Review my Combi-Boiler design for my 1923 house?

squatch

Hi all,

I'm planning on renovating the unfinished basement of our 1923-era Minneapolis home this summer (~2000 sf if the finished basement is included). The house is currently heated by cast-iron radiators fed by a ~20-year-old Slant/Fin boiler. We've been in the house about 3 years and haven't had any issues with it.

As part of the basement renovation, I'll have to relocate the boiler and WH, and will also
have to move all the boiler piping as it hangs down too far from the ceiling. I figure as longs as I'm doing that, I might as well replace the system with a Combi-Boiler to save a bunch of space.

I'm a structural engineer by practice with zero experience in the hydronics world. But I'm a fast learner once I sink my teeth into something. I've been devouring everything I can read/watch/listen to over the past couple weeks --- I find the Coffee with Calieffi webinars to be supurb.

So I set about designing the new system for a Combi-Boiler, did the whole Manual J calculation and the whole nine yards. I think I'm settling in on a Navien NCB-240/110H (AHRI 89 kBtu/hr).

I put together the attached hydronics system layout over the past week and am humbly requesting some feedback on my design. I'm smart enough to know that it's probably pretty good, but I'm humble enough to know that this is not my area of expertise, and that there are certain to be some things that I've overlooked.

Questions

1. Overall thoughts // comments // questions // concerns // suggestions?
   
2. I've set it up as a true primary/secondary layout, with one secondary circuit for the hot radiators and another secondary circuit for the cooler under-floor radiant heating. Is this overkill?
   Navien has a design example (also attached) in which they put two temperatures on one secondary circuit, limiting the primary to a small loop right at the boiler.
   
3. Is my placement of the expansion tank okay? All of Navien's example layouts have the tank in the secondary loop (I think mostly because their primary is limited to a small manifold connected to the boiler).
   
4. The Navien has a direct make-up water port, but I've seen examples where people have skipped that and put in their own feed circuit. Any reason not to just use the Navien port directly?
   

Thanks in advance, I really appreciate you taking the time to read and comment!

hot_rod

A hydraulic separator is another excellent option.
It provides high efficiency air, dirt, magnetic function in addition to primary secondary. Cleans up the piping a bit also.
Delta P ECM circulators for P1 & P2. Thermostatic or motorized mix valve for radiant

Ironman

A couple of recommendations:
1. Skip the combi and go with a boiler + an indirect water heater. The reasons are numerous but mainly because it’s gonna be way over-sized for space heating and short cycle itself into any early grave.
2. Use a smart mixing valve or variable speed injection mixing on the radiant floor - especially if it’s a slab.
3. It doesn’t matter if the expansion tank is on the primary or secondary as long as the circulators are pumping away from it. In a p/s setup, the loop that has the expansion tank is considered the primary, and the secondary loop “sees” the entire primary as the point of no pressure change.

hot_rod

That Navien, regardless if it is a combi or just boiler can be de rated down to 13,000 btu/ hr for the heating mode. What did your manual J show for heat load?

Also calculate the DHW production based on the coldest temperature the water is in winter. 70- 77 temperature lift is more realistic for your area, so DHW may be more like 3-4 gpm with that size combi

If you want more DHW capacity, maybe go up 1 size, to the 130,000 the minimum turndown is still 13,000

Hot_water_fan

A combi would work well for this application with the space savings and turn down ratio if you’re using showers. If you’re expecting to use a higher flow fixture, like a tub, a combi won’t keep up. You’ll have the occasional cold water “sandwich” if a shower is on simultaneously with a hot washer cycle or something similar. 

squatch

Thanks for the thoughtful comments so far. I do appreciate it.

hot_rod said:

A hydraulic separator is another excellent option.
It provides high efficiency air, dirt, magnetic function in addition to primary secondary. Cleans up the piping a bit also.
Delta P ECM circulators for P1 & P2. Thermostatic or motorized mix valve for radiant

I'd been considering this as a potential option as well. Will certainly keep it in mind. Seems like most of the diagrams I'm seeing for residential applications (including the sketch you sent) don't use the closely-spaced tee design for multiple secondary loops for different temperatures. So I'm gathering that my design is a bit overkill in that regard.

Ironman said:

A couple of recommendations:
1. Skip the combi and go with a boiler + an indirect water heater. The reasons are numerous but mainly because it’s gonna be way over-sized for space heating and short cycle itself into any early grave.
2. Use a smart mixing valve or variable speed injection mixing on the radiant floor - especially if it’s a slab.
3. It doesn’t matter if the expansion tank is on the primary or secondary as long as the circulators are pumping away from it. In a p/s setup, the loop that has the expansion tank is considered the primary, and the secondary loop “sees” the entire primary as the point of no pressure change.

I'd not put much consideration into an indirect tank, but will definitely spend some time looking into it now. Seems like about the smallest tank available is about 30gal, which provide an incredible 200gal first hour delivery. With such high first-hour deliver rates, I wonder why there aren't smaller tanks on the market (20 or even 10gal)?

hot_rod said:

That Navien, regardless if it is a combi or just boiler can be de rated down to 13,000 btu/ hr for the heating mode. What did your manual J show for heat load?

Also calculate the DHW production based on the coldest temperature the water is in winter. 70- 77 temperature lift is more realistic for your area, so DHW may be more like 3-4 gpm with that size combi

If you want more DHW capacity, maybe go up 1 size, to the 130,000 the minimum turndown is still 13,000

This is my first ever heat loss calculation, but I did my best. I figure a heat loss of around 72,000 Btu/hr from my Manual J calcs. We're considering a future addition to our home, which might add another 15,000 Btu/hr demand.

Our current boiler has a maximum output of about 66,000 Btu/hr. It doesn't quite keep up on the coldest day of the winter, and I'll be adding two under-floor radiant heating loops to whatever new setup I end up going with. So I think the Navien's 89,000 Btu/hr is about right (the next lower size only outputs 64,000 Btu/hr).

I plan to use an outdoor reset and make full use of the 10:1 turndown ratio to keep efficiency up.

Hot_water_fan said:

A combi would work well for this application with the space savings and turn down ratio if you’re using showers. If you’re expecting to use a higher flow fixture, like a tub, a combi won’t keep up. You’ll have the occasional cold water “sandwich” if a shower is on simultaneously with a hot washer cycle or something similar. 

Good to know re: keeping up with a tub. Our kids are young and still take baths, and my wife likes to take a bath almost every day. So I suppose that's an upvote for going with an indirect heater.

hot_rod

Here are the DHW outputs for a 130K.

IF you water is as low as 35F in winter and 105 is adequate to fill a tub or take a shower 4.5- 5 gpm is reasonable. 10 minutes to fill a 50 gallon tube. Or a very large, carwash type shower flowing 4 gpm continuously

5 gpm is plenty to run two loads at the same time, dishwashing, shower, washer, etc.

What size is the current boiler? What temperature does it run?
Does it run non stop on the coldest days? If it does cycle off, it may be inadequate heat emitters?

squatch

hot_rod said:

Here are the DHW outputs for a 130K.

IF you water is as low as 35F in winter and 105 is adequate to fill a tub or take a shower 4.5- 5 gpm is reasonable. 10 minutes to fill a 50 gallon tube. Or a very large, carwash type shower flowing 4 gpm continuously

5 gpm is plenty to run two loads at the same time, dishwashing, shower, washer, etc.

What size is the current boiler? What temperature does it run?
Does it run non stop on the coldest days? If it does cycle off, it may be inadequate heat emitters?

Current boiler is 90,000 Btu/hr input (66,000 output). On the coldest days it runs continuously (or nearly so), but obviously cycles on all other days.

The aquastat is set to ~170, but the temperature gage on the unit only reads about 145 when it's running at full blast. This is confirmed with an IR temp gun. Not sure what the deal with that is... maybe the aquastat is zeroed out wrong...? Or maybe my current boiler is smarter than I think it is and can do some sort of temperature modulating based on input/output temperatures? Not sure.

Heat emitters should be plenty sufficient. They're big old cast-iron behemoths, which I reckon have been around since the house was built 100 years ago. They're reasonably well balanced to each other size-wise, but are more than large enough to heat the house.

hot_rod

You’ve stumbled on thermal equilibrium. The heat emitters will dictate the operating condition of the boiler! Always😗

If the emitters can transfer more btus than  the boiler can provide, the boiler will seek and find that temperature operating point

The operating control on the boiler is powerless to change this condition

More about thermal equilibrium here

https://www.caleffi.com/sites/default/files/file/idronics_32_na.pdf

squatch

This old dog sure has been learning a lot of new tricks in the past few weeks --- in large part to you guys at Caleffi.

Hat's off to you and your colleagues.

squatch

Another question ---

After all the copper pipes and manifolds in the vicinity of the boiler, I gather there's no issue running Pex-Al-Pex to the cast-iron radiators, and Oxygen Barrier Pex for the under-floor radiant?

Currently it's all big old black-iron piping in the basement. Plan would be to remove whatever I can and replace it with PAP, and only pig-tail into the existing pipes where they turn vertical on their final runs to the radiators.