Boiler sizing using HDD Therms

mtnheat

perhaps well trodden but i'm stuck trying to determine natural gas boiler size. 2000 sq' duplex south facing in CO mountains at 8000' with baseboard heat three zones. Using January 2016 data from the gas company, over 34 days and average temperature of 20 degrees, HDD = 1517, Therms =180. I don't have exact figures for my area for 99% Heating Temperature, Leadville is 2,200' higher with (-14) Eagle is 1,400' lower and (-2). Guessing (which i'd prefer not) that somewhere in the middle would be OK. Current boiler powers 40 gallon TurboMax indirect. Gas usage includes laundry dryer for household of three. I gotten three proposals, two with 110,000 BTU Triangle Tube, one with 155,000 Lochnivar. Everything i read here and elsewhere suggests these are way oversized. Please weigh in. Include the proper formula if possible. Many, many thanks
https://www.energystar.gov/ia/partners/bldrs_lenders_raters/downloads/County Level Design Temperature Reference Guide - 2015-06-24.pdf

nicholas bonham-carter

You can use the SlantFin App, on your android, or IOS tablet, which will take the measurements of walls, and windows, and calculate the heat loss for you on a room-by-room basis, although it will not account for solar gain.
Then it will select a boiler for you, capable of handling the load.
The problem with making a calculation based on gas usage is the other loads, (clothes-drying, DHW, and cooking), which have to be subtracted from the total, to obtain the heating usage.
If you use a lot of hot water, and the boiler will produce it, then a size bigger than the heat loss may be needed, but those boilers do seem oversized, and will probably short-cycle.--NBC

mtnheat

thanks NBC. on the one hand there's the need to account for solar gain and it's variability vs gas usage which would be actually representative and still get a bump (from what i've read 1.4% larger). with only the clothes dryer also using gas, (the existing boiler also provides DHW) say the bump is 1.3ish but we get actual usage and then need to find the boiler that meets that load. i am having problems with the calculation...the proper formula. so far the guys up here balk on any warranty for their work unless their boiler sizing is agreed to.

Henry

The natural gas utility here has a number of charts with rule of thumb annual consumption:
oven 3531 cu ft
dryer 3531 cu ft
hot water 22,954 cu ft

Also for determining heat load:
HxWxL for cubic feet, tight construction is 3 BTU per, old windows and old construction 3.5 BTU per

Using the actual gas bill and going backwards makes the above fairly accurate. Not as accurate as a proper heat loss evaluation but it puts it in the ballpark..

Paul48

I'd think that HDD is a way of figuring averages, but with those extreme cold conditions, you'd better know the low, and be able to satisfy that. I'd do a proper heat loss, and size accordingly. That's my unprofessional, common-sense opinion.

Gordy

Don't forget boilers get derated for altitude.

mtnheat

hi Hatterasguy: in my area code, 81657, 8000' Colorado mountains, degreedays.net HDD data is spotty. when i extrapolate the 7 months available and add it to Eagle County (1400' lower) the HDD sum is 8123. i data from the gas company yields an HDD of 8038 from Sept. 2015 to August 2016. Gas usage for the same period = 977 therms. gas usage for 32 days (July +) = 14 therms. the boiler is NTI T150 with a TurboMax 40 gallon water heater. the clothes dryer is gas serving three people. many thanks for weighing in. i will need this data to get a properly sized boiler. much appreciated.

mtnheat

Hatterasguy: shouldn't a portion of my concern be with max recorded Therms and being able to cover that load with a buffer ? for instance, the data from the gas company shows 180 therms over 34 days and 152 therms over 29 days as the two max readings.

ChrisJ

Hatterasguy said:

We'll use the 8123 HDD as valid.

977 therms for total usage with 168 therms for DHW and cooking. 809 therms for CH annually.

We're going to call the NTI 90% efficient and, therefore it delivered 728 therms to heat the building.

72.8M BTU.

373 BTU/degree hour

I'm going to use -10F as your design day temperature and use 64F as your base temperature. These can be modified with additional information.

Heatloss on design day: 27633 BTUH

28,000 btu/h at -10F in a 2,000sqft building?

ChrisJ

Hatterasguy said:

ChrisJ said:

Hatterasguy said:

We'll use the 8123 HDD as valid.

977 therms for total usage with 168 therms for DHW and cooking. 809 therms for CH annually.

We're going to call the NTI 90% efficient and, therefore it delivered 728 therms to heat the building.

72.8M BTU.

373 BTU/degree hour

I'm going to use -10F as your design day temperature and use 64F as your base temperature. These can be modified with additional information.

Heatloss on design day: 27633 BTUH

28,000 btu/h at -10F in a 2,000sqft building?

Didn't I just calculate it for you? Did you pay attention to the HDD number?

You know sometimes you're as pleasant as shaving with a cheese grater?

Gordy

The ti150 has an output of 136k. That calcs at about 91% eff.

You must derate output by 24% at an elevation of 8000'. That would be 103,360 output. If on propane output drops more. However I see you are using therms so NG.

Gordy

Hat your calculations are based on the boilers "input". When sizing a boiler you use the output rating. In other words his usage is gross, not net. That's what the emitters get to heat the house. So his actual heat loss is lower.

Gordy

The OP has a indirect tank. Recovery will be as good as a standard 40 gallon ng water heater. Which is fine so long as the poster understands the decreased output of the indirect with that small of a boiler. We have no idea of the posters DHW needs.

Gordy

It becomes a little more involved when altitude is a factor. As you can see it takes a lot of the output away from the rating at 8000'. So this must be taken into account. Every boiler is slightly different.

Gordy

I'm just stating for the OP to think about.

Rich_49

ChrisJ said:

Hatterasguy said:

We'll use the 8123 HDD as valid.

977 therms for total usage with 168 therms for DHW and cooking. 809 therms for CH annually.

We're going to call the NTI 90% efficient and, therefore it delivered 728 therms to heat the building.

72.8M BTU.

373 BTU/degree hour

I'm going to use -10F as your design day temperature and use 64F as your base temperature. These can be modified with additional information.

Heatloss on design day: 27633 BTUH

28,000 btu/h at -10F in a 2,000sqft building?

Gordy

Good choice since the nti has a high restriction HX, and was properly piped the cdn should slip right in.

Make sure to get the high altitude model. Derate on that is 16% for 8000'.

Gordy

mtnheat

there is a good thread regarding derating for altitude:
http://forum.heatinghelp.com/discussion/151238/hydronic-baseboard-altitude-derate
spot on Gordy, getting a contractor to install a boiler approaching 50,000BTU up here may be a pipe dream. best estimate at the moment is 110,000 BTU. the west walls of the duplex are a common wall, the north side of the first floor (bottom floor is 3/4 below grade due to slope). the data suggests significant solar gains from the southern exposure. the envelope will be tightened and solar likely to be introduced. unfortunately there is not a readily accessible 99% design temp. extrapolating suggests an extreme of (-5 to -10). if the heat loss is 40,000 BTU an hour, adding a buffer at 1.4% puts a boiler size at 56,000 BTU... basically 1/2 of the lowest bids so far...the existing NTI T150 is 13.5 years in service...likely on the lower side of efficiency for the unit specs. i will need to sell a boiler size to an installer UNLESS they buck up and do the appropriate work and in addition to really good components, definitely need a knowledgeable adept installer. any thoughts ? thanks

Zman

I would not recommend sizing your boiler based only on a HDD number. There are simply too many variables.

The assumptions being used for DHW and other loads are estimations, your seasonal usage may vary a bit.

Additionally, every home is effected differently by the sun and wind. I am certain that there are many days that are technically 10-20 degree heating days where the boiler does not fire at all due to solar gains. At other times, you could have design temps with no sun and lots of wind.

On average it all works out, it just does not give you a very accurate number for any particular day.

I am not discounting the use of HDD as a design tool, I would just caution you to compare those numbers to a heat loss calc before making a decision.

What is the age and general type of construction?

mtnheat

oops, 1.4 x 40,000 BTU =56,000 BTU the proposal for a EFTC-140F seems crazy large top end.
most recent proposal is for a HTP-80 boiler and a HTP Superstor 60 gallon indirect water heater. moving in a better direction...still need to sort the jumps and install..

NY_Rob

The 60 gal indirect may be overkill- which won't hurt, but it's just considerably more expensive than a smaller indirect tank.
I have a HTP UFT-80, I went with a 30 gal SS Ultra indirect and have yet to run out of hot water. I have two college age girls at home, myself and my wife and I've noticed that when someone's taking a shower and the boiler fires up.. it actually satisfies the indirect demand and shuts down while they're still taking their shower. The SS indirect uses 1" boiler piping vs. some indirect tanks that use 3/4", so you can get a lot of BTU's into the SS Ultra's quickly.

mtnheat

thanks NY_Rob...are you pleased with the UFT-80 ? anything you'd do differently ? what is your design temp for comparison. think i'll be using close to (-10) is the SS Ultra setup as a separate priority zone ?

Rich_49

Put your space heating on a relay and the end switch to the UFT or EFT . The boiler has the capability of turning that circ on and run the AquaStat or sensor wire directly to the boiler DHW circ . The boiler will do your priority for you .

Design temp has little to do with anything other than can your boiler make hot enough water to heat the house with your emitters . 50K is 50K ,130* is not 180*

mtnheat

NY_Rob...i have baseboard fin tube with 3 zones, did you go with programmable thermostats for your zones ? appreciate your thoughts on the SS indirect. definitely will downsize from the 60.

NY_Rob

Mtnheat- I only installed the UFT-80 three months ago, so I haven't used it for space heating yet, but it should serve adequately.
My heatloss for our 1960's era 1'700 sq/ft home on Long Island was around 30k BTU's at 15F. The shortest of my three baseboard zones came in at 8.5k BTU's with 130f supply water, so the UFT-80 should be able to handle it without short cycling- hopefully. If it becomes a problem- I can combine zones.

As Rich mentioned, the HTP UFT-80 is already provisioned to have an indirect circulator as it's own priority zone- just wire it up and you're good to go. You can set the amount of time DHW is a priority via the boiler's control panel. I used a temp sensor in the indirect well vs. an aquastat because the boiler software actually makes more accurate fire rate adjustments because it sees the DHW temp from the tank sensor rather than just the on/off info if I used a DHW thank aquastat. I used a Grundfos 3-speed pump for the DHW loop.

My research, advice from here and on other forums- have indicated that programmable t-stats are really not needed or desirable for mod-cons as you really don't do much nite-time setback with mod-cons.
The idea is to just deliver enough constant BTU's day and night to meet but not exceed the heatloss by employing an outdoor temp sensor and a boiler that can modulate it's firing rate based on outdoor temps. If you start using severe setbacks, you start chasing the heat and efficiency/comfort is lost. I used a DP ECM (Grundfos Alpha) pump for space heating.

FWIW- the HTP UFT-80 has been flawless in the three months I've been using it for DHW. It's quiet and efficient. My gas bill is literally 30% lower than the same period last year when I had a stand alone gas fired atmospheric DHW heater.

mtnheat

NY_Rob: curious, why the UFT-80 over the EP-80. any need to hire a system designer or are these foolproof enough ? my setup should be pretty straight forward but i definitely want it done properly. thanks

Rich_49

What is an EP 80 ?

NY_Rob

I just looked up the high altitude notation for the HTP UFT-80W...
"The appliance will de-rate by 4% for each 1000 feet above sea level."
So the 80k BTU boiler is actually a 54k BTU boiler at 8'000 altitude.

Keep that in mind when selecting your new boiler.

mtnheat

Rich: HTP: EP-80 boiler is characterized as a low mass boiler
http://www.htproducts.com/elitepremier.html
the HTP: UFT-80W is characterized as a medium mass boiler...
i'm not sure why to choose one of the other..

mtnheat

NY_Rob: that de-rating makes me a bit nervous but the existing NTI T150 de-rates per 1000' at 3.6%
according to calculations from actual natural gas usage, even with the UFT-80 de-rated to 54k BTU the boiler is over 2x oversized...here's the calculations plus a buffer of 1.4x
Annual: 977 Terms
July = 14 Therms
efficiency 28.8- 91.9 = 63.1%
12 x 14 = 168
977 -168 = 809
50 x 809 = 40,450 x .63.1 = 25,524
25,524 x 1.4 = 35,733

Rich_49

Elite Premier is a water tube boiler . Although all boilers require good attention to water quality and properties the water tubes can bite you in the proverbial **** if not paid very close attention to . They got a very bad rap being the first HXs basically available in the infancy of mod cons . Undeserved I might add as for the most part .
A system designer is never a bad idea . One that has the ability to vett your available contractor pool would be best for your protection .
Project specific drawings are also a great asset as opposed to manufacturer generic stuff .

NY_Rob

If you do end up going with the HTP boiler.. make sure whom ever suggested the HTP boiler is able & willing to maintain and service it.
Also make sure parts are locally available for HTP products.

mtnheat

thanks Rich: i'll stick with the UFT-80 unless it seems too big from my usage calculations.
think i'll inquire with the HTP distributor if he has a system designer.
there was a guy i read about in Minneapolis (badgerboiler or something) that did hydronic design work.
probably some guys on here as well. may need a separate thread.
appreciate your input...all the best

mtnheat

NY_Rob up here there is a love affair by installers with Lochnivar. seems Triangle Tube has gained a foothold as well. there is a distributor in Denver (100 miles) of HTP. one or two guys up here (wish it was a larger pool) that install service. HTP seems kinda plug and play. my NTI has been a struggle finding techs. i'll check with a couple of the supply stores..

Gordy

Also remember your low end modulation gets derated. Don't know much about your zoning requirements.

The potential beauty of the high tdr when talking indirects is having the horse power with a dhw call. Then less power to meet ch demand.

NY_Rob

^ With that in mind, you really need to seriously consider your choice of boiler manufacturer.

I have a HTP distributor literally 1mi from my job... but upon my questioning them- it turns out they don't stock parts for the UFT-80W. You have to press them on the details.

Unlike the old simple cast iron boilers where many parts were common/shared between brands the mod-cons don't have that luxury.
Last winter when I had my 1960's cast iron boiler serviced for the last time before replacement.. the tech said the thermopile was in really bad shape and needed to be replaced before the winter season. He walked out to his truck with the old part in hand- matched it up with the common ones he carried onboard and it was swapped out within 10min.

If that was literally any part from a mod-con the outcome would have been completely different. Even within a brand name lineup- parts aren't shared in many cases.

Here's a good test to see what would be involved getting a common part for a HTP UFT-80w.. call the installer who suggested the HTP boiler- tell them you want to get a spare flame sensor (literally a $5 but crucial part) for that boiler... let's see what happens.

I'm not trying to steer you away from HTP... but if you can't get parts locally- that's trouble waiting to happen.

SWEI

NY_Rob said:

I just looked up the high altitude notation for the HTP UFT-80W...
"The appliance will de-rate by 4% for each 1000 feet above sea level."
So the 80k BTU boiler is actually a 54k BTU boiler at 8'000 altitude.

While it may be in the manual, that number is almost certainly incorrect. I have ASHRAE papers from 1933, 1952, and 2007 on the subject and current consensus among industry experts is something like 1.8% per 1,000 ft ASL for a natural draft appliance. The "high altitude versions" may or may not run better than the standard versions depending on burner design, gas composition, and altitude.

Fan-driven neg/reg appliances typically quote 2% per 1,000 ft but the real number is lower and really depends on the specific fan performance (it's close to zero in some.)

Rich_49

Rich said:

ChrisJ said:

Hatterasguy said:

We'll use the 8123 HDD as valid.

977 therms for total usage with 168 therms for DHW and cooking. 809 therms for CH annually.

We're going to call the NTI 90% efficient and, therefore it delivered 728 therms to heat the building.

72.8M BTU.

373 BTU/degree hour

I'm going to use -10F as your design day temperature and use 64F as your base temperature. These can be modified with additional information.

Heatloss on design day: 27633 BTUH

28,000 btu/h at -10F in a 2,000sqft building?

I could not even imagine being able to find a designer out there . LMAO

ChrisJ

Rich said:

Rich said:

ChrisJ said:

Hatterasguy said:

We'll use the 8123 HDD as valid.

977 therms for total usage with 168 therms for DHW and cooking. 809 therms for CH annually.

We're going to call the NTI 90% efficient and, therefore it delivered 728 therms to heat the building.

72.8M BTU.

373 BTU/degree hour

I'm going to use -10F as your design day temperature and use 64F as your base temperature. These can be modified with additional information.

Heatloss on design day: 27633 BTUH

28,000 btu/h at -10F in a 2,000sqft building?

I could not even imagine being able to find a designer out there . LMAO

All of this just makes me depressed over how bad my house is.

I mean, not depressed enough to spend a lot of money on insulation and go and actually insulate it.

But pretty depressed!

mtnheat

Gordy: (3) zones and two floors (1032 sq ft each). thinking of combining to just have (2) zones. one of the zones only has 10-12' of baseboard.