Over sizing a HTP UFT boiler in anticipation of basement remodel?

[regrebleo]

I am interested in replacing my aging boiler with one of the HTP UFT boilers. Based on two years of heating data run through Dana's formulas I have calculated a load of about 45k. I sanity checked this with a heat loss which came in at around 50k. Adding the ASHRAE 1.4x multiplier it gets me up to around 75k so the UFT-80W seems perfect. That said, at present I have an unfinished basement that I imagine in about 5 years or so will be at least partly finished. With this in mind, would it be prudent to get the UFT-100W instead so I have a little more headroom and will definitely be able to support heating the new space. I would usually not considering upsizing but because the UFT has a 10:1 turn down ration, the difference between the on the low end is just 2k (8k vs 10k). So while I will be taking a efficiency hit in the shoulder season over the next few years, it seems like a better option than having to buy a new boiler in five years if the 80w cannot keep up with the extra load.

Or, does the ASHRAE multiplier provide me with enough headroom that I should stick with the 80W?

I understand that this is not a simple question because of all of the heat load variables involved in the current basement, vs the finished space. To that end, at present the space is not technically heated, but does contain a cast iron boiler and the over sized pipes of an old gravity conversion. These provide quite a bit of heat to the space so I would say that in the winter the basement is currently 10 degrees colder than the thermostat upstairs. In the eventual remodel, these pipes would be replaced, and the new boiler will have much lower jacket loss, but since that is not the case now, and my heat load calculations have included the heat that has been leaking into the basement, the 10 degree difference at the 45k load value is valid, correct?

Anyway, the simple question is, since the UFT boilers have such a large turndown ratio which makes the difference between the 80k and 100k only 2k on the low end, does that make them a more valid candidate to oversize?

Thanks in advance for any comments. This is a wonderfully useful community.

 

[NY_Rob]

The length of fintube (not length of total piping) in your shortest zone will determine if you can go with 10K BTU low fire or you'll need to go down to 8K BTU's.

 

[Sponsor]

 

[regrebleo]

NY_Rob, thanks for your quick response. I should have clarified that my heating is cast iron radiators all on one zone. Cheers.

 

[NY_Rob]

^ Sweet! Sounds like a perfect combo.
FWIW- you can limit the high fire rate on the HTP UFT boilers - that 100K boiler can be derated to 50% of 100K BTU output if desired.


If you're considering an indirect tank for domestic hot water.... the 100K UFT would heat the water a little faster.

 

[regrebleo]

It is interesting to hear that the UFT boiler can be derated up to 50%. I am a little confused as to what advantage that offers however. Doesn't a mod con constantly derate itself to match the load? If I went with the 100k wouldn't it just never reach high fire rate since it would not need to? This is assuming my system is flying off the outdoor reset curve. Would derating the boiler allow a little more flexibility in tweaking the outdoor reset curve? Or does it offer other advantages I am not aware of. Cheers.

 

[NY_Rob]

Limiting the fire rate can stop the boiler from reaching it's high limit too quickly because it can't modulate down fast enough once the supply water makes a complete loop and the DT between SWT and RWT narrows.

The boiler will ramp all the way up to 80K or 100K BTU's quickly if it sees a big difference between target temp and SWT. Unfortunately, it doesn't modulate down fast enough in some cases. Limiting the fire rate just stops the boiler from throwing everything it's got at the heat exchanger only to find it overshot the mark.

With the mass of your CI emitters, you may be able to run at 100% fire capacity, but it's nice to know you have the option of derating if desired.

I currently have my UFT-80W limited to 60%, I'm keeping an eye on it... I may go to 50%, but we haven't had enough cold days yet to make a proper observation.

 

[Dana]

The low-fire numbers are key to determining the lowest water temperature at which it begins to cycle. Derating the high fire of a bigger version does not change the low fire output.

Stick with the smaller boiler- the ASHRAE multiplier is already overkill, since the fuel-use numbers are probably higher than reality (since old oversized boilers don't actually hit their nameplate efficiency numbers.)

When you get rid of the cast iron beast and seal up the flue you will have reduced the 24/7 parasitic heat load flue-driven outdoor air infiltration, and when you finish the basement you will likely insulate the basement, reducing the load even further. Odds are pretty good that your true load is more like 40K, now, and would be under 40K (even when heating it to a higher temp) after turning it into insulated living space.

An 80K condensing boiler heats 50 gallon indirect in about half the time of a standalone 50 gallon tank(!)- you'd hardly notice the difference in recovery time by going with a 100K boiler. To swing from a tepid-showering temp of 100F to a fully recovered storage temp of 140F takes about 16,000 BTUs. With 70K/hr of output (non-condensing) that takes 16K/70K hours = less than 14 minutes, and less than 5 minutes for it to recover to a piping-hot showering temp. With the 100K boiler it would take 4 minutes to recover from tepid to piping-hot showers- do you really care? In practice an 80K boiler would sustain a 24/365 full flow shower if it didn't need to pause to heat the the house every once in awhile.

Describe the basement in it's current condition, in terms of how much above-grade exposure on the walls, insulation levels, and the average temp in winter. Are there radiators in the basement right now?

Basements don't balance well with above grade rooms when operated as single zone, so you'll probably want to zone it separately when you turn it into living space. With the outdoor reset curve dialed in you can probably still keep it as a single zone from the thermotat upstairs, micro-zoning the basement by giving it a thermostatic radiator valve & bypass.

 

[BadgerBoilerMN]

The ASHRAE multiplier is for cast iron. In other words, obsolete.

Bumping up the boiler output for DHW is also old-school. Size the water heater to the largest tub or as you would a conventional water heater.

Don't mix old school with a ModCon boiler.

 

[Dana]

The ASHRAE 1.4x oversizing factor is also really an upper bound or "optimum" for cast iron, not a minimum. I'd never up-size a mod-con to clear that hurdle, but I might upsize it to be at least a 1.1x oversizing factor.

My current system is radiation limited to about a 1.25x oversizing factor relative to the Manual-J or fuel use load calculation at the fixed ~125F water temp I'm running. Earlier this year we experienced the coldest temperatures of the past 35 years (more than 20F cooler than the 99% temperature bin) and I didn't feel a need to bump up the system temp, or even grab a sweater, though one of the rooms dropped below 65F (too much window area, or not enough radiant floor) for several hours.