Solar Assist For A New Boiler

[john2459]

I am replacing my 20+ year old boiler with a new more efficient combi type that supplies DHW and baseboard heat.
I would like to be able to pre heat the water coming into the boiler (checked with NTI and that is not a problem) using the solar preheated water for the baseboard heat and the DHW.
I have done the research but can’t find many examples of this type of system.
The solar installer seems this would just be good for the pre heat to the DHW but a $7k pre heater for an on demand DHW seems a bit much.
What I’m asking is should it be possible to utilize the solar hot water to heat (not to operating temp but enough to minimize the cycling of the boiler) in the baseboard heat AND the DHW?
From what I can see if anything I would need another/different water tank with 2 coils vs 1.
Any info, ideas, experience would be very helpful.

The major parts:
3 bed 2 bath 1400 sf rancher in central Colorado
2 adults

Apricus ETC30 Evacuated tube panel.
40.9kBtu/day
4 gpm max flow rate
Rheem Solaraide 81VR80U-T 80 Gal

NTI TX151 C modulating combi boiler

 

[Dana]

If the combi boiler hasn't been installed yet, DON'T- it's mistake!

First, the TX 151 C is a ridiculously oversized boiler for any 1400' house. Even the smallest of the line TX51 could probably heat your house at -50F at sea level, probably down to the -30F at 10,000'. Combi boilers are rarely a good fit even for houses 2x that size.

The minimum fire output of the TX 151 C is about 18,000 BTU/hr. Your heat load at -14F (Leadville's 99% outside design temp) is likely to be between 25,000-35,000 BTU/hr (and if it's 35K it can probably be cost effectively reduced.) If the minimum-fire output is more than half your design load, it's going to be well above your average heat load, and thus will spend the majority of the season cycling on/off rather than modulating efficiently. There's no point to a modulating boiler that rarely (or never) actually modulates.

Then there's the issue as to whether you have enough baseboard to run it so that it can CONDENSE to hit the 90%+ efficiency without short-cycling the boiler into low efficiency &/or an early grave. To hit the low-90s in efficiency the entering water temperature coming back to the boiler has to be under 110F, which means an average water temp (AWT) under 120F. At an AWT of 120F typical fin-tube baseboard puts out about 200 BTU/hr per running foot of baseboard. So to emit the min-fire output of 18,000 BTU/hr into the house without cycling at condensing temps you would need more than 18,000/200= 90' of baseboard PER ZONE. The more radiation-deficient the zone, the more often it will cycle on/off during calls for heat, and with every ignition cycle & flue purge it's blowing extra fuel out the exhaust, and heat out the heat exchanger, and putting wear & tear on the boiler.

Since you have a heating history on this place, run a fuel use based heat load calculation on your place. If you know how to use spreadsheet tools and have some mid-winter fuel bills this takes less than 15 minutes in most cases (about the amount of time it takes to read that blog bit on how to do it.)

I'll walk you through that here if you don't want to bother, but by doing it yourself you can become more comfortable with the reality of the numbers. Most boilers in the US are 3x or more oversized for the loads, and oversizing a modulating condensing boiler to the point that neither modulates nor condenses without destroying itself would be a waste. (Unfortunately that's more common than a correctly sized mod-con boiler.)

To figure this out also requires knowing how many feet of baseboard (by zone, if broken up into zones), so measure that all up too.

With the load & radiation information we can estimate what your average space heating water temperatures need to be, and from that determine if any kind of solar-boost option can be configured that makes any technical or economic sense at all.

[edited to add]

You can see from the illustrative graph that any solar assist is better applied to the output of the boiler rather than on the return, unless the place has so much radiation that return water temps are expected to be well under 100F (not likely, with fin-tube). Running the solar at higher temps lowers the solar efficiency, but raising the entering water temp of the boiler to above the condensing zone reduces combustion efficiency. Adding heat to the return water may just end up going out the flue as water vapor that didn't condense- where on the curve the system is operating is consequential:

 

[Sponsor]

 

[RoxanWright]

Listen to the guy above me. He is telling you the truth and know what his explaining to you. I'm also reading his reply to you and learn more about solar haeter.

 

[john2459]

If the combi boiler hasn't been installed yet, DON'T- it's mistake!

First, the TX 151 C is a ridiculously oversized boiler for any 1400' house. Even the smallest of the line TX51 could probably heat your house at -50F at sea level, probably down to the -30F at 10,000'. Combi boilers are rarely a good fit even for houses 2x that size.

The minimum fire output of the TX 151 C is about 18,000 BTU/hr. Your heat load at -14F (Leadville's 99% outside design temp) is likely to be between 25,000-35,000 BTU/hr (and if it's 35K it can probably be cost effectively reduced.) If the minimum-fire output is more than half your design load, it's going to be well above your average heat load, and thus will spend the majority of the season cycling on/off rather than modulating efficiently. There's no point to a modulating boiler that rarely (or never) actually modulates.

Then there's the issue as to whether you have enough baseboard to run it so that it can CONDENSE to hit the 90%+ efficiency without short-cycling the boiler into low efficiency &/or an early grave. To hit the low-90s in efficiency the entering water temperature coming back to the boiler has to be under 110F, which means an average water temp (AWT) under 120F. At an AWT of 120F typical fin-tube baseboard puts out about 200 BTU/hr per running foot of baseboard. So to emit the min-fire output of 18,000 BTU/hr into the house without cycling at condensing temps you would need more than 18,000/200= 90' of baseboard PER ZONE. The more radiation-deficient the zone, the more often it will cycle on/off during calls for heat, and with every ignition cycle & flue purge it's blowing extra fuel out the exhaust, and heat out the heat exchanger, and putting wear & tear on the boiler.

Since you have a heating history on this place, run a fuel use based heat load calculation on your place. If you know how to use spreadsheet tools and have some mid-winter fuel bills this takes less than 15 minutes in most cases (about the amount of time it takes to read that blog bit on how to do it.)

I'll walk you through that here if you don't want to bother, but by doing it yourself you can become more comfortable with the reality of the numbers. Most boilers in the US are 3x or more oversized for the loads, and oversizing a modulating condensing boiler to the point that neither modulates nor condenses without destroying itself would be a waste. (Unfortunately that's more common than a correctly sized mod-con boiler.)

To figure this out also requires knowing how many feet of baseboard (by zone, if broken up into zones), so measure that all up too.

With the load & radiation information we can estimate what your average space heating water temperatures need to be, and from that determine if any kind of solar-boost option can be configured that makes any technical or economic sense at all.

[edited to add]

You can see from the illustrative graph that any solar assist is better applied to the output of the boiler rather than on the return, unless the place has so much radiation that return water temps are expected to be well under 100F (not likely, with fin-tube). Running the solar at higher temps lowers the solar efficiency, but raising the entering water temp of the boiler to above the condensing zone reduces combustion efficiency. Adding heat to the return water may just end up going out the flue as water vapor that didn't condense- where on the curve the system is operating is consequential:

Good afternoon Dana,

Sorry it has taken so long to get back but work and farm stuff is always on the plate.
OK, so the current configuration is 2 zones, zone 1 is 379" (31.58') and zone 2 is 277" (23.08') and they are all fin tube. So what is the next step?

BTW I really appreciate the detailed answer.

Thanks
John

 

[Dana]

I've spelled out most of what follows in detail on this blog piece, which I encourage you to read, but I'll fine tune it a bit to your particular.

In order to get any condensing efficiency out of the boiler the return temp has to be under 125 F. At low flow you might be able to do that at an average water temp of 130F but to get into the 90s it really has to be under 120F, with 115F water entering the boiler. At water temps a few degrees under 120F the fin tube only emits 200 BTU/hr per running foot.

You have only ~55' of fin-tube total, so at condensing temps it can emit at most 55' x 200 BTU/ft= 11,000 BTU/hr even with both zones calling for heat. If the boiler is putting out more heat than that the temperature rises until the controls boiler's internal controls turn off burner as the pumps keep running, and re-fire when the water cools to below the the water temperature setpoint (usually governed by an "outdoor reset" control with condensing boilers, lowering the temp as outdoor temps rise, and conversely.) If the minimum firing rate isn't a large fraction of the heat emittance at condensing temps, it will cycle on/off a lot, losing both efficiency and longevity.

In your case it's broken up into zones, the smaller 23' zone can only emit 23' x 200 = 4600 BTU/hr into the zone. That is more than half the minimum firing rate of any current boilers out there, but not a LOT more. About the best you are going to do with modulation alone is the samllest of the TX line, the NTI Trinity TX51. It can throttle back to 7,100 BTU/hr-in at min-fire, but can ramp up to 57,000 BTU/hr when necessary. At 95% efficiency that 7100 BTU/hr input becomes 6745 BTU/hr out, so with just the 23' zone calling for heat the fin-tube can still emit 2/3 of the min fire output. With some tweaking of settings and flow that's probably going to be enough, but if you can add enough radiation such that each zone can emit the full 6745 BTU/hr at condensing temps it would be better. If fin-tube that would be about 6745/200= ~34'. Zone 1 is fine, but you'd need to add radiation equivalent to 10-12' of fin-tube to zone 2.

If you're concerned that the TX51 might not be enough, consider that baseboard can't emit more than ~500BTU/ft-hr with 180F entering water temp, a typical setting on a cast iron boiler, and even at most boiler's maxiumum temperature settings not more than 750 BTU/ft-hr. With 55' of fin tube at an EWT of 200 F and very high flow you'd be getting 700 BTU/ft out of it, x 55'= 38,500 BTU/hr, which is well below the non condensing output of the TX51- you have a lot of boiler capacity to spare.

Setting up the TX51 to supplement the solar tank operating as a third zone, with the hot water zone given "priority" by the zone controller should work. A priority zone suppresses delivering heat to the space heating zones whenever the tank needs heat, but even with an 80 gallon tank the recovery time is pretty quick, and not a comfort issue unless you take continuous showers during the absolutely coldest hours of the year. A 57,000 BTU/hr boiler delivers far more heat than a typical 50 gallon standalone HW heater. It's not enough burner to run an "endless shower" in the dead of winter, but it almost is. (With a drainwater heat recovery heat exchanger and low flow shower heads it could be, if that's a goal.) The heating system water needs to be isolated from the potable with a heat exchanger, so without know much about the Rheem Solaraide 81VR80U-T I would assume that would require specifying a plate-type external heat exchanger & appropriate pumps & controls, but this is not a difficult design problem.

 

[john2459]

I've spelled out most of what follows in detail on this blog piece, which I encourage you to read, but I'll fine tune it a bit to your particular.

In order to get any condensing efficiency out of the boiler the return temp has to be under 125 F. At low flow you might be able to do that at an average water temp of 130F but to get into the 90s it really has to be under 120F, with 115F water entering the boiler. At water temps a few degrees under 120F the fin tube only emits 200 BTU/hr per running foot.

You have only ~55' of fin-tube total, so at condensing temps it can emit at most 55' x 200 BTU/ft= 11,000 BTU/hr even with both zones calling for heat. If the boiler is putting out more heat than that the temperature rises until the controls boiler's internal controls turn off burner as the pumps keep running, and re-fire when the water cools to below the the water temperature setpoint (usually governed by an "outdoor reset" control with condensing boilers, lowering the temp as outdoor temps rise, and conversely.) If the minimum firing rate isn't a large fraction of the heat emittance at condensing temps, it will cycle on/off a lot, losing both efficiency and longevity.

In your case it's broken up into zones, the smaller 23' zone can only emit 23' x 200 = 4600 BTU/hr into the zone. That is more than half the minimum firing rate of any current boilers out there, but not a LOT more. About the best you are going to do with modulation alone is the samllest of the TX line, the NTI Trinity TX51. It can throttle back to 7,100 BTU/hr-in at min-fire, but can ramp up to 57,000 BTU/hr when necessary. At 95% efficiency that 7100 BTU/hr input becomes 6745 BTU/hr out, so with just the 23' zone calling for heat the fin-tube can still emit 2/3 of the min fire output. With some tweaking of settings and flow that's probably going to be enough, but if you can add enough radiation such that each zone can emit the full 6745 BTU/hr at condensing temps it would be better. If fin-tube that would be about 6745/200= ~34'. Zone 1 is fine, but you'd need to add radiation equivalent to 10-12' of fin-tube to zone 2.

If you're concerned that the TX51 might not be enough, consider that baseboard can't emit more than ~500BTU/ft-hr with 180F entering water temp, a typical setting on a cast iron boiler, and even at most boiler's maxiumum temperature settings not more than 750 BTU/ft-hr. With 55' of fin tube at an EWT of 200 F and very high flow you'd be getting 700 BTU/ft out of it, x 55'= 38,500 BTU/hr, which is well below the non condensing output of the TX51- you have a lot of boiler capacity to spare.

Setting up the TX51 to supplement the solar tank operating as a third zone, with the hot water zone given "priority" by the zone controller should work. A priority zone suppresses delivering heat to the space heating zones whenever the tank needs heat, but even with an 80 gallon tank the recovery time is pretty quick, and not a comfort issue unless you take continuous showers during the absolutely coldest hours of the year. A 57,000 BTU/hr boiler delivers far more heat than a typical 50 gallon standalone HW heater. It's not enough burner to run an "endless shower" in the dead of winter, but it almost is. (With a drainwater heat recovery heat exchanger and low flow shower heads it could be, if that's a goal.) The heating system water needs to be isolated from the potable with a heat exchanger, so without know much about the Rheem Solaraide 81VR80U-T I would assume that would require specifying a plate-type external heat exchanger & appropriate pumps & controls, but this is not a difficult design problem.

That being said, would the fact that the TX 151 is a combi and the TX51 is not have anything to do with it? Does that DHW coil make up for the huge difference / overage?
Also the solar side of this equation is no longer an issue. The solar installer said this can't be done for both he could install it for the DHW but not both. I can't see 7k for a DHW heater so I decided to keep the 7k and rethink that side. So now all we have is a boiler and DHW. So in your professional opinion would I be better off with combi boiler, a regular boiler and regular DHW heater or a regular boiler and an on demand DHW heater?

Thanks for all your patience
John

 

[Dana]

The ~18,000 BTU/hr minimum modulation of the TX151 is the key issue- the fact that it's a combi is of no consequence. 18,000 BTU/hr into 55' of baseboard is 327 BTU/ft-hr, which means it can literally never run at condensing temperatures with your radiation without short-cycling itself into an early grave.

Most of the new fire-tube heat exchanger based boilers & combis are good for at most a 10:1 turn down ratio, which is a limitation of the heat exchanger design. That's actually a major improvement over typical water-tube condensing heat exchangers, which struggled to hit a more than a 4:1 ratio efficiently. The TX151 modulates between 18.9-151K BTU/hr in which is about an 8:1 ratio. The TX51 modulates between 7.1-57K BTU/hr-in, which is also about an 8:1 ratio. That's really about as good as it gets. There are some 10:1 ratio boilers out there, but none that I'm aware of that modulates below the minimum firing rate of the TX51.

The fact that it takes a minimum of ~75KBTU/hr of tankless water heater to run a 24/7 shower even at low flow, and you only have enough radiation to emit ~4500 BTU/hr of heat in your smaller zone means you would need a turn down ratio of well over 15:1 for a combi to make any sense at all. Combis are usually only a good fit for houses with larger than average space heating loads and relatively modest hot water loads. What you have is a modest heat load and a modest HW load.

Solar thermal systems have really lost luster now that solar PV has gotten cheap, along with arrival of heat pump water heaters. For $7K even if you spent $3000 all in on a heat pump water heater and have enough left over for 1200-1500 watts of grid tied rooftop PV, which would produce more than enough power to cover that used by the heat pump water heater, especially in sunny high-altitude locations in CO.

 

[Dana]

BTW: Depending on your location and outside design temp, you can probably heat your house more cheaply with a 1 or two ductless mini-splits than a new propane fired boiler. At 33' of baseboard, 500 BTU/hr per foot implies the heat load of the bigger zone isn't more than ~16,000 BTU/hr, which is below the output of a 1.25-ton Mitsubishi @ +5F, and the output of the 1.5 tonner @ -13F.

The 23' of baseboard on the other zone implies a heat load of no more than ~11,500 BTU/hr, which is about the output of the 1-ton unit somewhere in negative single-digits.

Fujitsu has a similar series of cold climate mini-splits with somewhat higher output per ton.

In my area mini-splits run about $3.5-4K per ton, all-in when put up to competitive bidding. With better analysis of the heat load you can probably hone it down to something less than 2.5 tons of mini-spit, maybe even less than 2 tons. You have a heat load history on this place, so you should be able to use the existing boiler as the measuring instrument to come up with the whole-house load, and use the baseboard size a rough guide to how the load is apportioned. If the baseboard is about right, it's a 60/40 split, so it could be a 1 ton or 1.25 ton + a 3/4 ton (2 tons total) or at worst a 1.5 ton and a 1-ton as outlined above. Two tons of cold climate mini-split is under $8K, installed, in my neighborhood. If mostly DIY hiring a tech for the final test & commissioning you can probably bring that down to the $5.5K range (4.5-5K in raw hardware cost, plus an hour or two of refrigeration tech time.)

 

[RoxanWright]

If you used solar as a heat it will work well since there is a big rebates today. On the other hand, installing mod/con boiler will operate at minimum fire due to warm water from solar system. I happened to have one and the DHW was done mainly by solar.