needed without also heating the upstairs. Tekmar advertises "zone synchronisation" with some products, could that help me out?

The boiler room is under a staircase, so even the vertical space is limited. There is room for a manifold and some circulators but not for an extra storage tank.

I am not very familiar with reverse indirects, but I get the general gist. Are they considered a fairly niche product or is this a mainstream solution to increase the thermal mass? In other words, am I likely to run into problems by going this route? What about the tank in tank solution, do they provide any significant buffering, or is there too little boiler water in them to make a difference?

I appreciate all the thoughts and advice. In the end I think I may have to live with some sort of compromise solution.

Cheers,
Paul