New to boilers... looking for general advice

[Greenmountain]

considering either a combi unit or boiler/indirect hwh for a house/workshop at high altitude (9000') in CO.
This will be a completely new system (no existing equipment) for an old house.
Does get pretty cold in deep winter up here... and winter can last 9 months out of the year.

not quite ready for all the hard math... just hoping to get some general advice for a somewhat unusual situation.

I will shoot to get an accurate heat load #, but generally speaking, living area is one bed/one bath, approx 1500sq'. insulation is currently very poor... 2x4 ext walls/fiberglass insul, single pane windows, etc. Workshop below stick-framed living area is cinderblock.

Workshop below needs heat only to keep water pipes and paint/etc from freezing.
So ideally the whole workshop would be one zone set to around 35-40F.

Given the smallish size of the living space, I'm not sure how appropriate it would be to split it into zones, but I prefer to not run heat when I don't need it. I generally use no heat overnight while sleeping, have heat for early morning, then off most of the rest of the day, then back on for dinner/bed time, but that can vary.
I use a pretty sophisticated programmable thermostat for my other residence running a forced air furnace, so it's easy to have the heat on only when I want it, but not sure if that type of control is appropriate for a hydronic system.

Am installing a wood burning masonry heater for the living space, but would like a system that will suffice when not burning wood.
Also thinking about integrating some sort of (high thermal mass) hydronic heat exchange into the masonry heater. I could set it up as an independent system, but would be interested to hear if anyone has tried to tie a wood heater into a boiler system.
Also have plans to add a greenhouse off of the workshop, so will likely have it as another zone in the future.

Given the lack of energy efficiency I'm already facing due to poor insulation (will be working on improving it over time, but it is what it is for the short term), I'd like to strive for as much efficiency in any and all other possible options, so having a boiler functioning at its highest efficiency would be a priority.

Just looking for any advice, basic strategy, etc to get me started. Thanks!

 

[Fitter30]

Combi heater in boiler mode not water heating is 95% efficient with return water temperature 130* or below. Above 131* efficiency drops to 86%. Size heat emitters accordingly. In water heating need to size looking at coldest incoming water temperature and max gpm flow you will need. Just one of many articles wood and gas boilers.
https://www.phcppros.com/articles/1565-wood-boiler-meets-modern-hydronics

 

[Sponsor]

 

[Dana]

Combi boilers with burner output sufficient to get decent hot water flow rates have a substantial minimum-fire output- typically 18,000 BTU/hr.

If there isn't sufficient radiation on EVERY ZONE to emit that much heat at water temperatures cool enough for condensing the burner will constantly cycle on/off during continuous calls for heat. If it's cycling something like 8x or more per hour with burn times under two minutes it takes a toll on both efficiency and boiler life. The dumb napkin math model on how that works lives here. There are boilers out there that can throttle to under 8000 BTU/hr, which won't need nearly as much radiation to tame he short cycling issues, but even in a less than well insulated 1500' of living space it should probably set up as a single zone. The workshop can be it's own separate zone, but still needs enough radiation to emit the full min-fire output to not run into short cycling issues.

Domestic hot water can be served up by an indirect fired tank water heater running off the same boiler. Typical standalone tank water heaters have burners with 30-35,000BTU/hr, so even a 50-80,000 BTU/hr water heater will still have plenty of capacity for heating the water quickly as the "priority zone", delivering 100% of the boiler's output to the indirect before resuming space heating function. Unless you're taking 5 back-to-back showers when it's -10F outside the space heating won't be suppressed long enough to be a comfort problem.

To maximize condensing efficiency for the living space it's better to NOT use deep setbacks, since it requires the boiler to run at a higher water temp and higher burn rate during the recovery ramp, both of which take a toll on efficiency. With modest setbacks and a "boost" mode on the boiler the efficiency hit is small, and some fuel savings can be had, but not much. The most important thing is size the boiler and room radiation correctly for the loads, and carefully dial-in the "outdoor reset" function to run the water temps at the lowest possible temperature that still heats the space.

Setting the workshop thermostat to 35-40F isn't enough margin to keep things from freezing up, especially if you're not going to air seal and insulate the walls and improve the windows. Shoot for 45-50F. At the very least low-E windows over the old single panes would bring performance close to current code minimums. The CMU walls can be brought up to current code minimums with 3" of rigid polyiso foam board. (Reclaimed roofing polyiso is a pretty cheap way to get there usually $15-$30 per 4' x 8' sheet for three inch, depending on source and condition.) I've covered in detail how to go about that on multiple threads on this site, and won't go into it here unless you're really going there.

A 1500' reasonably tight (not super-drafty, not hermetically sealed) 2x4/R11 type house with clear storms over single panes (or non low-E replacement windows) and R19-R25 in the attic would typically have a heat load of~35,000 BTU/hr @ -10F (a typical 99% outside design temp for 8000-9000' elevation locations in Colorado), give or take, and if the workshop is partially or mostly below grade it's load would be substantially less, once insulated. I suspect even as-is the whole house block load (workshop included) is no more than 60,000 BTU/hr. While (based on old school atmospheric drafted equipment) the industry standard is to derate boiler's capacity 4% for every 1000' of elevation above 5000' that is too severe for mod-cons. They take some hit, but nowhere near 4% per 1000. Since 9000' is 4000' above 5000', you'd only be derating a boiler by 16% (84% of nameplate rating capacity) even in the worst case.

With a load of say 60,000 BTU/hr it means a cast-iron boiler's output rating would need to be greater than 60,000/0.84= 71,429 BTU/hr, which is about where most 80-85KBTU/hr-in mod-cons come in. An 80K-in boiler with a 10:1 turn-down can throttle all the way down to 8000 BTU/hr-in, 7500 BTU/hr out, which can be emitted by as little as 40' of 9-10" tall fin-tube baseboard, making it substantially more "zone-able" than a combi boiler with a minimum heating output of 12-18000 BTU/hr.

But run real load numbers on the "after upgrades" version of your house before picking a boiler- size it for the eventual heat load, not the where-is-as-is condition of the house or it will be suboptimally oversized once you fix the house. Even if the boiler may a a bit shy of heating all zones to 70F on the coldest 1% of hours of the year, an 80K-in boiler will have NO problem keeping 1500' of sorta-insulated house warm, and the basement 45-50F, even with the lower level uninsulated.

Don't trust HVAC contractors to get the heat loads correct, or to properly size the boiler & radiation- the industry average is WOEFULLY out of touch. Run your own room by room Manual-J-ish numbers using either LoadCalc or CoolCalc, and be super-optimistic (read "aggressive") about your actual air tightness, R values, and U-factors or they will overshoot reality by quite a bit. Insufficient aggressiveness on input assumptions is a common error for newbies and HVAC "pros" alike, but would be unforgivable for a third party doing load calculations a service (such as an engineer or architect). Even with aggressive inputs those two online freebies will overshoot by a double-digit percentage, but not 50% higher than reality. With conservative inputs you might as well throw your hands up and take a WAG, or use some ridiculous BTU per square foot rule of thumb, since they can overshoot by 2x or more.

 

[Greenmountain]

thanks for the detailed responses!
... have a few more questions about some of the basics.

One, I've found that the cost of combi-boilers seems to be only very slightly (like $100) higher than boiler only.
The cost for even fairly small indirect-dhw tanks ($1000+) is higher than even a new tankless heater, so from at least a cost standpoint, I'm not sure I see much benefit to an indirect dhw system (other than reduction of venting pipe)... I'm sure I'm missing something here but not sure what?

Also, I do see that the combi units have a higher minimum fire rate (around 11-12MBH), but regarding being mindful to not oversize the boiler, what about the idea that highest efficiency is had when running the boiler at it's lowest fire rate?

"The raw efficiency of a mod con is determined by the entering water temp coming back from radiation, and the firing rate of the burner. With most the sweet spot on the firing rate is at or near the lowest modulated range, and going more than 50% of full fire cuts into condensing efficiency at any return water temp by a significant more-than-theorectical amount, which you'd somehow have to make up for with lower return water temps:"

Doesn't this suggest that a relatively higher btu boiler would be more efficient since it would be running at it's lower fire rate more often (assuming enough radiation to keep return water temp<130)?

 

[Fitter30]

Dhw using one without a storage tank has to be in the range of heating water load. Dhw need to take in configuration coldest incoming water temp and gpm. If dhw needs a 199k for a garden tub or multi head shower and heating load is 75k your modulation at a light load just isn't there. There isn't a sweet spot on a modcon boiler as long as return water temp is under 130*. Short cycling will kill any boiler efficiency and life. Modcon boilers use for dhw do require yearly service when not using a storage tank to clean heat exchanger along with air and water screen .

 

[Dana]

The cost for even fairly small indirect-dhw tanks ($1000+) is higher than even a new tankless heater, so from at least a cost standpoint, I'm not sure I see much benefit to an indirect dhw system (other than reduction of venting pipe)... I'm sure I'm missing something here but not sure what?

A major benefit of an indirect over a combi or tankless is maximum flow. With a tank the maximum is determined by the plumbing size and water pressure. With a combi the maximum is determined by the maximum firing rate- it's burner- limited.

Also, I do see that the combi units have a higher minimum fire rate (around 11-12MBH), but regarding being mindful to not oversize the boiler, what about the idea that highest efficiency is had when running the boiler at it's lowest fire rate?

Firing rate is only one parameter affecting efficiency. The average length of burn and incoming water temperature also matter (a lot). With short draws of hot water the burn lengths are very short, taking a significant toll on as-used efficiency.

"The raw efficiency of a mod con is determined by the entering water temp coming back from radiation, and the firing rate of the burner. With most the sweet spot on the firing rate is at or near the lowest modulated range, and going more than 50% of full fire cuts into condensing efficiency at any return water temp by a significant more-than-theorectical amount, which you'd somehow have to make up for with lower return water temps:"

Doesn't this suggest that a relatively higher btu boiler would be more efficient since it would be running at it's lower fire rate more often (assuming enough radiation to keep return water temp<130)?

That would be the case if didn't also suggest there would be more short cycling. Modulation ranges are not infinite.

 

[Greenmountain]

in late winter, incoming water temp is very low... around 36F.
gpm needs are generally pretty low, currently just one occupant, but would not mind a little extra potential capacity.

another question re short cycling... In summer when the only demand on the boiler is DHW, how is short cycling avoided?
if dhw is used at random, brief periods throughout a day... say hand washing, maybe rinsing some dishes or whatever... even if pulling from or through a larger storage tank, the boiler is only going to replenish whatever heat was taken out, regardless of how the dhw is stored and/or heated, right? Are there strategies/equipment to prevent short cycling in this scenario? Seems some sort of relatively high volume tank with a wide storage temp range? Not sure if this is the correct terminology, but what I mean is that the boiler control is set with a low cut-in temp and a high cut-out temp so that it doesn't kick on and off every time just a little bit of hot water is used.

On that note, I'm curious about systems using a reverse indirect dhw/buffer tank like the Ergomax/Turbomax units... Seems like something like this would help with short cycling but wouldn't that also depend on where the temp sensors are and what the temp range setting would be?

Also read that there are euro appliances that are essentially a boiler and reverse indirect dhw in one unit (ex: Rotex, ACV - http://www.duluthenergydesign.com/C...tionMaterials/2014/Day1/siegjo-bespra-001.pdf ).
Anything like this in USA?

 

[Dana]

in late winter, incoming water temp is very low... around 36F.
gpm needs are generally pretty low, currently just one occupant, but would not mind a little extra potential capacity.

another question re short cycling... In summer when the only demand on the boiler is DHW, how is short cycling avoided?

If you have a big tub to fill the gpm needs are high regardless of the number of occupants. If you don't, it's not such a big deal.

Short cycling is NOT avoidable with a tankless or a combi boiler in DHW-only mode. With a tank there is substantial thermal mass to work with, but it comes with the tradeoff of standby losses.

Rather than an indirect running off the boiler, at about the same (or lower) money sometimes it's better to use a heat pump water heater (more true in cooling dominated climates than in CO, but is still favorable when the hot water needs aren't high. With a heat pump water heater 2/3 or more of the heat going into the water is coming from the room air (including the heat of vaporization of humidity in the air, making it a slam-dunk for the southeastern US, not so much in CO.) Any standby losses simply get "re-harvested" by the water heater on the next heat pump cycle. It does lower the temperature in the room where it's located- great in summer, but an additional heat load during the winter. The down side to heat pump/hybrid water heaters is slow recovery times, but that's usually only an issue for high volume hot water users such as a family of 6 or more, not households of 4 or fewer. If there are any spa tubs to fill the larger size heat pump water heaters are also pretty pricey compared to an up-sized indirect. (But filling a spa tub with a combi is an exercise in tedium- be prepared to chant several verses of the Rig Veda to stay chill. )

if dhw is used at random, brief periods throughout a day... say hand washing, maybe rinsing some dishes or whatever... even if pulling from or through a larger storage tank, the boiler is only going to replenish whatever heat was taken out, regardless of how the dhw is stored and/or heated, right?

Not exactly. With a combi/tankless it blows away a fixed amount of heat out of the exhaust vent with every flue purge & ignition cycle, whether drawing 0.1 gallons in a quick rinse or 100 gallons in a showering marathon. Multiple random draws of 0.1 gallons distributed in time is the least efficient way to operate it, since the residual heat left in the heat exchanger at the end of the draw has time to dissipate into the room & flue.

Are there strategies/equipment to prevent short cycling in this scenario? Seems some sort of relatively high volume tank with a wide storage temp range? Not sure if this is the correct terminology, but what I mean is that the boiler control is set with a low cut-in temp and a high cut-out temp so that it doesn't kick on and off every time just a little bit of hot water is used.

On that note, I'm curious about systems using a reverse indirect dhw/buffer tank like the Ergomax/Turbomax units... Seems like something like this would help with short cycling but wouldn't that also depend on where the temp sensors are and what the temp range setting would be?

The typical high/low limit differential on the aquastats that come with the Ergomax series is about 7-8F. On a 48 gallon/~400lbs version (like the one in my basement) that is a 7F x 400lbs= 2800 BTU storage buffering range. At a fixed input of 7000 BTU/hr (the minimum fire from an 80K boiler with a 10:1 turn-down) that's a 0.4 hour (=24 minute) burn. Setting up the flow rates to adequately serve both heating and hot water takes a bit of arithmetic and maybe some after the fact tweaking, but if you're planning to use the outdoor reset function of the boiler to max out heating season efficiency you won't necessarily want to buffer the heating system at DHW temperatures, since that will deliver less efficient entering water temps at the boiler. It's a compromise, but often a compromise worth making.

I run my Ergomax at a storage temp of ~130F, which works OK. The radiation in my somewhat 5 zone house (the smallest zone has a design load of only ~2000 BTU/hr) was designed to deliver design day heat to all zones with 125F average water temp (AWT), and the largest (a ducted air zone with a hydro-air hander) pushing 20,000 BTU/hr. The minimum firing rate on the (now 10+ year old) burner is about 20,000 BTU/hr, but I've set up the flow through the Ergomax and the fixed output temp of the burner to guarantee ~44,000 BTU/hr at a tank temp of 125F (somewhat more than the design heat load) but it will run higher fire as the tank temp drops. With all zones calling for heat AND the shower running for long periods it would end up delivering very tepid <100F water to the shower, but an aquastat on the biggest zone (the air handler) is set to suppress that zone when the water temp is under 110F, freeing up more heat for the shower load. Yes, it's kludgey, but it works, and is capable of "endless showers" while still heating the house, at the flow rates and temperature targets it was designed for. There's a 3-5% hit in steady-state efficiency going that route, but there is roughly an order of magnitude fewer burn cycles per year compared to doing it with a combi (which would have short-cycled insanely on my smaller zones.)

Going forward I'm expecting to yank the gas meter and use a full VRF heat pump such as a 4 or 5 ton LG Multi-V S for heating the house. The air handler would be replaced by a 2- 2.5 ton refrigerant-only (no hydro-air) handler, and the Ergomax would be simply a buffer and DHW- pre-heat, using the outdoor reset function on the heat pump to keep temperatures low and efficiency high, feeding the cold side a heat pump water heater. During the cooling season the heat being pulled out of the house is dumped into the Ergomax rather than the outdoor air. The heat pump water heater would be running at 120-130F, temperatures that the Ergomax would not likely except during very cold or very warm outdoor temperatures.

Also read that there are euro appliances that are essentially a boiler and reverse indirect dhw in one unit (ex: Rotex, ACV - http://www.duluthenergydesign.com/C...tionMaterials/2014/Day1/siegjo-bespra-001.pdf ).
Anything like this in USA?

The well engineered HTP Versa Hydro series is exactly that sort of system (and featured in several pictures in that document). (Their headquarters are less than a 2 hour drive from my house.) Had it been available at the time I probably would have gone that route rather than an Ergomax.

 

[Fitter30]

The more hi tech/ higher efficiency equipment parts are not hour away there days. Also if a service person is needed good luck finding one that has the knowledge and tools ( laptop with correct program)

 

[Fitter30]

Ergomax 99% efficient don't think so. A plate heat exchanger is only 90-95% at best they have so much more surface area. Boilers are usually most efficient at high fire. What brand and model of your boiler?

 

[Dana]

Ergomax 99% efficient don't think so. A plate heat exchanger is only 90-95% at best they have so much more surface area. Boilers are usually most efficient at high fire. What brand and model of your boiler?

Buffer tanks like the Ergomax typically rate their efficiency by the standby loss rate in degrees per hour @ 180F (just like any indirect) not a percentage.

It's not all about surface area either. The Ergomax & Turbmax reverse-indirect water heaters do NOT incorporate plate heat exchangers, but use multiple parallel-plumbed ~-3" diameter 1/4" -3/8" copper tubing coils fully immersed in the (non-potable) tank water. The input port from the boiler also has a narrowing injector directed at the coils to increase turbulence on the storage-water side of the heat exchanger coils to increase heat transfer efficiency. The tight winding of the coils induces turbulence of the potable water inside. While physically much larger than plate heat exchangers the transfer efficacy is quite good even when the boiler loop is at idle (with turbulence only on the potable side), better still when the boiler loop is flowing.

The importance of turbulence is significant for a couple of reasons. While more coils increases surface area and will handle higher potable flows, the heat transfer efficacy at low flow falls off as velocity & turbulence diminishes, then begins to increase at ultra-low velocity (due to the amount of time the water is in contact). At the lower turbulence the potential for developing lime scale on the potable side also increases. (Like a tankless water heater, the coils can be descaled with mild acid solutions if that becomes a problem.) Most residential apps will be better off with the Ergomax E23 (3 internal coils) rather than the E24 (4 coils, more surface area) especially if most of the hot water draws are <<2 gpm. Keeping storage temps low also reduces scale-up potential, both for the physical chemistry reasons as well as the fact that lower temp = higher flow on the hot side when mixed down to 105-110F for most residential uses. In the notes regarding sizing on the manufacturer's website they state:

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3. Do not oversize. The flow of water through the coils should be kept high to ensure a high level of turbulence within the coils (eg. high velocity). (This turbulence scrubs the inside of the coils keeping them free of scale.)
4. ERGOMAX should be compared to units produced by other manufacturers based on performance and not on tank size.
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In many multi-family or commercial applications buffer storage temps run north of 160F, increasing the likelihood of scaling at low flow or oversized coils. But with condensing mod-cons or a hydronic heat pumps combined with low temp heating radiation the scaling problem fades into the background.