Engineer (Non HVAC) looking for some straight technical talk (MOD-CON)

jav

Great forum... I've been reading and learning a lot on here and will no longer just accept sales fluff. As an engineer, I'd like to "understand" what most don't tell you up front so here goes. I'm considering a new mod con boiler.

I need to replace a relatively new system (less than 7 years old) because it's a maintenance and reliability nightmare. Current system is a 140KBTU 80% eff, gas, direct vent cast iron boiler (non modulating) with separate indirect hot water tank on one of the 4 zones. The symptom is the boiler eats hot surface igniters and control boards but, the real problem is (I believe) short cycling and perhaps even piping /pump layout (single circulator- 4 zone valves) . The property is an old 3 family house (3-1200 sq ft units) in Massachusetts- minimal insulation with the indirect tank serving all 3 floors. The boiler was sized due to the max demand with water tank. When the tank is calling plus just even 1 other zone, the boiler doesn't short cycle. However, when just one or even 2 heating zones are calling but no tank demand, it short cycles badly.

I've decided I don't want to band-aid it by adding thermal mass (storage tank) and was "initially" very excited about the MOD CON 10:1 & 15:1 TDR and began reading here only to temper my expectations. Would someone be so kind as to provide a rough primer on the key system gotchas on specing a good, reliable and efficient system with a true service life expectancy of 20+ years? I understand the difficulty in supplying such a large min-max firing range of (20Kbtu min -120Kbtu max-Net) and I'm not looking at this solely from a cost stand-point but, that is a consideration as well.

My supply house moves a lot of Navien gear and they suggested the NHB-150 but- I was leaning towards the HTP UTF-140 because of it's supposed medium mass heat exchanger, they're a local company... I already own several of their superstor tanks (which I hope to keep- so no combi boiler is needed) AND- my reading suggests the Navien requires more ancillary gear on install to do it "right".

So. 1) Are MOD CONS the way to go given the min-max firing range? 2) What are the must have sub systems (water filtration/treatment, Variable flow circulators, additional mass storage) etc. to do it right 3) Should I be considering other types of systems for reliability/longevity reasons? Since this system services a 3 family building as the sole heat and hot water source, boiler problems in the dead of winter are a big deal... should spare parts be kept on site such as igniters, control boards or other difficult to source parts?

Thanks

hot_rod

4 zones of baseboard or low temperature emitters?

jav

2 zones base board, 1 zone hydro-air handler, 1 zone indirect hot water tank. Smallest zone is 56 feet of standard baseboard.

Ironman

Zoning up a mod/con can be problematic. If it's causing short cycling with a cast iron boiler, it will only be worse with a mod/con. Modulation in a mod/con is NOT there to compensate for over-sizing or zoning. It's there to match the firing rate of the boiler (as nearly as possible) to the actual load based on outdoor temp. This, coupled with ODR, produces the most efficiency. The rationale is to run the burner for as long as possible, with as low of a water temp as possible, on as low a fire as possible while maintaining the temp of the structure.

You really need to do a heat loss calculation to determine what size boiler is actually needed. SlantFin has a free app that's easy to use.

Unless you have an unusually high domestic load, that is not added to the sizing of the boiler. Put in a larger or multiple indirects, but don't up size the boiler. Or, add a second boiler whose primary load would be domestic.

If you must keep the zoning, you'll probably need a buffer tank. An electric water heater with the elements removed is an inexpensive alternative to more costly buffer tanks.

I'd recommend a properly sized UFT. If your domestic load is unusually high, you could twin 2 UFT 080's together and still have an 8k minimum firing rate because you'd have a 20 to 1 turndown.

hot_rod

If you do not have enough, or the correct heat emitters to work at temperatures below 140- 150F, then a mod con maybe is not the best choice.

The load calc is always the key to answer that question.

Cast iron boilers by default give you some buffering in the water content and mass of the iron, it may still be a best choice properly controlled with odr.

Buffer tanks are not necessarily a band aid, just a tool to help with multi/ micro zoning in wide temperature spread conditions.

Viessmann and HTP have high mass offerings to best deal with zoning, if condensing is doable.

Your situation is not unusual, there is no perfect answer or product until we get 0- full output modulation

jav

Thanks gents. I have run heat loss (Slant-fin App & others) but I struggle to trust them because each one produces different results and my config is awkward- top floor has knee walls and slopped ceilings/partial attic- balloon framing. For example- the top unit in this building has 56' of base board (slantfin 30). The boiler went down Friday nite and the unit got to 48 degrees before we were able to re-fire it.

Did several heat loss calcs on that unit with the highest showing about 14KBtuh (using a 60* delta T). Boiler was running at 180 degrees & with 56' of base board- my calcs suggest around 28Kbtuh output. After 8 hours of running, the unit's temp only rose 4 degrees (to 52). Now outside temps that evening were -3- +9 but I would have expected more rise.. as did the tennant.

I suspect the base boards aren't transferring very efficiently due to age, dirt, carpet-convection obstructions because the return water temp was still quite high (around 170-172 if my IR gun can be trusted... but all of this makes be question everything from the heat loss calcs to the base boards true BTU emmisions etc.

On the water tank... that's a tough one. I own a bunch of rental property and water heaters last 5-6 years at best. The stainless indirects last much longer but if I have to feed then with a separate boiler, that becomes a real expensive water heater. When the tank calls (especially on a true cold start), it will use all the boiler can put out to the point where I had to stop hot water priority or apartments would get too cold too fast as people were getting out of the shower.

Given that I think I need to run water temps near 180 to get the Btu I need out of the baseboards, perhaps condensing is out of reach in this application and that's good to know now. how about non condensing modulation? Am I expecting too much from that technology? Am I better served by sticking with what I have and just adding thermal mass?

Ironman

Using the water heater for a buffer means it will be in a closed loop, not an open one. That, plus it will have no direct fire means it should last much longer.

If your radiation is minimal and you can't get return temps below 140* when it's 35* + outside, then you're probably better off just adding mass to what you have.

hot_rod

If the existing boiler is failing due to frequent short cycles, look for a fix.

Maybe the boiler is oversized
Not enough heat emitter connected to deliver the load.
Improperly piped.
Fins plugged or covered by furniture and drapes

Do the number crunching before you spend more $$

It may be worth investing in a better load program. I use this one, well worth the 90 bucks if you have more occasions to use it. Or download the free demo and try it out.

It allows you to build up the walls for the simulation a bit more input details allowed to the online freebies. The number you get out is only as good as the data you enter.

https://www.hydronicpros.com/downloads/index.php?id=23

jumper

Each "zone" requires its own temperature and flow. Zone valves alone cannot do the job.

jav

thank you for the feedback. Any suggestions on storage tanks? An electric water heater sound interesting but 40 gallons is pretty large... comment on size and thermal efficiency versus a purpose built storage tank? Also- the highest flow loop is the indirect which is piped with 1" copper. I presume a storage tank must not be a restriction to the highest flow loop?

hot_rod

most all electric tanks now are foam insulated, good insulation. You can buy those wraps cheap i9f you want more R-value.

usually 40 gallon tanks are the least expensive, 6, 12, and 20 are often as much or more $$ then 40gallon.

Use the 1" element holes if you can for best flow. A two pipe like this or 3 pipe makes it simple to pipe.

This is a 6 gallon electric tank on my Lochinvar Nobel to give it some capacity.

I have a 4.5 KW element for times when I run out of LP, gives me some back up.

Ironman

@hot rod
I have to ask: why do you have two gas cocks on that boiler? Did you run out of couplings?

Larry Weingarten

Hello, Two things. First, your base board doesn't seem to be working as well as it should. It might be over pumped, but it would be good to figure out, (using flow and temp differential) how close it is to rated output. That will tell you if it needs to be cleaned, have better air flow around it, or whatever to increase output.

Next, your comment about water heaters lasting only 5-6 years: Than can be fixed! By simply replacing anodes as needed, making sure water pressure is in range and not working the heater too hard, you can get 50 years from a heater. I've done it many times.

Yours, Larry

nicholas bonham-carter

Why not hire a good hydronic expert, and first make what you have work properly, using a buffer tank, if needed.
In the repiping, leave room for a quick replacement of the present boiler, which may have another 20 years in it.
The expert, (maybe from the find a contractor button here), will find out what has caused so many problems with the igniter, (maybe combustion air contaminated with vapors from something stored nearby?).
He can install a number of gauges for temperature and pressure which you can study in operation, as a welcome relief from watching the news on the box.
Time spent next to a boiler, especially when firing can be so satisfying.—NBC

jav

forgive me... over pumped? I understand how over pumped could account for higher than expected return temps but shouldn't that increase baseboard BTU output? The base boards are rated at 580 BTUH/ft at 4GPM and I ran my calcs at 500BTUH/ft output and I don't believe I'm getting that based on ambient temp rise vs heat loss calcs.

Flow is hard to control with a single circulator and 4 zone valves, I'm not a fan of the setup and it makes trouble shooting each loop difficult since the returns all merge into a single larger return riser running between the floors. The return delta I measured was at the boiler and that could have been impacted by flow from another heating zone. I haven't thought of it but... that might be my failing! I need to measure the delta right in each unit in different situations IE- 3rd flr calling with none other, then with 2 calling also and then 1 calling also, then with indirect as well. I wonder if my flow to that unit is being adversely impacted and I'm not seeing it because of where and how I'm measuring delta!

On the storage tank pic... I'm not following the line routing and do I see a cord? I figured a storage tank would go in series in the heating loop (I'd presume) on the return side nearest the boiler just to increase thermal mass?

Zman

You don't need to size the boiler to some rating on the DHW indirect. Those are just calculations based on example boiler sizes.
You need to come up with a solid heat loss calculation and size the boiler to that. Then look at it from zone by zone point of view, comparing those numbers to the minimum firing rate.
You absolutely want to make sure your DHW is sufficient, just don't let it drive your design.

Ironman

jav said:

On the storage tank pic... I'm not following the line routing and do I see a cord? I figured a storage tank would go in series in the heating loop (I'd presume) on the return side nearest the boiler just to increase thermal mass?

Buffer tank piping: 2 pipe vs 4 pipe.

https://www.pmmag.com/articles/100544-the-finer-points-of-applying-a-2-pipe-buffer-tank

Gordy

I think you need to do a detailed analysis on the baseboard convectors, and inspect for things blocking output. If that's not rectified no boiler any size will remedy poor, or insufficient radiation.

The cycling is from hitting high limit. If btus are not dispersed by the radiation they go back to the boiler.

With set points not being maintained, or below you should see lower return temps. Btus given up to the space.

Jamie Hall

Two random thoughts...

First, the only reliable way to determine how much heat is being put out by a hot water radiator -- of any configuration -- is to determine the flow through the radiator and the temperature drop from inlet to outlet. The good old hydronics formula will give it to you: BTUh equals 500 times flow in gpm times delta T in degrees F (as an engineer, you should have an amusing few minutes figuring out where that constant comes from -- and what the units on it are!).

Trying to estimate BTUh from the rate of temperature change of a space and the estimated heat loss is, frankly, a mug's game -- there are just too many variables which are difficult or impossible to measure.

Second, a word about "mod/con" boilers. One needs to keep in mind that there is a slash in there, and modulating boilers do not need to be condensing, nor vice versa. The modulating part of the phrase refers to any boiler which can vary the firing rate, hopefully to match the load. In our present usage of the term, it refers to burners which can smoothly alter their firing rate over some range -- a rather neat trick, since one needs to keep the air/fuel ratio correct. There is no particular reason why the term couldn't also be used to refer to boilers which pulse modulate -- that is turn on and off with a cycle timing suitable to match the load -- except that that is not how the term is applied usually. The condensing part of the phrase refers to boilers which operate at a low enough temperature in a portion of the boiler to condense the water vapour formed by combustion, thus recovering the higher heating value of the fuel as opposed to the lower heating value. This is how condensing boilers achieve their higher efficiencies (if you run the numbers, you will find that the difference in efficiency ratings of a modern condensing boiler as opposed to a modern non-condensing boiler is just that latent heat fraction of the HHV). There are two problems, though, with achieving that condensing higher efficiency. The first is a purely materials engineering problem: the condensate is ferociously corrosive, even with low and ultra low Sulphur fuels. Whatever the heat exchanger is made of must withstand that. The other is more an installation and control problem: the return water from the heating system must be cool enough (conventionally below 140 F) to cause condensation of the flue gas to occur. A well designed and installed system can do that over a fairly wide range of loads, though very few can do it at maximum load. A poorly designed or inadequately sized system may not be able to do it at all.

Just some things to think about...

Brewbeer

@jav you can do a room by room heat loss calc in a spreadsheet and account for atypical construction. See link in sig for example. The emitter survey is just as important as the heat loss calc.

EBEBRATT-Ed

In short all of the advise above is good advise. I think the consensus is a good heat loss the size the baseboard then the pipe and pumps and then the boiler. There are no shortcuts. Replacing a boiler with a mod con will not fix a heating problem

A cast Iron direct vent boiler is a decent boiler. Maybe it's oversized causing short cycling

Rich_49

Jav asked , " forgive me... over pumped? I understand how over pumped could account for higher than expected return temps but shouldn't that increase baseboard BTU output? The base boards are rated at 580 BTUH/ft at 4GPM and I ran my calcs at 500BTUH/ft output and I don't believe I'm getting that based on ambient temp rise vs heat loss calcs. "

Common mistake made by many , this thought . Delta T is an answer to a physical question , it is also a number used while testing for the mythical AFUE number on all boilers . Moving 4 gpm as opposed to 1 gpm for an increase in output of 5-6% is a fools errand . While the AWT may be a bit higher in the baseboard it is not a good idea since it DOES not promote boiler efficiency .

1 gpm = 10000 BTUh @ a 20* Delta while 4 gpm = 10000 BTUh @ 5* delta . If you do not leave those BTUs in the reservoir ( rooms) it returns to the boiler , the AWT in the boiler climbs , aquatsta reaches high limit , boiler shuts down (short cycles) , boiler comes back on after a few minutes . It really is that simple . Ultimately this will happen with any boiler operating outside it's output (lower) rating . You and everyone needs to make sure your SYSTEM is a SYSTEM .

hot_rod

Larry Weingarten said:

Hello, Two things. First, your base board doesn't seem to be working as well as it should. It might be over pumped, but it would be good to figure out, (using flow and temp differential) how close it is to rated output. That will tell you if it needs to be cleaned, have better air flow around it, or whatever to increase output.

Next, your comment about water heaters lasting only 5-6 years: Than can be fixed! By simply replacing anodes as needed, making sure water pressure is in range and not working the heater too hard, you can get 50 years from a heater. I've done it many times.

Yours, Larry

And higher operating temperatures tend to shorten tank style WH life.

More and more we see the "crank them up" suggestion to address legionella potential.
That's gonna cost you, both on operating cost due to higher standby loss and shorter tank life.

jav

Thanks for the great feedback guys! I need to process and re-read to fully grasp the info.

OK-

I'm confident that I do have 2 problems 1) boiler short cycling due to zoning and 2) heat transfer to certain zones in certain conditions. I'm also reasonably convinced that a Mod/Con isn't the answer.

ironman THANK YOU for that buffer tank article!! Not what I had envisioned and to be honest, it took me a while to grasp the impact and benefit of stratification versus cycling the entire tanks mass in series. It wasn't apparent from my initial read that the primary heat source loop circulator only ran while the boiler was firing- and was off during zone circulation after the boiler hit high limit. A buffer does seem to be a reasonable solution to the short cycling problem.

I'm struggling with how to measure a loops output because it's difficult to define flow in the current configuration. I need to better investigate that but I confess I'm not sure how. There's a single pump and multiple zone valves. If I isolate 1 zone, I can define head and look at the pump curve and make a reasonable deduction BUT, once another zone opens up, flow through that first zone will change AND the rate of change will vary depending on which other zone AND how many zones open (spreading pump flow through multiple paths with varying resistances). Am I not correct here?

I'm thinking my best option maybe to measure Temp deltas, not at the boiler, but inside each unit at the input and output of each radiation loop- with and without other zones calling. Since there are no flow control devices that I can see (other than zone valves), It makes sense in my pea brain that flow may be diminished to some loops in certain conditions. That could account for the insufficient heat transfer, despite having more than enough baseboard for the expected heat loss. Sound reasonable? Any other thoughts?

nicholas bonham-carter

Obviously, just replacing the hot surface igniter has not cured the problem. Is there enough combustion air for a clean light off? Maybe a vented dryer starving the burn. Maybe there is a bad connection between the panel and the boiler.—NBC

hot_rod

Here is a handy, inexpensive temperature monitor, battery powered or 24VAC. It also captures some data, so you could attach and let it monitor for a few hours or more.

Insulate around the sensors, use stainless hose clamps to get a tight connection of the sensor to pipe.

https://www.azeltec.com/images/brochure.pdf

hot_rod

Rich said:

Jav asked , " forgive me... over pumped? I understand how over pumped could account for higher than expected return temps but shouldn't that increase baseboard BTU output? The base boards are rated at 580 BTUH/ft at 4GPM and I ran my calcs at 500BTUH/ft output and I don't believe I'm getting that based on ambient temp rise vs heat loss calcs. "

Common mistake made by many , this thought . Delta T is an answer to a physical question , it is also a number used while testing for the mythical AFUE number on all boilers . Moving 4 gpm as opposed to 1 gpm for an increase in output of 5-6% is a fools errand . While the AWT may be a bit higher in the baseboard it is not a good idea since it DOES not promote boiler efficiency .

1 gpm = 10000 BTUh @ a 20* Delta while 4 gpm = 10000 BTUh @ 5* delta . If you do not leave those BTUs in the reservoir ( rooms) it returns to the boiler , the AWT in the boiler climbs , aquatsta reaches high limit , boiler shuts down (short cycles) , boiler comes back on after a few minutes . It really is that simple . Ultimately this will happen with any boiler operating outside it's output (lower) rating . You and everyone needs to make sure your SYSTEM is a SYSTEM .

So a radiant system running a comfortable 10∆ causes inefficient boiler operation??

If the boiler is short cycling it is oversized to the load regardless of the ∆ it is running at. And yes frequent, short cycles is bad operation for any equipment.

Better to modulate, ramp delay, or buffer to lessen the cycling, not increase the ∆.
Comfort and efficiency are obtainable with proper design, component selection, and installation.

Boiler efficiency in mod cons, any boiler really is driven by lowest possible return temperatures and operating temperatures.

Rich_49

Here's another nice little tool for checking Delta at multiple points .

https://www.trutechtools.com/hilmor_electronics

jav

To close the loop on the hot surface igniter/control board issues... I decided to dig in rather than just throwing parts at the system- which was done for too long. Being an electronics test engineer, I sat with the boiler for hours monitoring it after making some repairs. The end cause/effect was a bit interesting.

I did find that on the current/last control circuit board, one of the 120v traces going to the igniter relay was burnt off. This is what ultimately lead to the controller being inoperable. The hot surface igniter had also failed and my initial thought was that the igniter failed in a short circuit mode and burnt the PCB trace. However, after repairing the PCB trace, and then replacing the hot surface igniter, I noticed something interesting. The controller has some LED indicators that monitor the ignition sequence- power, purge, igniter, gas valve , flame. I noticed on some start cycles that the igniter LED would go out BUT- the igniter would continue to receive 120V AFTER the LED went out. Sometimes 120V would drop immediately with the LED and other times it would hang for seconds... one time it just got stuck on and I shut down power. I removed the Controller PCB again and took apart that relay (which was the one with the burnt trace) and found that the contactor was heavily pitted. So I now suspect that:

1) Hot surface ignitors failed prematurely due to short cycling.
2) The frequent igniter failures likely began to damage the relay contact which eventually, caused the relay to begin sticking.
3) The relay sticking likely accelerated hot surface igniter failures by sending power to it for too long ... repeat cycle until both the igniter and controller failed.

Gordy

A radiant slab is usually running much lower awt

Jamie Hall

On the flow with multiple zones... you are so right. In a single zone it's simple, as you note -- get your delta P and read it right off the pump curve. Pretty close.

Now for where it gets interesting. Do that for each zone singly, and use each zone's delta P and flow to compute an equivalent length of straight pipe. Any straight pipe. Use something reasonable, so the numbers are manageable. Now your problem is to determine the flow in each of two or more parallel pipes. The algebra gets a bit messy (!) but the fundamental equation is a rearrangement of the Hazen-Williams equation: head loss = K times flow raised to the 1.85 power. Since you know your total head loss, you know your flow. From your fiddling with the individual zones, you know K for each zone. For multiple zones open, then, you measure the head loss, look at your pump curve, get the total flow -- and then the flow in each zone can be calculated.

Note: all English units -- feet for head loss and gpm for flow.

Have fun!

jav

Replacing the ignitor and controller has fixed the symptom... but not the underlying problem. Rather an interesting failure cycle on those 2 parts but in the end related to short cycling - I think.

Ironman

It's doubtful that both the HSI and the control board were both bad - particularly on multiple occasions.

Multiple failures of either indicate you have voltage problem (too high). A loose neutral connection at the service or lateral will cause the phase voltage to spike.

jav

Ironman-

I can't speak to prior occasions but as an electro-mechanical engineer, I did trace this instance pretty deeply.

Voltages were fine (primary and secondary). Controller was non operational and HSI was bad (short circuited). I found a burnt trace on the controller boards primary HSI relay circuit & figured ignitor shorting load took out the control board trace.

I replaced the HSI and repaired the trace on the controller PCB. Tested controller in situ with a scope on the outputs. The controller tested fine without loads and upon connecting the loads, the boiler started and ran fine. I decided to continue to monitor the output voltages through several start cycles and found that the controls HSI relay was sticking "ON" intermittently- not always- but through 10 start-ups, it hung and released twice after a few seconds of overrun and stuck for 5+ seconds1 time where I killed power to save the HSI. I disassembled the PCBs HSI relay and found the contact to be heavily pitted and oxidized.

My best guess is, short cycling likely causes premature ignitor failures. Frequent igniter failures, and the probability that some failures will be in a short circuit state, causes cumulative controller relay contact damage. The relay eventually begins to stick, causing longer glow times and further decreases igniter life. Rinse and repeat this cycle until either the relay sticks on and takes out the igniter OR a shorted igniter/stuck relay takes out the controller.

hot_rod

jav said:

Ironman-

I can't speak to prior occasions but as an electro-mechanical engineer, I did trace this instance pretty deeply.

Voltages were fine (primary and secondary). Controller was non operational and HSI was bad (short circuited). I found a burnt trace on the controller boards primary HSI relay circuit & figured ignitor shorting load took out the control board trace.

I replaced the HSI and repaired the trace on the controller PCB. Tested controller in situ with a scope on the outputs. The controller tested fine without loads and upon connecting the loads, the boiler started and ran fine. I decided to continue to monitor the output voltages through several start cycles and found that the controls HSI relay was sticking "ON" intermittently- not always- but through 10 start-ups, it hung and released twice after a few seconds of overrun and stuck for 5+ seconds1 time where I killed power to save the HSI. I disassembled the PCBs HSI relay and found the contact to be heavily pitted and oxidized.

My best guess is, short cycling likely causes premature ignitor failures. Frequent igniter failures, and the probability that some failures will be in a short circuit state, causes cumulative controller relay contact damage. The relay eventually begins to stick, causing longer glow times and further decreases igniter life. Rinse and repeat this cycle until either the relay sticks on and takes out the igniter OR a shorted igniter/stuck relay takes out the controller.

As you probably well know a manufacturer designs a product like that to so many cycles. The parts used inside are specified to that target cycle life.

I agree frequent short cycling may be the cause of your electrical gremlins. Pitting on the contact would confirm that the relay has been over-worked and or contaminants are stressing the contact closure.

We manufacturer some electro-mechanical components and have found a 5000 cycles per heating season is common, so a product designed for a 20 year cycle life would want components designed for 100,000 cycles. Cost comes into the equation of course

All this assumes the component was built, tested and approved to a cycle life. Sometimes going off shore for components and manufacturing has an effect on quality and longevity of a product, regardless of the appropriate engineering involved.

HSIs are probably considered a consumable, the modules should last for years with proper, "clean" power applied to them.