Modulating boiler

[Alan Dunn]

Good day,
Sourcing out a new mod/con boiler, but am stuck on this question for myself. I couldn't get any info on this online anywhere.

How does a boiler know to modulate? What triggers or determines a modulating boiler to fire at high and low rates? From its supply/return water temperatures?

thanks in advance for the knowledge.

 

[Jac04]

Hi Alan. Yes, it is the water temperature setpoint that determines the firing rate. Depending upon how the unit is set up, you could be using either supply or return temperature. So, on a 'cold' start the unit fires at a fairly high rate to get the water up to the setpoint temperature, and then it throttles back the firing rate and modulates to try to maintain that temp. Some units will also have a maximum firing rate that can be set.

Ideally, the firing rate will modulate down to where the heat input equals the heat output, and a steady water temperature can be maintained. This is why it is important to not oversize the boiler. Too large of a boiler can not modulate down far enough, then you overshoot the water temperature, causing the boiler to shut down and then repeat the cycle.

Again, getting the size of the mod/con boiler is very important. There are a bunch of great people here who will help & guide you with this critical step.

 

[Sponsor]

 

[Dana]

Most modulating condensing boilers are operated under "outdoor reset" control, with a programmed curve to raise the output temperature of the boiler with lower outdoor temps and conversely, to roughly match the changes in actual heat load of the house. The difference in return water temperature is the only thing the modulating controls is operating from.

With cooler output the return water is also cooler, but the difference in temperature shrinks and expands with boiler output temperature. This is because the rate at which heat is emitted from the radiation into the rooms is higher at higher tempertures than at lower temperature.

When buying a mod-con boiler the minimum firing rate is more important than the maximum, as long as the maximum still covers the design heat load. If the radiation can't emit the full minimum fire output at low condensing temperatures the boiler will cycle on/off rather than modulate, which takes a toll on both efficiency and longevity of the boiler. The napkin-math on that lives here.

If you have a heating history on the place, it's possible to measure the heat load using the existing boiler as the measuring instrument, using wintertime fuel use and weather data. Details on how to do that are here.

It's a sad but true state of the industry that most modulating boilers currently installed are oversized for their radiation, installed by well intentioned folks who don't run the math on either the 99% load OR the heat emittance of the radiation at condensing temperatures, and just replace the pre-existing oversized cast-iron beast with a condensing boiler of similar size, since they know that will at least heat the place. Many installed cast iron boilers are also oversized for the radiation and cycle on/off, but the thermal mass of the water volume + cast iron is often (but not always) enough to limit the short-cycling to tolerable levels. It's harder to get away with that with a low-mass mod-con unless the radiation is high-mass, such as radiators with significant water volume.

 

[Alan Dunn]

Appreciate this detailed in-depth knowledge, Jac04 and Dana! I think I understand most of the points there.

We live in a relatively mild winter climate in Canada. Rarely get below 32F freezing. The three winter months here are averaging 40F.

Our current Weil-Mclains gas cast iron boiler (now having intermittent firing issue) is 21 year-old at 80K btu output. Set-up is a high design temperature low mass joist-suspended radiant system with thin aluminum extruded plates. Our house is 2-floor with 2x6 wall 2,700 sqft built in year 1997 divided by 7 zones with a 60 gallon indirect for DHW.

Using last Dec.'s gas bill at 128 therms of usage, following Dana's link for calculation, I figure our heat loss is around 27k btu.(probably subject to discrepancy due to my inexperience).

One contractor (referred by IBC) quoted IBC's SL 14-115 G3 MON/CON boiler (with 13k-109k btu output and a turndown ratio of 10:1). They didn't do a heat loss calculation.

Since our house is micro-zoned, I wonder if their one step down and lowest model SL 10- 85 G3 ( with 10k - 79k btu output) is better for our set-up?

Also, these models have Supply & Return connections on both sides of the boiler. Wonder what pros and cons this arrangement would have in terms of installation and piping? Is prim/secon piping required regardless (which we don't have now)?

I understand the return water temperature at the boiler needs to be 125F or lower before condensation occurs on the heat exchanger. So I wonder how or what can be done to ensure we take full advantage of the high 95% efficiency condensing boiler when our system requires high temperature to operate? Will this built-in ODR help a bit to control return temp, but still not enough?

 

[Dana]

Vancouver/Nanaimo/Victoria?

With micro-zoned low mass radiation and a sub-30K heat load you'll need a boiler with the lowest possible minimum firing rate, and probably a high-mass hydraulic separator/buffer tank, such as a Boiler Buddy or Ergomax. Even the SL 10- 85 G3 is oversized for the zone radiation, and there isn't sufficient thermal mass in the system to keep it from short cycling.

The ODR control curve is programmable, and you can tweak the curve until it's delivering the absolute minimum water temperature needed to actually heat the place. But the lower the water temp, the more cycling it will experience.

With the low heat load and micro-zoning may make more sense to use a modulating stainless condensing water heater such as HTP's PH76-50 50 ("Ph0enix Light Duty"), which is inherently self-buffering. It's minimum firing rate is about the same as your heat load, but it has a lot more thermal mass than a Boiler Buddy, and is probably easier to set up and install than a SL 10- 85 G3 + buffer tank. HTP's Versa PHE130-55 is already set up as a combi boiler and also self-buffering but is more expensive- probably overkill.

 

[Alan Dunn]

Bingo! We live in Victoria, BC. That's just amazing, Dana!

We actually have a combi boiler quote on a NTI VMAX 110 Plus with a 16-gallon indirect built-in as well as this IBC quote. The existing SS 60g indirect in our house seems to be still in good shape and we are concerned how multiple hot water demand will be managed with a combi. My family of five is very pleased with our current HW supply.

We'd like to see our 21-year old cast iron conventional 80,000 kbtu-output boiler to last a little longer. Even at 75 to 80% efficiency, this boiler has served us well in the sense that our summer gas bill (just for HW) is around $50 and winter season (which isn't long) averages $210 monthly.

It is too bad that it has intermittent and random firing issue (without warning or pattern) stuck at pre-purge stage that all I needed to do is flip the power switch to get it going again even after someone had changed out the pressure switch. Touch wood. It hasn't hiccupped since Sept 1st this time around......

The wife pushes for more options. She wants me to look into mini ductless heat pump now and gives up our radiant all together thinking an expensive MOD/CON boiler like IBC may not benefit our setup.

It has been a daunting and confusing task so far!

Thank you for all the education and advises.

 

[Dana]

Combi boilers are better suited to homes with high space heating loads and low to moderate domestic hot water needs. If you've been fully utilizing a 60 gallon indirect you have the converse- low heat load, moderate to high hot water needs..

If The minimum-fire output of the NTI VMAX 110 at condensing temperatures is 16,000 BTU/hr. That might be nearly your 99% heat load. Have you run a fuel-use based heat load calculation? It's max fire it's output is about 96,000 BTU/hr.

A16 gallon internal tank will keep it from short-cycling on HOT WATER calls but wouldn't be enough buffer tank to keep from short cycling a micro-zoned house. At high fire it won't support more than one full-flow shower with any margin. Filling a bathtub in winter could be pretty tedious.

A right-sized electric boiler for the backup heat and a mini-split or two can do really well in your climate. It's important to not oversize the mini-splits for their zone loads or you'll give up some efficiency & comfort. A room-by room, zone by zone heat load calculation using a freebie online Manual-J tool could help figure out what makes sense (and what doesn't) for ductless solutions. That tool tends to estimate high by 25% or more, so be aggressive on the input assumptions (eg: if it's 2x4 framing assume R13, not R11, etc.) and assume the house has NO ventilation and NO air leakage. It'll still over shoot some, but it'll be good enough for sizing ductless correctly, and avoiding big ductless errors. (That worst thing you can do is put a ductless head in every room on a single Medusa of a multi-split compressor.)

 

[Alan Dunn]

Not sure if we fully utilize the 60 US gallon indirect, but we never run out of hot water. The ladies do spend a long time in the shower.

I thought that NTI is 10:1 turndown. So shouldn't min-fire output be around 9,600 btu instead if max-fire is at 96,000 btu?

I did follow Dana's fuel-use based calculation. That's how I figured the sub-30k btu heat load for the house. To be precise, 25k btu on HDD 65F or 27k btu on HDD 60F. It seems quite low, but our 2-storey house is only 2,700 sqft with 2x6 framing and is newer. We used 128 therms according to last Dec's gas bill and this included HW.

To alleviate the house being micro-zoned, I'm supposed we can combine some zones which I believe is doable from the zone controller. From there, we can "daisy-chain" thermostat wiring to activate more than one zone using one thermostat.

I relentlessly sourced out this contractor who quoted this IBC SL G3 series which I really like. No one else quoted IBC whose manufacturer happens to be very near us at Vancouver BC.

The mini split option is viable and even preferable (due to its rapid responsiveness in heat distribution and summer cooling), but definitely leaves us with one dilemma and that's HW which could set us back at least a few thousands more to put in a standalone gas-fired HW tank.

Thanks Dana advice on the sizing for the mini split.

 

[Dana]

On page 4 of the NTI VMax brochure (the bottom of the page under "Specifications") it indicates that the VM110 has a maximum input of 110MBH, minimum input of 17MBH. ( 1 MBH=1000 BTU/hr) I looks like the VM153 has a 10:1 turn down, but the -110 does not.

If the minimum fire output is more than half your design heat load, even if it were operated as a single zone it would be cycling more than modulating with load, but it could at least be set up to where it wouldn't short-cycle. With a 2-story you'd usually want/need to have at least 2 zones, one for each floor. Depending on the total volume of water in the radiation on each floor that can usually be set up to not short-cycle without resorting to buffer tanks.

A 21 year old cast iron boiler is approaching middle-age, but isn't usually on it's last legs even if right-sized for the load. If the burner controls are repairable it's worth fixing. Age derating tables notwithstanding, pretty-good gas boilers that have been operated at a low duty cycles (as yours has been- never more than 30% even at design condition, given your heat load) will probably still hit within a percent or two of their original steady state combustion efficiency, even if it's standby losses are lowering the as-used AFUE by quite a bit. With a heat purging economizer control bypassing some of it's aquatstat controls it should be able to hew pretty closed to it's steady-state efficiency as it's AFUE, with fewer, longer burns, lower average boiler temp. Consider fixing it, and consider installing a simpler-cheaper variation of a heat purge controller such the Intellicon HW+ (which is designed to over-ride the economizer function when serving the indirect.) Given your micro-zoning & 3x+ oversize factor it'll probably cut fuel use by more than 10%, maybe more than 15%.

For the relative effects of heat purge vs dumber control at 3x oversizing, see Table 3 of this document. Only system #3 had heat purge control, giving up about 1.5% below it's steady state efficiency. All other systems lost 8% or more just to oversizing (and that's in a laboratory test with adequate radiation on each system- no short-cycling on zone calls.)

 

[Alan Dunn]

Good eye Dana on that NTI Vmax 110 min. output.

Just to verify the whole 2,700 sqft house load again this time by online Slantfin heat load calculator vs historical gas usage calculation from last winter I got prior. (result attached). Not 100% sure, but I think we had a really mild winter last year. So I think it is fairly consistent.

And according to this online emitter output chart I got (as attached), our system (#10 on the list) generates 5.6 btu/hr-sqft output at 110F input water temp. With this info along the heat load calculation, will this IBC SL 10-85 G3 boiler work for us in terms of providing the heat and yet modulating and condensing efficiently the same time?

In addition, I wonder how this Slantfin calculation result will work for mini ductless sizing? Can I use this result to size mini?

 

[Dana]

While the Slantfin tool usually shoots high by about 25%, it's still good enough for sizing ductless heads. It looks like you ran the numbers at 0C/32F whereas the 99% outside design temp for Victoria is actually -3C/26F, which probably cancels most of the internal error for that tool.

The only individual rooms that would call for it's own ductless head are:

Living------------------7832---------------------1/2 ton head

Kitchen & Family-----9940---------------------3/4 ton head

Master----------------6162---------------------- 1/2 ton head

The above could either be done with individual mini-splits, or a 2-ton multi-split, such as the Fujitsu AOU24RLXFZ with a couple of 7RLS heads and 9RLS head. The combined output of those heads on that compressor would be over 25,000BTU/hr @ -3C, and is still delivering 24,000 BTU/hr @ -8C.

If the living room is adjacent to the kitchen & family that could be combined into a single 1.25 ton head.

The Mitsubishi multi-zone alternative would be the MXZ-3C24NAHZ , and some FH06 or GL06 heads for the half ton rooms, or a FH09/GL09. (0r with combined zones, a FH15)

The down side to any multi-split solution is that the heads don't really modulate. If you oversize a head by very much for a tiny-load space it can overheat/overcool the room even when "off". When other zones are active there would still be refrigerant pumped through the "off" heads, even though the zone isn't running, with no active blower.

If it's an individual single zone mini-split there could be a rationale for:

Chris Bedroom------3008----------------------half ton Mitsubishi FH06NA

The FH06NA on a dedicated compressor can modulate as low as 1600 BTU/hr @ +47F, and wouldn't short-cycle on that zone.

With the doors to the smaller load rooms open, with 2-2.5 tons of ductless, keeping the rooms with the heads at a slightly elevated temperature can distribute heat to the other rooms via convection. I have a relative on Whidbey Island (Puget Sound) who heats his whole house with a 1.5 ton single head Mitsubishi, but some of the remote rooms can get a bit chilly when it's -5C out or cooler, even with the doors open.

To make any headway on analyzing the micro-zoned radiant, calculate the room by room floor emittance with 125F water (assuming you're running it all at a single temperature.) If individual rooms can't emit more than half the minimum modulated output of the boiler it's in danger of short cycling unless zones are combined.

 

[Alan Dunn]

Great info. Dana. Appreciate the input. I'll take the time to digest. It is intitutive

but one thing I am not sure is where Dana would have obtained the 99% outside design temp for us is -3C/26F because I am not experiencing that.

This site https://www.currentresults.com/Weat...es/victoria-temperatures-by-month-average.php seems to suggest that days per month on average in Victoria when the minimum temperature drops below -2 °C are:
November 1
December 2
January 2
February 2

In fact the average temp for us is well above freezing (32F/0C) in the winter as shown in this pdf.

 

[Reach4]

So you are thinking that since 1.9% of your days are below -2C, probably your 1% design number would be a cooler temperature.

 

[Dana]

Great info. Dana. Appreciate the input. I'll take the time to digest. It is intitutive

but one thing I am not sure is where Dana would have obtained the 99% outside design temp for us is -3C/26F because I am not experiencing that.

This site https://www.currentresults.com/Weat...es/victoria-temperatures-by-month-average.php seems to suggest that days per month on average in Victoria when the minimum temperature drops below -2 °C are:
November 1
December 2
January 2
February 2

I

Great info. Dana. Appreciate the input. I'll take the time to digest. It is intitutive

but one thing I am not sure is where Dana would have obtained the 99% outside design temp for us is -3C/26F because I am not experiencing that.

This site https://www.currentresults.com/Weat...es/victoria-temperatures-by-month-average.php seems to suggest that days per month on average in Victoria when the minimum temperature drops below -2 °C are:
November 1
December 2
January 2
February 2

In fact the average temp for us is well above freezing (32F/0C) in the winter as shown in this pdf.

n fact the average temp for us is well above freezing (32F/0C) in the winter as shown in this pdf.

The 99% outside design temperature is the 99th percentile temperature bin over the most recent 20-25 years of weather data. In a "typical" year only 87-88 hours (out of 24 x 365= 8760 hours) will be cooler than the 99% temperature bin. Some years might not get that cool even for one hour, some other year with a major Polar Vortex event or two it could be a couple hundred hours.

This is the ACCA's most recent published list:

https://articles.extension.org/sites/default/files/7. Outdoor_Design_Conditions_508.pdf

(Find Victoria B.C. on p.22, p.28 in PDF pagination)

Even if you sized the equipment to EXACTLY the 99% load, it takes more than just a couple of hours below the 99% design temp to lose ground, and you can cover the difference by turning on some lights, baking cookies, or having a party (those warm mammalian bodies are good for over 300 BTU/hr even when they're sitting down drinking beer, more if they're doing a polka with the beer keg on one shoulder. )

But boiler sizing and mini-split sizing don't come in finely grained capacity increments- the next size up is usually substantially more capacity margin.

 

[Alan Dunn]

Thanks everyone. It's been very educational.

wonder how our low emitter output (6 BTU/hr- sq.ft) compared to 19 for gypsum cement overpour for instance should or shouldn't our mod/con boiler selection>

 

[Dana]

At what temperature is it emitting 6 BTU/hr per square foot?

Does it ever NEED to emit more than that? (...or even that much?)

 

[Alan Dunn]

According to the doc., that's with 110F water temperature.

 

[Dana]

According to the doc., that's with 110F water temperature.

That's well into the condensing zone. A RETURN water temp of 125F will deliver over 90% efficiency on the boiler, 115F return water temp is good for the mid-90s. For an average water temp of 110F the return would be 105F-ish , and there isn't much additional benefit to dropping the temperature further.