Tri-level replacement

[Ki23]

Good morning all, I've been reading these forums for weeks and getting great information from them. My situation is a little different as I have a 1978 tri-level house that has a furnace on the second floor only so we can remove that from the equation, I believe. I know how many therms I used per month but it's difficult for me to guess if the therm's are being used by the boiler or my gas furnace (see my dilemma?).

The first floor (1st zone) (900sq ft with 1 bed, 1 bath, and a living room. 100 feet of baseboard)
Third floor (2nd zone) (900 sq ft with 3 bed, 2 baths. 80 feet of baseboard)
Garage (3rd zone) 2.5 car with 45k btu modine hydronic heater running off boiler (heated to 45-50F only)

Gas bill was over $500 in January so I insulated the attic (previously had R19 in attic) and now it's R39. All windows and doors have been replaced with updated efficient models. My current boiler is original to the house and is 200k BTU input, 160k BTU output which seems VERY overkill. If I'm not mistaken I have my house needing just under 80k BTU to heat.

I have had a few contractors come in and they all seem to want to install the Navien 240E combi. After doing all my research and calculations I'm looking more at a mod con boiler with a direct fired 50 gal as we have two small children who need baths.

I was looking at the Lochinvar knight boilers and didn't know if the wall mount/floor mount made a difference. Also I'm thinking the 110N may be better than the 80N because the 110N has a net BTU of 89. Am on I base to make the correct decision here? I'd hate to have a boiler that can't warm my house during these -10 nights.

 

[Ki23]

I just had another contractor come in to quote an install and said that he determines BTU size by multiplying sq ft by 50, which puts me at 95,000. He recommended a 110 BTU unit which seems to make sense to me. Does this look alright?

 

[Sponsor]

 

[Dana]

I just had another contractor come in to quote an install and said that he determines BTU size by multiplying sq ft by 50, which puts me at 95,000. He recommended a 110 BTU unit which seems to make sense to me. Does this look alright?

That method is utter crap, and will oversize it by at LEAST 2x for an insulated 2x4 house in RI with clear glass storms over wood sash single-panes, and by more than 3x for a newer code minimum house, which I assume this isn't. Even the guys using 25 BTU/hr per square foot in central MA drive me nuts- 50 is over the moon nuts!

A more accurate rule of thumb for an insualted 2x4 framed triple decker would be 15 BTU/hr, but no rule of thumb is good enough to hit the comfort and efficiency sweet spot.

Using 15BTU/hr per square foot would give you 13,500 for the top floor, which might be low, if there isn't much attic insulation, and 13,500 BTU/hr for the first floor, which might be high unless the basement is air leaky as a tennis racquet. That's 27,000 BTU/hr. Adding the middle floor would make it 36,000 BTU/hr.

To get a handl on this, even with the mid-level furnace errors left in the equation, run a fuel use heat load calculation. I think you'll find that water use, second floor furnace, garage, and all, you the house doesn't need anywhere near 80,000 BTU/hr at the 99% outside design temp to stay warm.

After establishing the VERY clear upper bound, run a zone by zone run a Manual-J type load calculation using loadcalc.net, using the most aggressive assumptions on air tightness possible, see what it comes up with. That tool usually overshoots reality by 25-35% even when being aggressive, but it'll put another firm stake in the ground.

For reference, my reasonbly tight but sub-code 2400' 1.5 story 1920s bungalow with ~R20 in the attic , cellulose in the 2x4 walls, + 1600' of R15 foam insulated basement in Worcester MA with mostly original windows and 1980s vintage storms comes in under 40,000 BTU/hr at the local design temp of +5F. Before I tightened it up and fixed the insulation it was around 50,000 BTU/hr. That's a comparable amount of area as your three story, but a much less efficient shape to the house, with a lot more exterior surface area per square foot of conditioned space. At the water temps I'm running I'm radiation-limited to about 45,000 BTY/hr or a bit less, yet the place stays pretty comfortable even at -10F (except for the family room with way too much window area heated only by a radiant floor.)

At a boiler output temp of 180F the 180' of baseboard will emit about 90,000 BTU/hr, not more. The 100' zone emits no more than 50,000 BTU/hr, and it's probably enough baseboard to keep you warm well into negative double-digits at high temp. At condensing temps it'll deliver about 200 BTU/hr per foot- 20,000 BTU/hr for the upstairs, 16,000 BTU/hr downstairs. It wouldn't be insane to forgo the outdoor reset function and run it at a fixed 120-125F out, which would still deliver mid-90s efficiency. Fin tube baseboard output becomes very non-linear at average water temps below 115F, which results in unsatisfactory performance with 100F water, even though it might not need water even that warm for much of the season.

As a replacement boiler you'd probably do pretty well with a fire-tube boiler with a max firing rate in the 50s, but the if that make you nervous the Lochinvar KHB085 would still deliver pretty reasonable service due to it's 10:1 turn down ratio. Similarly HTP's UFT-080W would have you covered too, probably for less money than the Lochnivar. Lochinvar still insists on primary/secondary plumbing even on their fire tube boilers (that are fully capable of handling the radiation flows of your simple system), whereas the UFT boilers they even advertise pumping direct as a feature, simplifying the plumbing, and saving the cost of another pump.

Unlike most people installing them you at least have enough baseboard on each zone to not short-cycle an Navien combi into an early grave, the min-fire output is probably way more than than half your design heat load for the whole shebang, and more than the design heat load of any individual zone (except the Modine in the garage, which is also ridiculously oversized unless you leave the garage door open.) Don't even bother.

 

[Ki23]

Dana, thank you for the advice. I downloaded an app by Slantfin and entered all the calculations as best I could. I am showing a heat loss of just under 50k btu/hr according to this. This closely resembles your 15 btu/sq ft measurement earlier. I talked to the contractor into going with the 110 model because he originally wanted to do the 150btu knight boiler.

At low fire the different between the 80 and 110 model seems minimal (8075 for the 80 model, 10,450 for the 110 model). I'd hate to hear the "This is why I wanted to go bigger" line if I have an issue later on. He said if I got the 110 I would be able to fire it down to a percentage when it's not under 0 outside etc but if the temp did drop the 110 would handle it without a problem where the 80 may struggle.

Thank you again

 

[Dana]

The 15 BTU per square foot isn't a measurement- it's a rule of thumb, and only useful for sanity checking other methods. It's the approximate load/area ratio of most tightened up 2x4 framed houses. If you're looking at two floors of 900' each, 1800' total that would be 1800' x 15= 27,000 BTU/hr, not 50K.

Slantfin's tool usually oversizes by about 40%, sometimes more, which is fine if you don't size the boiler any higher than a Slantfin load number. If Slantfin says 50K and you install the 110K you're hitting the ridiculously oversizing level- more than twice the amount of burner you'd need even a 10 below 0F.

Note that at high fire the 110K boiler produces more heat your 180' of baseboard can even emit. It probably only has that much baseboard to prevent the insanely oversized boiler you're replacing from short cycling, not because that's what it needs to heat the place.

Even with the 10:1 turn down fire tube boilers if your real load is under 30K (it should be), the difference in min-fire between 11K and 8.3K has some relevance in how much it modulates during the shoulder season. It's not going to be an efficiency killer- you have enough baseboard to keep it from short cycling, but it's still pretty far from the ideal. The 55K Lochinvars can probably heat your place down to negative double-digit temperature- the 110K can probably cover you down to negative triple-digits. Are you expecting an ice age soon?

If it's one of the Lochinvar water tube boilers with only a 5:1 turn down the 110K boiler is a problem- don't do it! The min-fire output would be way more than half likely load!

Seriously, pull out your January or February gas bill, note the exact meter reading dates and amounts, and post it here along with the first three digits of your ZIP code (for weather data and design temperature purposes.)

 

[Ki23]

It is a fire tube Knight model boiler from Lochinvar. First three digits of my zip are 028. Dec 22-Jan24 I used 363.6 therms. Jan 25 - Feb 22 I used 311.1 therms. I insulated the attic (previously had 6in of insulation and now it's R38) on Feb 20th so that should help. I actually have an app from my electric company that tells me when my boiler calls for power. It runs on average of 4-6 minutes each time it fires up lately. I hope this information helps.

Also, my 80% furnace on the second floor runs for 15-20 min and then shuts off for 15-20 on average.

 

[Dana]

Looks like I need the full ZIP code- 028 covers all of RI except Providence & Cranston! (Not that it will make a whole lot of difference.)

Using data from the weather station at the Newport State Airport from 12/22 - 2/21 they logged 1988.5 heating degree days, and from 12/23-2/22 it was 1996 HDD. Averaging them to null out the meter-reading time of day issue figure it was 1992 HDD.

Over that period you burned 647.7 therms at a presumptive 80% efficiency, for net (0.80 x 647.7=) 518.16 therms of heat going into the heating system (the rest went up the flue.)

That's (518.16 therms /1992 HDD=) 0.26012 therms/HDD or (/24=) 0.010838 therms/degree-hour. Converting to BTU that's (0.010838 therms x 100,000=) 1084 BTU per degree hour.

The 99% outside design temps in RI run about 9-10F, so lets use 10F. The presumptive heating/cooling balance point in the degree-day base is 65F, so you have (65F-10F=) 55F heating degrees, and an implied heat load of 55F x 1084 BTU per degree hour= 59,620 BTU/hr.

And that's for EVERYTHING, including the garage, the second floor with the furnace, the boilr standby and distribution losses (which will go down with a condensing boiler), the domestic hot water, etc. And that was before the insulation upgrade in the attic, which probably knocked at least 1K off the total load.

Using ASHRAE's recommended 1.4x multiplier, if going with a cast iron boiler you would never need anything more than 1.4 x 83,468 BTU/hr, which would allow deep overnight setbacks, and have you covered for Polar Vortex disturbance cold snaps, etc. With a modulating condensing boiler you don't need that much, since you'll usually be running at a constant indoor temp, modulating the boiler at the lowest possible water temperature for maximal efficiency rather than using a setback strategy.

The DOE output of the KHB-085 is 79,000 BTU/hr. That means even before the insulation upgrade the =085 has covered down to an outdoor temp of 65F- (79,000 BTU/hr) / (1084 BTU per degree hour)= -8F before it even begins to lose ground, assuming you were also heating the second floor with the boiler. So you're really good well into negative double-digits F with a KHB-085

Has it ever hit negative double-digits at your house in the past 25 years?

That's not "struggling", that's "right-sized.

The DOE output of the UFT-080W is a bit lower at about 70K, and that could heat the whole shebang down to 0F without losing ground, -10F or colder if you continue to use the furnace to heat the second floor.

You're easy driving distance to HTP's headquarters in New Bedford, MA if it ever crapped out on you, whereas you'd have to drive to Tennessee to give 'em what-for if the Lochinvar crapped out. But the track record of both of these boilers is pretty good so far.

 

[Ki23]

Wow. No wonder no one sizes these things appropriately, that's alot of math! Thank you for that explanation.

I believe my old boiler is running sub 60% efficient. That's according to the energy company who tested it for national grid gas.

It did drop to negative's here during the polar vertex in January but it for about a week where we didn't get above freezing. It def wasn't -8 for the whole time though so that makes sense.

As far as HTP vs Lockinvar....my contractor has never installed HTP, only lockinvar. I'm going to speak with him this week and order that 80k btu model. Thank you for everything, this was fantastic information.

 

[Dana]

Wow. No wonder no one sizes these things appropriately, that's alot of math! Thank you for that explanation.

I believe my old boiler is running sub 60% efficient. That's according to the energy company who tested it for national grid gas.

It did drop to negative's here during the polar vertex in January but it for about a week where we didn't get above freezing. It def wasn't -8 for the whole time though so that makes sense.

As far as HTP vs Lockinvar....my contractor has never installed HTP, only lockinvar. I'm going to speak with him this week and order that 80k btu model. Thank you for everything, this was fantastic information.

If the heating source was only 60% efficient that reduces the implied 99% heat load from ~60K to ~45K, and with the ASHRAE recommended oversize factor a boiler with 45K x 1.4= 63KBTU/hr is more than adequate to cover everything, including the zone heated by the furnace, the garage, and all your hot water.

The KHB-055 would be enough, but it's min-fire input is only 200 BTU/hr lower than the KHB-085. (8300 BTU/hr instead of 85o0 BTU/hr) which isn't enough to worry about. (That's less than the body heat heat rate emitted by one sleeping human.) If your domestic hot water is served by the the boiler using an indirect tank, the faster recovery rate of the -085 might make the upcharge "worth it", if it's a 3 bathroom house (one bath per floor.)

 

[Ki23]

It is a 3 bathroom house. I apologize if I didn't explain it correctly.

My house is a 2600 sq ft tri level. The first and third floors are 900 sq ft each (hence the 1900 sq ft) and the second floor is 700 sq ft but that's covered by the furnace mentioned earlier. My garage isn't factored into those sq ft numbers as it has it's own zone and only runs once in a blue moon (thermometer is set at 45 in the garage)

There are three full bath rooms in the house and the boiler would be heating that indirect. All things considered, like you said, the 085 should be plenty without being grossly oversized.

Thanks again

 

[Dana]

You may be in for some push-back from the rules-of-thumb type contractors- especially a contractor who initially proposed a ~150,000 BTU/hr boiler. But stand your ground- show them them the math if you have to.

 

[Ki23]

That method is utter crap, and will oversize it by at LEAST 2x for an insulated 2x4 house in RI with clear glass storms over wood sash single-panes, and by more than 3x for a newer code minimum house, which I assume this isn't. Even the guys using 25 BTU/hr per square foot in central MA drive me nuts- 50 is over the moon nuts!

A more accurate rule of thumb for an insualted 2x4 framed triple decker would be 15 BTU/hr, but no rule of thumb is good enough to hit the comfort and efficiency sweet spot.

Using 15BTU/hr per square foot would give you 13,500 for the top floor, which might be low, if there isn't much attic insulation, and 13,500 BTU/hr for the first floor, which might be high unless the basement is air leaky as a tennis racquet. That's 27,000 BTU/hr. Adding the middle floor would make it 36,000 BTU/hr.

To get a handl on this, even with the mid-level furnace errors left in the equation, run a fuel use heat load calculation. I think you'll find that water use, second floor furnace, garage, and all, you the house doesn't need anywhere near 80,000 BTU/hr at the 99% outside design temp to stay warm.

After establishing the VERY clear upper bound, run a zone by zone run a Manual-J type load calculation using loadcalc.net, using the most aggressive assumptions on air tightness possible, see what it comes up with. That tool usually overshoots reality by 25-35% even when being aggressive, but it'll put another firm stake in the ground.

For reference, my reasonbly tight but sub-code 2400' 1.5 story 1920s bungalow with ~R20 in the attic , cellulose in the 2x4 walls, + 1600' of R15 foam insulated basement in Worcester MA with mostly original windows and 1980s vintage storms comes in under 40,000 BTU/hr at the local design temp of +5F. Before I tightened it up and fixed the insulation it was around 50,000 BTU/hr. That's a comparable amount of area as your three story, but a much less efficient shape to the house, with a lot more exterior surface area per square foot of conditioned space. At the water temps I'm running I'm radiation-limited to about 45,000 BTY/hr or a bit less, yet the place stays pretty comfortable even at -10F (except for the family room with way too much window area heated only by a radiant floor.)

At a boiler output temp of 180F the 180' of baseboard will emit about 90,000 BTU/hr, not more. The 100' zone emits no more than 50,000 BTU/hr, and it's probably enough baseboard to keep you warm well into negative double-digits at high temp. At condensing temps it'll deliver about 200 BTU/hr per foot- 20,000 BTU/hr for the upstairs, 16,000 BTU/hr downstairs. It wouldn't be insane to forgo the outdoor reset function and run it at a fixed 120-125F out, which would still deliver mid-90s efficiency. Fin tube baseboard output becomes very non-linear at average water temps below 115F, which results in unsatisfactory performance with 100F water, even though it might not need water even that warm for much of the season.

As a replacement boiler you'd probably do pretty well with a fire-tube boiler with a max firing rate in the 50s, but the if that make you nervous the Lochinvar KHB085 would still deliver pretty reasonable service due to it's 10:1 turn down ratio. Similarly HTP's UFT-080W would have you covered too, probably for less money than the Lochnivar. Lochinvar still insists on primary/secondary plumbing even on their fire tube boilers (that are fully capable of handling the radiation flows of your simple system), whereas the UFT boilers they even advertise pumping direct as a feature, simplifying the plumbing, and saving the cost of another pump.

Unlike most people installing them you at least have enough baseboard on each zone to not short-cycle an Navien combi into an early grave, the min-fire output is probably way more than than half your design heat load for the whole shebang, and more than the design heat load of any individual zone (except the Modine in the garage, which is also ridiculously oversized unless you leave the garage door open.) Don't even bother.

Dana,
When you run it at 120-125 out...will this be hot enough to heat the fins on the baseboard? I have the unit installed and I’m having issues with it going into lockout. The “outlet” is getting too hot and it shuts off. It’s set at 176 until they can install the outdoor reset

 

[Dana]

Dana,
When you run it at 120-125 out...will this be hot enough to heat the fins on the baseboard? I have the unit installed and I’m having issues with it going into lockout. The “outlet” is getting too hot and it shuts off. It’s set at 176 until they can install the outdoor reset

If it's going into lockout the radiation may not be getting sufficient flow, or the primary loop may be overpumped.

You can try setting it to some lower fixed temperature output- you will never need anywhere near 176F out to heat those zones (even when hell freezes over.)

At an average water temperature of 120F (125F out, 115F back) typical baseboard puts out about 200 BTU/hr per running foot of baseboard, about 1/3 of what it delivers at an average water temp of 180F (190F out, 170F back.) Use this chart for reference.

(I hope the new boiler is something smaller than a ridiculously oversized -110. Even the KHB-085 is probably close to 2x oversized for your likely loads. But even if the -110 is ridiculously oversized it isn't as bad as LUDICROUSLY oversized like the -150 initially proposed.)

 

[Ki23]

If it's going into lockout the radiation may not be getting sufficient flow, or the primary loop may be overpumped.

You can try setting it to some lower fixed temperature output- you will never need anywhere near 176F out to heat those zones (even when hell freezes over.)

At an average water temperature of 120F (125F out, 115F back) typical baseboard puts out about 200 BTU/hr per running foot of baseboard, about 1/3 of what it delivers at an average water temp of 180F (190F out, 170F back.) Use this chart for reference.

(I hope the new boiler is something smaller than a ridiculously oversized -110. Even the KHB-085 is probably close to 2x oversized for your likely loads. But even if the -110 is ridiculously oversized it isn't as bad as LUDICROUSLY oversized like the -150 initially proposed.)

I ended up getting the Wall hung 085 Knight. (glad we didn't go bigger!) I know we could have went smaller but there's always the possibility of adding an addition etc on this house.

We were getting lockout because of a simple problem with the primary grundfos pump. That problem has been resolved and the boiler seems to be working perfectly now. I'm very happy with the installation.

Also, I was completely unaware that normal 1970s era baseboard would emanate heat at such low temperatures (120s) I was always under the assumption that they would need 165F or higher. I will try lowering the space heating 1 set point temperature to 125 and see how that goes. It's colder at night this week so it should be a good test for it.

Thanks again for the information, I have learned a ton in this process.

 

[Dana]

7-10" baseboard will still deliver some heat with 100F water, but the output becomes unpredictable. Fin tube baseboard relies on convection driven by the temperature difference of the air at floor level and the temperature of the fins/water to deliver the heat. A 100F it needs a taller "stack" than 10" to be able to design reliably with water that cool, since the temperature difference is so low, but taller convectors can work just fine even at fairly low temperatures.

Even 115F can work just fine with some fin tube, but much lower than that it's a bit funky. This is important to consider when setting up the outdoor reset curves.

To be able to get condensing efficiency out of it the entering water temperatures at the boiler need to be 125F or lower. At 125F it'll be about 90% efficiency, but that rises fairly quickly to the low to mid-90s as the entering water temp drops to 115F, falling more slowly as temperatures drop even more. A graphic representation of that looks like this:

 

[Ki23]

So they hooked up my outdoor reset on the new boiler and it's creating the set point super low. I attached an image to show you guys. There is no way my baseboards are giving off heat at 83F. It's been running at 10% for a while now (an hour) and the room doesn't seem to be getting any warmer...any suggestions?

 

[Dana]

Try reading the manual (start on p.23), then program the curve to something more reasonable. The default settings at not-so-cold outdoor temps are super-low, more appropriate for a heated slab type radiant floor than fin-tube baseboard.

Since it isn't really getting very cold at night it won't be possible to fully fine tune the Low Outdoor Temperature, but you should be able to find the range of High Outdoor Temperature that heats the place and doesn't short cycle.

With fin tube baseboard the High Outdoor Temperature setpoint should be set no lower than 115F , and program that happen at an outdoor temp of 60F or 65F. (The default setting is 70F output water @ 70F outdoor temperature) If it's not short cycling with 115F water (it should be fine with 80' of baseboard on the shorter zone) you can experiment by dropping the outdoor temp at which the setpoint hits 115F.

For now set the Low Outdoor Temperature setpoint to 135F, at an outdoor temperature of 0F. (The default is 180F output @ 25F outdoors, which is way overkill.) With 180' of baseboard and an average water temp of 130F (135F out, 125F back) it would be emitting ~45,000 BTU/hr which is probably way more than your actual load at 0F. When colder weather arrives you can fine tune it to find the lowest temperature that still works.

 

[Ki23]

Dana, just updating on this post. My boiler has been working great all spring, summer, and so far this fall. I've been heating the house for the past few weeks (we have a three month old) with water around 130. It is working great, never ran out of hot water with our 50gal indirect. The summer gas bills have been under 30 dollars as compared to 45 with the old gas fired water heater.

My only complaint is my garage heater. It needs water hotter than 130F to heat my garage. I can get the aquastat to turn the fan on but it doesn't blow hot air due to the low water temperature. Depending on the cost, I may install a second temperature loop to get water of 170F flowing to the Modine hydronic heater. The installer said the lochinvar is capable of having different temperatures for different loops.

 

[Dana]

My only complaint is my garage heater. It needs water hotter than 130F to heat my garage. I can get the aquastat to turn the fan on but it doesn't blow hot air due to the low water temperature. Depending on the cost, I may install a second temperature loop to get water of 170F flowing to the Modine hydronic heater. The installer said the lochinvar is capable of having different temperatures for different loops.

That's true- the new Lochinvars will support something like 4 different zone-dependent water temps, so if it takes 150-180F water to heat the garage zone, so be it.

If the garage currently isn't air-sealed & insulated it might be "worth it", if you don't already have 10 other house projects on your plate.

 

[Ki23]

That's true- the new Lochinvars will support something like 4 different zone-dependent water temps, so if it takes 150-180F water to heat the garage zone, so be it.

If the garage currently isn't air-sealed & insulated it might be "worth it", if you don't already have 10 other house projects on your plate.

The projects never stop! The garage is a 2.5 car space under my main living area. There is insulation in the walls and ceiling and a concrete slab for a floor. I have two garage doors that I will be replacing within the next year or so.

I'm hoping I just add a loop without changing too much of the electrical work. The manual calls for a "multi-temp loop control" module but I am hoping the one I was using previously will work. We will find out!