Designing a new hydronic system for my home, cast iron radiators and radiant floor?

[carriagehousereno]

So i'm about to disconnect my old Weil-McLain boiler and hook up a new Navien NCB 240 combo boiler which we've been using for DHW since January.
I know, I've already been told elsewhere on here that the NCB 240 is overkill for the size of the house and may likely not work efficiently because of this. Unfortunately I was consulting a local plumber when starting the project and purchasing the boiler instead of consulting this community, He recommended it based on our DHW needs, the boiler already has been purchased, installed and in use for the DHW since January.

I was interested in the possibility of using radiant floor heat on the first floor since I have access to the underside of the subfloor and keep the cast iron radiators and cast iron baseboards on the second floor. My main reason for wanting to do away with the cast iron on the first floor is to free up the space that they are taking up. I'm not opposed to reusing them, but think I would prefer not to. I know they don't take up that much space, but the rooms could be configured much differently if they weren't a factor. I guess it all depends on how easy it would be to utilize both radiant and cast iron with this condensing boiler that we have.

The home is a little over 1700 square feet. The first floor includes a small 450sf one bedroom apartment. I was hoping to have 1 radiant zone for the apartment, 1 radiant zone for the remainder of the first floor and 1 zone using cast iron radiators for the 3 bedrooms and bathroom on the second floor.

The first floor is 3/4" pine tongue and groove subfloor with 1/4 oak hardwood face nailed on top of it.
The crawlspace under the subfloor is approximately 4 feet high with 90% of the floor being concrete and the other 10% under the boiler dirt floor.(most likely will be pouring concrete there as well as soon as the boiler is removed). The foundation around the crawlspace is uninsulated stone and brick.

I haven't done the load calculation for each room/zone yet, but I'm planning to use one of the online calculators to do this early this week.

Any advice would be appreciated.
Thanks in advance.

 

[WorthFlorida]

Just a note that the temperature for floor radiant heat is critical. Too hot and you can ruin your flooring and the feet can get a little toasty. Having to run lower temperature throught the floor radiant heating system will take longer time to heat up the space. Plenty of information online as what is the best temperature.

When costing out your project, in addition to insulation under the floor, also consider installing Reflectix radiant barrier between the floor joist. https://www.homedepot.com/b/Building-Materials-Insulation/Reflectix/N-5yc1vZasbsZ19n

 

[Sponsor]

 

[Dana]

With the thermal mass of the water in the cast iron rads you have at least a shot at running the NCB 240E at condensing temperature without short-cycling it into low efficiency, but with low-mass radiant systems that may not be possible. The minimum fire output of that boiler is about 17,000 BTU/hr, and you would need enough radiation on each ZONE to be able to emit that much at condensing water temperatures, or sufficient thermal mass in the system to guarantee sufficiently long burn times at low temp to keep it from short cycling.

For the 450' apartment zone, forget about radiant as the sole heat emitter. There is probably less than 400 square feet of available floor to use that isn't covered with cabinets and appliances, and even at 25 BTU/hr per square foot (which would take high temp water for an under-floor solution even when using aluminum heat spreader) that only adds up to about 10,000 BTU/hr, not 17,000. If there are radiators in that space they might have similar limitations on emitting the heat, but in combination with under floor heating there may be sufficient thermal mass that it won't short cycle. So start by estimating how many square feet of equivalent direct radiation (EDR) to you have in the radiators on that zone?

If you have pictures of the radiators to share, that might help in figuring out the water volume and iron thermal mass you have to work with too. Maybe you can find similar radiators in size and style with published weights and water volume per section. Both the iron weight and water volume count, but the water volume is usually the lion's share of the thermal mass. (Water has about 9x the specific heat of cast iron- a gallon of water is "worth" about 76lbs of cast iron, from a thermal mass perspective.)

Add up the EDR for the rads on the other first floor zone, and estimate how much available floor space is available for radiant floor. Unless it's a very well insulated house with better than code windows you may not be able to get there with under-the floor radiant on it's own anyway, but we can try to figure that out.

Add up the EDR for the second floor (treating it as a separate zone, for now) as well.

To emit the full min-fire 17,000 BTU/hr at condensing temperatures takes something on the order of 350' EDR of radiator, which you may not even have for the whole house. But if there are 25 gallons of water (= ~200 lbs) and 100' EDR in a zone it will be emitting about 5000 BTU/hr with 12,000 BTU/hr (=200 BTU/minute) of excess heat going into the zone that isn't being emitted, so the water temperature will rise, causing the burner to kick off when it's sufficiently above the boiler's setpoint. But with 200lbs of water mass a 10F rise would take 200 lbs x 10F = 2000 BTU total. So that zone would have a minimum burn time of about 10 minutes at condensing temps, which is just fine. It would be able to run 3-4 burns/hour, which isn't going to wear the boiler out prematurely, and the losses from flue purges and ignition cycles become less than 1% of the total heat released in the burn.

But if's 20lbs of water in a radiant zone (~250' of half-inch PEX) that emiting the same 5000 BTU/hr it becomes a 1 minute burn and a short-cycling boiler-destroying disaster. So it's very important to get a handle on the rough thermal masses and radiation we're working with when the boiler is oversized for the zone radiation.

Operating as a single zone with radiant floors in some rooms, radiators others can be done, but you have to do the math on the total radiation and total heat loss in each room to be able to get the room to room temperature balance right. Doing a room-by-room I=B=R type load calculation is good enough to get the balance right.

As a sanity check on your IBR load calculations, run a whole house load calculation based on wintertime fuel use. If the existing rads have been keeping the place warm enough, that, combined with the total EDR for the whole house would allow a good estimation of the peak and average water temperature requirements, which may affect which way you go on the radiant. Odds are pretty likely that the Weil-McLain is 3x oversized for your actual 99% outside design temperature load and that you could heat the place with boiler output less than 140F, but maybe not. These are important numbers to have to start narrowing in on what makes sense, and what really doesn't.

It's almost impossible to toast your feet with an under-floor radiant floor solution, and truly impossible to ruin the flooring if running it all at condensing temperatures (=less than 130F out of the the boiler, less than 125F returning to the boiler) except when it's sub-zero F outside. Even 180F water for half a day at a time during a cold snap isn't going to do anything to the finish flooring when the tubing is under the subfloor.

Reflectix and other radiant barriers are next to useless low-temp radiant floors, other than for somewhat isolating zones from one another. Radiant barriers become more useful at fairly high temperature differences. If the low-temp radiant is aluminum heat spreaders (either sheet metal or extruded) both the emissivity of aluminum and the low temperature make the radiant barrier solutions FAR less effective than low-density fiberglass batts. Adding radiant barrier to batts doesn't add much marginal improvement in this situation either- the temperatures, emissivities, and temperature differences are just too low to make it worthwhile.

 

[carriagehousereno]

Thanks for all the good info, its hopeful to here that we may be able to set up a system that won't short cycle even though our boiler is oversized for the application. I'll be spending some time this week working on the calculations, the links you offered look like they will be really helpful.

I have a mix of cast iron baseboard, column and tube type radiators, so I'll be evaluating each of those for their thermal mass.

Again thanks, I'm sure I'll be back to this thread for more feedback once I have the calculations complete.

 

[carriagehousereno]

@Dana
So I've begun to work through your list,

so start by estimating how many square feet of equivalent direct radiation (EDR) to you have in the radiators on that zone?

Total EDR for apartment zone is, 151.9sf with a total of 221sq feet available for underfloor radiant. It would be 253 if I include the bathroom, but that would have subfloor and tile floor for the heat to radiate through.

Add up the EDR for the rads on the other first-floor zone, and estimate how much available floor space is available for radiant floor.

Total EDR for first-floor zone is 69.4sf of EDR with 258sf or Let available for radiant.

Add up the EDR for the second floor (treating it as a separate zone, for now) as well.

Total EDR for the second-floor zone is 185.2sf.

If you have pictures of the radiators to share, that might help in figuring out the water volume and iron thermal mass you have to work with too. Maybe you can find similar radiators in size and style with published weights and water volume per section.

Here are the pics and dimensions
see if I'm coming to the right numbers

1st - Apartment Kitchen - 63sf, 535.5cf, one 6' x 8.5' north facing exterior wall with one 12sf window.
Fero 2.5" center Tube style, 25" high, 32" long, 5"wide, 21 sections, 3 tube = 48.9sf of EDR ?lbs, ?gallons.

The second is similar to the Burnham Classic. I have four of these, the dimensions are:

1st - Apartment living Room (pictured below) 130sf, 1105cf, 0ne north-facing exterior wall 13' x 8.5' with one 12sf window and one 16.25sf door, one west facing exterior wall 10'x 8.5' with one 18sf window:
Column Style, 38" high, 40" long, 9" wide, 16 Sections, 3 Column = 80 sf of EDR, 403 lbs, 9.75 gallons total.

2nd - Master Bedroom - 172sf, 1376cf, one exterior south-facing wall 13' x 8' with one 11.25sf window, one east facing exterior wall 15' x 8' with two 11.25sf windows
Column Style, 38" high, 33" long, 9" wide, 13 Sections, 3 Column = 65 sf of EDR, 332 lbs, 7.93 gallons total.

3rd - Bedroom #1 - 157sf, 1256cf, one south facing exterior wall 16' x 8' with one 11.25sf window and one west facing exterior wall 9'x 8' with one 11.25sf window.
Column Style, 38" high, 2" long, 9" wide, 12 Sections, 3 Column = 80 sf of EDR, 306lbs, 7.32 gallons total.

4th - Bedroom #3 - 157sf, 1256cf, one south facing exterior wall 16' x 8' with one 11.25sf window and one west facing exterior wall 9'x 8' with one 11.25sf window.
Column Style, 38" high, 27" long, 9" wide, 10 Sections, 3 Column = 80 sf of EDR, 258lbs, 6.1 gallons total.

This one is odd, doesn't look like any of the pics
Best Guess is

Apartment Bedroom - 108sf, 918cf, one west facing exterior wall 9'x 8.5' with one 18sf window, one south facing exterior wall 12'x 8.5' with one 18sf window.
Tube Style, 26" high, 30" long, 10" wide, 12 Sections, 4 tubes = 33 sf of EDR, ?lbs, 1.44? gallons total.
This didn't really match any of the dimensions on the document at :

find similar radiators in size and style with published weights and water volume

I've got two of these:

1st - Second Floor Bathroom 76sf, 608cf, one north facing exterior wall 13' x 8' with one 11.25sf window.
Baseboard Style, 10" high, 36" long, 2.5" wide, = 10.2 sf of EDR, 45lbs, 0.9 gallons total.

2nd - Main Living Space(open floor plan) - 361sf, 3429.5cf, one south facing exterior wall 19'x 9.5' with 4 - 6sf windows, One east-facing exterior wall 19' x 9.5' with 3 - 6sf windows and one 12sf window, One North Facing exterior wall 19' x 9.5' with one 18sf window and one 16.25sf door.
Baseboard Style, 310" high, 72" long, 2.5" wide, = 20.4 sf of EDR, 90lbs, 1.8 gallons total.

Finally
Main Living Space
Tube Style, 26" high, 38" long, 8" wide, 14 Sections, 5 Tube = 49 sf of EDR, ?lbs, 9.75 gallons total.

For all I know these aren't even appropriately sized for their spaces, the largest radiator in the whole place is in the apartment living room which isn't even close to the largest area to heat.

I'm open to the idea of switching them around or even getting different ones if needed.
let me know; I'll include some of the sq feet/cubic feet calculations for each room as well.

 

[Dana]

At"reasonable" condensing temperatures the radiators emit ~50 BTU/hr per square foot EDR. So...

Apartment: with 152' total when just the apartment zone is calling for heat it would only be emitting 7600 BTU/hr, which is less than half the minimum fire output of the NCB240.

First Floor: The ~70' EDR of rads would emit ~3500 BTU/hr, and assuming 5-10 BTU per square foot of the ~260' of radiant floor you'd get at most another 2600 BTU/hr out of it, call it 6000 BTU/hr total, about 1/3 of the minimum fire output.

Second floor: The 185' EDR of the second floor zone would emit about 9250 BTU/hr.

So if you add up all three you're looking at a total of about 23,000 BTU/hr at condensing temperatures, which means with all three zones calling for heat at the same time it could indeed operate in condensing mode without short cycling.

Without doing the math, the apartment and second floor almost certainly have sufficient thermal mass to suppress short cycling even when only one of those zones is calling for heat. The zone of highest risk is the First Floor, since it has the least amount of emitter AND the least amount of thermal mass. So, what are the radiators in the first floor zone? (It's a little hard to figure out from your post, since it's not clearly broken down zone by zone.)

 

[carriagehousereno]

So, what are the radiators in the first floor zone? (It's a little hard to figure out from your post, since it's not clearly broken down zone by zone.)

The first floor zone in the main house will have 21’ of cast iron baseboard radiators with a 71.4sf of EDR and one six tube 20” high slender radiator at 72sf of EDR.

The baseboard rads will have an added thermal mass of 315lbs of iron and 52.5lbs of water. The slender rad will have a thermal mass of 34 lbs of water and I can’t find a weight on it but I imagine it will be in the ball park of the baseboards since they have similar EDR.

With those specifics, what’s you guess as to suppressing short cycling if the first floor zone is only calling for heat?

Additional question: do you have a recommendation on what circulation pump, thermostats, zone valves, and valve control to use? I was looking at Taco products at supplyhouse.com, but really have no idea where to start.
Thanks again for your advice.

 

[Dana]

The 315lbs of cast iron is roughly the equivalent of 35 lbs of water, so with water content you're at about 90lbs total on that part, if including some of the distribution plumbing water mass.

The ratio of cast iron weight to EDR is much higher for baseboards than column or tube rads, so let's call the tube rad 15lbs water equivalent for the cast iron, 34lbs of water for about 50 lbs.

Adding the thermal mass of the water in the boiler and the rest of the distribution it looks to be about 150lbs total thermal mass, and about 150' EDR when you add what's radiating off the plumbing to the radiators.

The 150' EDR emits about 7500 BTU/hr at condensing temps which means the NCB240 is dumping excess heat at a bit more than 9,000 BTU/hr (= 150BTU/minute) into 150lbs of water-equivalent thermal mass. That means the temperature will rise at about 150BTU/150lbs = 1F per minute. So if the boiler's high/low control range around the outdoor reset set point is 10F, the minimum burn time will be on the order of 10 minutes, which is GREAT. If the range is only 5F it'll have only about a 5 minute minimum burn time, which is still fine, and you'll still be fewer than 10 burns per hour even in a worst-case. (Boiler & efficiency killing short cycling would mean burn times under 3 minutes, with a dozen or more burns per hour.)

With multi-zoned residential systems a smart ECM drive pump operating on pressure feedback and zone valves usually works well, but "hydronic design by web forum" takes a lot more time than most competent designers would give away for free. There are some designers who are willing and able to spec every valve and pump in a system via internet for a fee. (BadgerboilerMN used to do that, but I think he was talking about retiring last year.)