Navien NCB240e Questions

[jeff711981]

I apologize in advance for the length of this post, my first post here, but I moved into a new house about 3 months ago and had to replace the boiler immediately. The old Munchkin one had gas leaks in the valve train and apparently Munchkin was sued out of business so people won't work on them anymore.

Long story short, I ended up having the NCB 240e combi-boiler installed. My home is about 2250 sq/ft, parts of it date back to 1880 and the most recent addition is 40 years old, so insulation is not perfect - there are lots of drafts. The heating design is all fin-tube baseboard. There are three zones in the house. Family room (addition), first floor, and 2nd floor.

The plumbing for the fin-tube baseboard elements looks rather poor. There are some connections that look heavily corroded and last night I just had one spring a leak. I've got the company who installed my boiler coming out tomorrow to help bypass the section with the leak (it's a really janky looking series of fin-tubes under the kitchen floor... kind of a makeshift radiant floor heating system). Eventually I need to replace all this, but for now, I'm just having them bypass the leak so I can stop rotating buckets every 3 hours in order to keep the heat on.

But I have some questions about the system as I am new to hydronic heating systems.

First, I'm observing about a 10 degree F delta between supply and return. For example, right now I took a look at the unit and the supply temp is 127, and the return is 116. I have roughly 25-30 feet of fin-tube elements in a 20x20 room with a vaulted ceiling. With some research online, I estimate the heat load of the room to be about 12,000 BTU. Assuming my fin-tube elements are working as good as modern new ones, and they're roughly 300 BTU/ft at 120-130ish water temp, this means I'm roughly 4000 BTUs shy of what's required for a room this size. So by either adding more fin-tube elements, or swapping the existing for higher efficiency ones, I should see a larger drop in return temp compared to supply temp, right? This would explain why the system can't maintain the 70 degree temp I have set (it dropped from 70 to 66 when I set the space heating temp to 130 instead of 150).

Second, I have no outdoor temp sensor (I'm in the process of requesting that the installer provide it, since he even described its function to me when he sold me the boiler), but given that I need the supply water to be somewhere in between 130 and 150 to maintain my desired temperature, if I'm only getting a 10 degree temperature drop and my return water temp is above 130, the boiler is likely not condensing much, if at all, and so I'm going to be in the mid 80s in regard to efficiency, right?

Third, I've had the boiler in for 6 weeks and I've gone through 400 gallons of propane. For the first 3 weeks, the zones were mixed up and I had to play games with the thermostats to keep the different zones reasonable, but occasionally overnight things got out of whack and I woke up to temps in the high 70s on several occasions. So I wasted a lot of fuel due to that. Regardless... I'm really unhappy with the fuel consumption as it doesn't appear to have slowed down at all now that the zones are corrected, and I believe that's because the installer didn't install the outdoor temp sensor, and had set the supply temp to 180 during the initial install, and 170 after having come back to correct the zones. Regardless, this rate of consumption is over double what I expected... I expected to have to fill my 850 gallon propane take maybe twice per year. At this rate I'm going to have to fill it 2-3 times before this Michigan winter is over. Am I correct that this is excessive?

And finally (I think), going back to my first point/question, I have a (manual) variable speed circulator pump... I'm not an HVAC expert, but it seems to me that if I slow the pump down, the water is "out and about" longer and that should help achieve lower return temps - is my logic sound?

 

[Fitter30]

So you still have a leak? System all fin tube under the floors? What water pressure are youe running in your system?

 

[Sponsor]

 

[jeff711981]

So you still have a leak? System all fin tube under the floors? What water pressure are youe running in your system?

Yes, still have a leak - the company who installed the boiler said they'll send someone out today. The system is mostly fin-tube baseboards. There's this one section where a previous owner linked together a bunch of elements under the kitchen floor. The pressure varies - it's between 12 and 22psi for the most part.

Honestly, I'd like to do the work myself. I've worked with PEX before, and as a temporary solution for the season throwing 8 feet of PEX with push fittings in to get me through 3-4 more months of winter should work. What's keeping me from doing that myself is the lack of knowledge in regard to draining, then refilling and purging/bleeding the system. There is no air separator installed and there aren't valves to isolate zones - so I have to shut down and drain the entire system to fix this zone.

 

[Fitter30]

Take some pics of the boiler, pumps ,expansion tank and piping around boiler. Normal design for heating system is 20* - 30* max difference. With house that old you must have some drafty windows look at this cheap option .
https://www.amazon.com/Duck-10-Wind...=Insulating+Window+Film&qid=1609271144&sr=8-7

 

[jeff711981]

Here are some photos. The leak has been fixed/bypassed and the Outdoor Temp Sensor installed.

I'm still seeing only ~10 degree delta between supply and return temp (not that I expected these things to change that). This makes it really hard to keep the boiler in condensing mode because 130 degrees isn't enough to heat the house in order to get return water temps of <120. Basically right now with the Outdoor Temp Sensor installed, the boiler wants to run at about 150 degrees as the outdoor temp is 28F. This means water is coming back at 140 degrees and it's not condensing. Is this how it's expected to work? Basically below freezing the boiler can't be efficient and it's going to run at like 85% efficiency like a non-condensing boiler?

If not, what would I have to do to fix this situation? Reduce heat loss in the house? Install fin-tube elements capable of higher BTUs at lower supply temps? Based on the predefined curves, it seems a supply water temp of 150+ is normal for fin-tube baseboard heat... if I can't get a 30 degree delta, does that just mean I'm SOL in regard to efficient heating?

 

[Fitter30]

So theres only the one pump what is the brand and model. Couldn't find your supply water for heating loop is there a water regulator? Fin tube doesn't really start to heat till water temp is 120*. Your outdoor reset can be adjusted. Your 3 zone how are they controlled and the boiler. Is there a boiler/zone control? Didn't see a valve on the discharge of the pump.

 

[jeff711981]

So theres only the one pump what is the brand and model. Couldn't find your supply water for heating loop is there a water regulator? Fin tube doesn't really start to heat till water temp is 120*. Your outdoor reset can be adjusted. Your 3 zone how are they controlled and the boiler. Is there a boiler/zone control? Didn't see a valve on the discharge of the pump.

Yes, one pump: https://product-selection.grundfos....ca/ups-15-north-america/ups-15-58-fc-59896341

Not sure what you mean by supply water for heating loop... do you mean the make-up supply? If so, that's visible in the picture that ends in 3267 - it's the smaller of the two blue PEX lines that run into the bottom of the boiler and it does have a ball valve on it.

I'd be happy with 120 degree water heating my home... if return temps were <110 that would be great for efficiency, but 120 isn't enough to heat the "problem zone", which is a 20x20 room with vaulted ceiling and was built 40 years ago with a forced air system which failed and has been abandoned and retrofitted with the fin-tube.

My three zones are as follows:
Zone 1: First Level
Zone 2: Second Level
Zone 3: Family Room

Each zone has its own thermostat, and each zone has a corresponding control valve after the pump visible in the picture that ends in 6420. If a zone calls for heat, the boil fires and the control valve for that zone opens.

 

[Dana]

I have roughly 25-30 feet of fin-tube elements in a 20x20 room with a vaulted ceiling. With some research online, I estimate the heat load of the room to be about 12,000 BTU. Assuming my fin-tube elements are working as good as modern new ones, and they're roughly 300 BTU/ft at 120-130ish water temp...

Bad assumption.

Typical baseboard only puts out ~250 BTU/hr per foot at an average water temp (AWT) of 125F. With only 25-30' of fin tube it means the zone is putting out at most 8000 BTU/hr @ 125F AWT, while the NCB-240e is putting in about 17,000 BTU/hr, which means it will very likely be short cycling itself into low efficiency (despite condensing) and an early grave.

Measure the lengths of the burns to the nearest 5 seconds or so when only that zone is calling for heat.

How much baseboard is there on each of the other zones?

BTW: The hydraulic separator manifold always lowers the delta-T at the boiler, since it's mixing return water from the radiation directly with the boiler supply output. Measure the delta-T on the zone supply and return (at the manifolds, before & after the hydraulic separator) to get a better idea of what the radiation delta really is.

Note, the water being pumped through the boiler loop side of the hydraulic separator (aka the "primary loop" is being done by the NCB's internal pump. The external pump is what's driving the radiation flows (the "secondary loop"). The very purpose of that hydraulic separator is to minimize interactions between radiation flows and boiler flows, to guarantee that the boiler's minimum flow specifications are always met, but without over-pumping the water-tube heat exchanger (which would wear it out from erosion.)

 

[Fitter30]

Since the pump has three speeds try slowing the speed down to increase the temperature difference between supply and return.

 

[jeff711981]

Since the pump has three speeds try slowing the speed down to increase the temperature difference between supply and return.

I did that last night before going to bed and it's having no problems keeping the two zones at 70F.

The 3rd (problem zone) had been at 66 all day with the thermostat set at 70.

 

[jeff711981]

Bad assumption.

Typical baseboard only puts out ~250 BTU/hr per foot at an average water temp (AWT) of 125F. With only 25-30' of fin tube it means the zone is putting out at most 8000 BTU/hr @ 125F AWT, while the NCB-240e is putting in about 17,000 BTU/hr, which means it will very likely be short cycling itself into low efficiency (despite condensing) and an early grave.

Measure the lengths of the burns to the nearest 5 seconds or so when only that zone is calling for heat.

How much baseboard is there on each of the other zones?

BTW: The hydraulic separator manifold always lowers the delta-T at the boiler, since it's mixing return water from the radiation directly with the boiler supply output. Measure the delta-T on the zone supply and return (at the manifolds, before & after the hydraulic separator) to get a better idea of what the radiation delta really is.

Note, the water being pumped through the boiler loop side of the hydraulic separator (aka the "primary loop" is being done by the NCB's internal pump. The external pump is what's driving the radiation flows (the "secondary loop"). The very purpose of that hydraulic separator is to minimize interactions between radiation flows and boiler flows, to guarantee that the boiler's minimum flow specifications are always met, but without over-pumping the water-tube heat exchanger (which would wear it out from erosion.)

The boiler has been running constantly for 24 hours - it's unable to raise the temp in my problem zone to the set point of 70 - it's been 66 degrees for the past 24 hours. (I have the Outdoor Reset Curve enabled using a custom Heat Load - basically running at a max of 60% space heating capacity and max of 150 degree supply temp)

Zone 1 has roughly 35 feet of fin-tube. Zone 2 has roughly 30 feet. Those have no problem maintaining my set point of 70 degrees, specially Zone 2, which is upstairs (I can feel heat from Zone 1 rising into Zone 2, so Zone 2 doesn't run often).

I did slow the pump down to the minimum setting last night... seems adequate for Zone 1 and 2, but seems to have no effect on Zone 3.

Also, I'm not sure how I can measure the water temps of each zone's supply and return. There's no sensor in them, and my handheld IR sensor doesn't seem very accurate when I try to get it to tell me the temp of the pipes.

 

[Fitter30]

See your outdoor temp is 32*. You don't say what the water temp is now. Do you think the room stats are shutting the zone valves. What brand of stats and model number? Fin tube doesn't start to really heat till 140* water

 

[jeff711981]

See your outdoor temp is 32*. You don't say what the water temp is now. Do you think the room stats are shutting the zone valves. What brand of stats and model number? Fin tube doesn't start to really heat till 140* water

The thermostats are shutting down the zones when they should be (when room temp reaches the set point), but not before that. They're all different models (this house had lots of little additions over time).

When I went to bed last night the Outdoor temp was 28F, the Outdoor Reset Curve was calling for 126 degree water and the boiler was reporting about 122 degree supply temp and 110 degree return temp. I have the pump on the lowest flow setting.

Zone 1 and 2 were keeping up just fine like this all night, keeping the 70F set point.

Zone 3, the problem zone, is 62F this morning with a 70 degree set point. So this zone isn't able to keep up unless (based on my recent experience) I run the supply temp in the 150-160 range, which with the ~10 degree temperature delta, takes the boiler out of condensing mode. I'd really like to keep it in the condensing zone because I had my propane filled when we moved in roughly 2 months ago, and just yesterday I had it filled and it took 450+ gallons. The clothes dryer is the only other appliance running on propane.

I have a wood pellet stove I can fire in Zone 3 to help augment what the boiler & baseboards are doing, but it's somewhat loud and that's where the TV is... I don't really want to have to run that every day if I can help it, although I'm getting fuel for it at $3 per 40lb bag and a bag lasts about 3 days if I just run it in the evening when the zone is occupied. But I'd really like to figure out how to get the boiler to be able to fulfill the full load of the room. I'm thinking once this winter season is over and I shut the boiler down for the season, I might install some retrofit radiant floor heat... this sort of thing (I think the boiler has plenty of capacity, there just isn't enough fin-tube for the room):

 

[Dana]

The boiler has been running constantly for 24 hours - it's unable to raise the temp in my problem zone to the set point of 70 - it's been 66 degrees for the past 24 hours. (I have the Outdoor Reset Curve enabled using a custom Heat Load - basically running at a max of 60% space heating capacity and max of 150 degree supply temp)

"...running constantly..." doesn't mean the same thing as burning constantly- it's probably cycling the burner on/off, even though the pump to the radiant zone is running continuously.

Minimum burn times and the burns/hour are what matter- pumping times not so much.

Zone 1 has roughly 35 feet of fin-tube. Zone 2 has roughly 30 feet. Those have no problem maintaining my set point of 70 degrees, specially Zone 2, which is upstairs (I can feel heat from Zone 1 rising into Zone 2, so Zone 2 doesn't run often).

17,000 BTU/hr of minimum boiler output divided by 35' of fin tube is ~486 BTU/hr per foot, which takes boiler temps north of 160F to not cycle at all when just zone 1 calling for heat. Of course that takes it well out of the condensing temperature range.

For a single zone to be able to emit the full 17K minimum output of the NCB at condensing temps takes about 65-70' of fin-tube. Practically speaking as long as calls for heat from the different zones tend to overlap in time that could be reduced to 45-50' of fin tube per zone with very little short cycling.

To do it "right" a heat load calculation would be done on each zone, with the fin tube lengths adjusted to be proportional to the load. When the radiation is proportional to the load, after the ODR curve gets tweaked into the fine adjustment range the calls for heat from the zones will almost always overlap.

Using the Manual-J-ish freebie online load calculators such as LoadCalc or CoolCalc would be fine for getting the zone to zone proportional loads reasonably adjusted, even if the total load accuracy tends to be overstated by 25% or more with those tools.

I did slow the pump down to the minimum setting last night... seems adequate for Zone 1 and 2, but seems to have no effect on Zone 3.

It's far more likely that getting the heat into zone 3 is a too-cool water temperature issue, not a flow adequacy issue. If you look at the charts for fin-tube the difference in output between 1gpm and 4 gpm isn't very much.

Also, I'm not sure how I can measure the water temps of each zone's supply and return. There's no sensor in them, and my handheld IR sensor doesn't seem very accurate when I try to get it to tell me the temp of the pipes.

If the fin-tube zones are connected to the manifolds with copper/brass/bronze/galvanized pipe, take some paint (any non-metallic color) and paint a small section of zone supply/return pipes near the manifolds. In infra-red thermometer reading of the painted section will be reasonably accurate. PEX is sufficiently emissive to take IR measurements without painting the pipe (if any of it was done in PEX.)

Just randomly throwing some radiant floor under the Zone 3 room isn't the right approach. Doing at least the basic arithmetic (starting with the load calculations) would be far more likely to succeed. It's likely that you'd be better off spending the radiant floor money on an appropriately ductless mini-split heat pump would be a better investment, especially since your heating fuel is propane. In almost all US local markets, even those with expensive electricity a cold-climate mini-split is significantly cheaper to heat with than condensing propane, even when the propane boiler isn't short-cycling. There are many cold climate mini-splits with a specified output at -10F and colder. (Your 99% outside design temp in Fowler is still above 0F, so you'd have plenty of options.)

 

[jeff711981]

Yeah... I'm assuming its burning constantly because it can't hit the set point in the problem zone... why would it ever shut off the burner if a zone is still calling for heat? Every time I go look at it, the burner is on.

My intention with adjusting the flow was to keep the water out in the system longer, hopefully giving up more of its heat and creating a greater temperature delta so I can run the boiler at higher temps in order to reach the set point while keeping the return temps low enough to condense.

I don't intend to just throw random things around and hope it works - if that was the case there'd have been no reason for me to make this post. But I'm not sold on a mini-split being the way to go. First, it would destroy the aesthetics of the room - it's all reclaimed barn wood and brick. Second, I pay $1.39 per gallon for propane, and estimating 100 million BTUs per heating season that's roughly $1500... I pay $0.18 per kWh for electricity, which would require roughly 30,000 kWh to provide 100 million BTUs, for a cost of over $5000.

Anyway... condensing boilers are more efficient at low return temps. And since I don't think it's likely to achieve 40-50 degree deltas, the only option to keep return water temps low is to keep supply temps low. Since radiant floor heat is designed to run below 120 degrees, that seems ideal since the fin-tube baseboards only put off a couple hundred BTUs per foot at lower water temps. I've also been looking into some of the newer fin-tube designs that either have two rows of fin-tube per baseboard unit or larger fin designs to increase the BTU/ft. So I'm thinking of using the existing baseboard units, but sending the return water, which is around 110-120 degrees, through a radiant floor system. So I can probably send 130-140 supply water out, and get <110 degree water back. But yes, I'll have to do the math and make sure I don't install too much capacity and cause short cycling.

 

[Dana]

Yeah... I'm assuming its burning constantly because it can't hit the set point in the problem zone... why would it ever shut off the burner if a zone is still calling for heat? Every time I go look at it, the burner is on.

I can pretty much guarantee you that it's NOT burning constantly, even though the call for heat from that problems zone is constant. There is no way on earth that 30' of fin-tube is going to emit 17,000 BTU/hr at an AWT of 125F. It takes an AWT of over 170F to get there (even 180F+ for some fin-tube). The burner can't run any lower than 17,000 BTU/hr. When there's more heat going into the system than is coming out into the rooms the water temp rises, and at some temp above the boiler's setpoint it turns the burner off. (With most mod-con boilers and I believe the NCB included, the turn-off temp is programmable within some range of overshoot.)

The other two zones are being satisfied, when only the problem zone is calling for heat it must be cycling on/off. The important question is how often it's cycling, and how short are the burns.

My intention with adjusting the flow was to keep the water out in the system longer, hopefully giving up more of its heat and creating a greater temperature delta so I can run the boiler at higher temps in order to reach the set point while keeping the return temps low enough to condense.

Without doing the math you might be able to convince yourself that this approach would work, but it doesn't.

I don't intend to just throw random things around and hope it works - if that was the case there'd have been no reason for me to make this post. But I'm not sold on a mini-split being the way to go. First, it would destroy the aesthetics of the room - it's all reclaimed barn wood and brick. Second, I pay $1.39 per gallon for propane, and estimating 100 million BTUs per heating season that's roughly $1500... I pay $0.18 per kWh for electricity, which would require roughly 30,000 kWh to provide 100 million BTUs, for a cost of over $5000.

A better-class 3/4 ton ductless mini-split delivers about 13,000 BTU/kwh (= HSPF rating of 13.) Some do even better. That's a seasonal average- they do worse than than at temps below +10F, but much better than that at temps above freezing. At 95% efficiency would give you 87,000 BTU/gallon into the system (there rest goes up the flue), and has a not insignificant use of electricity too (much of which depends on your pump efficiency.) To deliver the same 87,000 BTU/hr with a mini-split takes 87,000/13,000= 6.7 kwh, which at 18 cents costs $1.20.

There are floor-mounted and ducted cold climate mini-splits too, if the high-wall blob look is offensive.

Since radiant floor heat is designed to run below 120 degrees, that seems ideal since the fin-tube baseboards only put off a couple hundred BTUs per foot at lower water temps.

Radiant floors are only designed to run below 120F if you actually DESIGN it to deliver the required amount of heat at 120F. My radiant floor zones are running 125F water (and don't always keep up). Most under the sub-floor systems like the one in your picture will need 140-160F water to keep up at the 99% outside design temp, and that's only if it's a pretty tight well insulated room with reasonably low loads. Many older homes would need 180F water to really cut it at sub-zero temps.

I've also been looking into some of the newer fin-tube designs that either have two rows of fin-tube per baseboard unit or larger fin designs to increase the BTU/ft. So I'm thinking of using the existing baseboard units, but sending the return water, which is around 110-120 degrees, through a radiant floor system. So I can probably send 130-140 supply water out, and get <110 degree water back.

Sure there are better fin-tube solutions and there are flat panel radiator solutions too. But yes, you absolutely DO have to do the math.

But yes, I'll have to do the math and make sure I don't install too much capacity and cause short cycling.

Huh? I think you've stood this on it's head. Short cycling happens when the boiler's minimum firing rate is still putting more heat into the radiation than the radiation can emit.

You already installed too much BOILER capacity, which is the source of short cycling. The more RADIATION capacity that gets installed the less potential there is for short cycling.

 

[jeff711981]

Don't mean to be combative, because I appreciate help and information - that's what I'm here for. But for as many assumptions as I'm making, you seem to be making quite a few yourself. My statements and assumptions are based on observations about what is actually going on in my home. And for as far as I can get without actually doing the math, only using the math without taking into consideration the actual behavior will only take you so far as well since the math would be based on assumption at this point as well given the age of the house and the way additions and renovations have been made over time by what appears to have been all DIY efforts. Coming to a more precise heat load calculation for the home or a particular zone wouldn't be possible without opening up walls to look at the construction methods and materials, using an IR camera to detect leaks, etc.

For as confident you are that my system is short cycling, I'm confident that it's not. I climb down into the basement multiple times a day, and even sit and observe it for 10-15 minutes at a time. In fact last night I was down there for over an hour working on my water softener. I have never ever observed the boiler circulating, but not firing when zones are calling for heat - the only time it turned off was when my wife ran some hot water for a few seconds and I could clearly hear it switch from space heating to DHW heating and back again - it's very obvious when the burner is off and it stops circulating fresh air and exhaust. Additionally, the rest of the time I'm in my home office which has a window right next to the boiler exhaust, so I can clearly see the (now much smaller since it's condensing) plumes of white exhaust every time I turn around in my chair.

Maybe heat is exiting the system somewhere other than the actual fin-tube elements. And maybe I'm using some incorrect terminology because I'm not an expert. But it's clear to me that regardless of boiler size, if I install too much radiation in a zone and the set point can be achieved after only 30 minutes of runtime before the call for heat ends, that's not good for the boiler either - maybe it's not called "short cycling" but the effect on the boiler is similar. Arguing semantics and terminology isn't what I'm here for...

 

[Fitter30]

Fin tube doesn't really start heating till 140* water heres a chart. Systems that have both radiate and fin tube have to piped differently with two separate loops. Boiler runs at higher temp for the fin tube and a lower temp the radiate. Max temp for a hardwood floor is 85* if your have any.
Radiate loop uses a separate pump and either a three or four way valve to run a lower temp. Do know how many feet are in the radiate loops and how many.
https://www.google.com/url?q=https:...FjAEegQIBhAB&usg=AOvVaw3JCJJXHviIZcDVWnMawsWN

 

[jeff711981]

Fin tube doesn't really start heating till 140* water heres a chart. Systems that have both radiate and fin tube have to piped differently with two separate loops. Boiler runs at higher temp for the fin tube and a lower temp the radiate. Max temp for a hardwood floor is 85* if your have any.
Radiate loop uses a separate pump and either a three or four way valve to run a lower temp. Do know how many feet are in the radiate loops and how many.
https://www.google.com/url?q=https:...FjAEegQIBhAB&usg=AOvVaw3JCJJXHviIZcDVWnMawsWN

The ~125 degree water is heating my first two zones and maintaining 70 degree set point without much drama. So while I've read those charts and understand them, it's clearly not that they're "off" below 140 and "on" above 140. Again, it's the 3rd zone which is a problem.

The fin-tube and radiant floor systems only need to be on separate loops if they run at different temperatures, right? So if I plumb in the fin-tube first, the return from the fin-tube might be at the proper supply temp for radiant floor... so I wouldn't need two separate loops... that's basically what I'm working toward.

I don't know the exact length of the loops yet... that involves a lot of wiggling through crawl spaces and tracing pipes between crawl space and basement which I'm just not prepared to do in this weather and when I can't shut the system down for days anyway to make improvements.

Basically, what I'm trying to do here is keep the boiler in condensing mode while heating the house efficiently. In order to do that, return temps have to be below 130 degrees F - the lower the better (within reason). I can't do that with a supply temp of 160 because it comes back at 150 the way things are set up right now, which is definitely too high to condense. So what I want to do is increase the delta for Zone 3 - but even if I do that and achieve a 30 degree delta, that's still just on the edge of condensing with a 160 degree supply temp. So I want to be able to run a lower boiler supply temp AND increase the delta. The only thing I can come up with is add more radiation in Zone 3. I have no more wall space to add baseboard fin-tube... so I can either replace fin-tubes with some with higher BTU/ft rating, or supplement existing fin-tube with something else. In this case, since I'm trying to run low boiler supply temps due to the delta being only 10 degrees or so (for every zone, not just Zone 3) to keep it in condensing mode, I figure why not supplement with a heating system that's designed to work at those lower temps?

Seems like a sound high level idea? Or no? I get that the math is important, and I'll definitely do the math to figure out exactly what and how much to add in terms of replacing fin-tubes with higher BTU/ft models or supplementing with radiant floor heating. But I don't want to get lost in the details if the overall idea isn't sound.

If it won't work, what am I missing?

 

[Fitter30]

All you do is try it the main problem does the pump produce enough head to over come the added length of piping. Could install three globe valves in the three zone and adjust them for the 30*. But without knowing what your loads are and what your heat emitters can put out its a guessing game. One pump three zones not the greatest set up. Three zone pumps and get rid of the single pump. Already have the primary/ secondary loop.