boiler choice for small house

Sweetheart

I want to replace my 34 year old gas boiler and take advantage of good rebates currently available. My house has about 1000 sq feet of living space to heat. It is a split entry, living area on 2nd story, lower story is unheated slab for laundry, storage, garage.
I did my own two heat loss calculations. One followed steps specified by Chester on Feb 9 on this forum, using two months of utility bills, heating degree days for my town found online, and outdoor design temp for my locale (also found online). I assumed that my current boiler is 80% efficient. The other used the calculator available at the US Boiler web site. I estimated that my 34-year-old house is neither extremely drafty nor tight. My results were: 26,460 MBH and 29,869 MBH respectively. These are in line with the "rule of thumb" I saw somewhere on this forum of 30 MBH per 1000 sq feet.
I got proposals from three contractors for a total of 5 different boilers, each also with an indirect DHW. Here are the boilers and their MBH ratings:

1. modcon Burnham Alpine 080W : input 16-80 MBH
2. modcon Viessmann 100 : 37-91 MBH
3. modcon Buderus GB 142/24: output 22.7 - 75.2 MBH
4. conventional (not modcon) Weil-MCLain GV90-3: Input 70 MBH, Heating Cap 65 MBH, Net AHRI 56 MBH
5. Bosch Greenstar Zwb28 (I don't know if this is similar to the first 3 or not. Online specs say it has a modulating fan...): Input 101 MBH, Heat capacity 91 MBH, Net Rating Water 79 MBH

I don't know how to compare the specs for the Weil-McLain and Greenstar. This is what the web sites showed.

I am most interested in one of the first 3. BUT - I am concerned that even though each is the smallest of its model series, they may be oversized. That is, should I look for a boiler where the low number is lower, e.g. around 10 MBH? How do I compare the numbers when one manufacturer gives the input, another gives an output? I don't know which it is for the Viessmann numbers. I am considering putting an addition of approx 400 sq feet to the living space (and an unheated garage space underneath) but that is down the road.
Should I remove any of these choices due to size alone? Can anyone recommend a more appropriate boiler, esp. regarding size?

Thanks for any ideas! I know next to nothing about plumbing except for my recent research online.

NY_Rob

Your location?
Type of heat (baseboard?)
Number of zones?
Length of finned baseboard for each zone?
Total loop length of each zone?

"30 MBH per 1000 sq feet" is quite high, a "modern" home should be about half of that....
Was your gas bill/usage for the period actual or estimated?

Paul S_3

Did the contractors themselves do a heat loss.... all seem oversized

SWEI

Shorter answer: Even at 30 BTUs per square foot, the smallest mod/con boilers currently available produce more heat than your house needs on the coldest day of the year. The minimum firing rate is the most important factor here.

Aaron_in_Maine

Those all look too big. How many zones do you currently have? Do you need to make hot water? I would not choose based on the boiler choose based on the contractor.

Dave H_2

Sweetheart,

Your boiler at this point is, yes, oversized for the heating load. However, the biggest factor here in sizing the correct boiler is the indirect DHW tank.
Yes your heating load is small, but the DHW needs may be larger which can then influence the size of heating appliance.

Dave H.

jonny88

Dave a ?.So now we have to oversize our 95% efficient mod/con for dhw.Now to ensure this doesnt cycle itself to death in a few years we have to put in a buffer tank.All this adds up and is a very hard sell as opposed to traditional water heater and CI boiler which the average household has.How can we make it more of a selling point when the competition gives the HO a water heater and Series2 or ES2.I have lost countless jobs over this.Sorry to derail thread.

Dave H_2

I agree jonny88, it does get a little crazy out there, I was just wanted to give the homeowner the reason why her boiler was much larger than the heating load.

I'm ecstatic that a heat load was done in the first place! Even though we are looking at a 1000 sqft home, maybe there is a large DHW demand, large soaker tub.......car wash shower.......

Not derailing the thread, this conversation needs to happen more often than not. Heat loads are much smaller than most realize and lots don't believe the numbers.

Dave H.

RJMCTAFO

I have found that a good quality 40 gallon indirect run at 145* wil satisfy the vast majority of homes. These are being driven by either a Biasi B10-3 or a Buderus 115-3. Have never had a customer complain about a loss of water temp. I had a customer with an 80 gallon soaking tub. Put in a 50 gallon tank run at 145* for more storage with the same small boiler. No issue.

I would rather see the storage size made larger and have a longer recovery than an oversized boiler.

SWEI

My current 'working rule' is to choose a boiler with a minimum modulation rate that is one third or less of the design day heat loss number.

A maximum rate of 80-85k will provide double the recovery rate of a standard gas tank water heater.

Robert O'Brien

It's a tough situation, selecting on purely turndown and no other factors implies every boiler is equal other than that and we all know that is far from the case. What good is 10-1 turndown without a good rep, inventory at wholesalers and parts support? Not to mention overall quality of the unit in general.

Jean-David Beyer

"1. modcon Burnham Alpine 080W : input 16-80 MBH "

I have an 1150 square foot CapeCod type house. I have a mod-con that runs 16K to 80K (input) Btu/hr, and also heats my domestic indirect hot water heater. I estimate it is about twice as large as it needs to be, but W-M who made it makes no smaller mod-con at the time I bought it.

I now heat my indirect to 140F (minimum, though it can go up to 160F sometimes), with a temperature control valve so it delivers at most 120F to the nearest tap (my shower). The indirect rarely runs more than 10 minutes at a time.

Design temperature is 14F around here, though I have seen the outside temperature go down to about 3F once this year and once last year. Perhaps for only an hour or so each time.

Zman

I would not recommend upsizing the boiler for DHW in this case.
You do not need to add the heating and domestic loads because the domestic can run as a priority.
Keep in mind that a typical electric water heater has an output of around 16k/btu hr. Most gas models are around 30k/btu.
All the boiler choices you are looking at will have more capacity than that. Yes, they are all too big!
If you need more hot water, get a bigger tank.

If you multiply the Input rating of the boiler by it's efficiency, you will get the output rating.

Sweetheart

More information...from someone who once spent two days choosing a bathroom faucet. I'm fussy after reading complaints related to oversize boilers.

Yesterday I asked all 3 contractors for their heat loss calculations. One gave me a Manual-J result of 61,434 MBH for an outside temp of 2 degrees F, indoor temp 70 F. He had estimated a floor area of 1440, for a house that measures 40 x 25 on the outside. OK, I could say 1440 is the size I will have if I extend one end with an addition. But comparing his calculations with the calculations used on the US Boiler site, the heat loss factors he used seem much too high. They seem to assume very little insulation. For example, his heat loss from 218 sq ft of glass windows and glass doors came to 18,861. This means he used a factor of 86.5. I have storm windows and a double-paned sliding glass door. The US Boiler calculator suggested a factor of 34 for storm windows. Big difference. Who's right?

The ultimate reason I doubt the 61K MBH is that my calculation based on actual, not estimated, usage (bills for Jan 14 - Mar 15) yielded 26,460 BTU/hr. This used a method recommended on this forum by Chester on Feb 9. I assumed 80% efficiency for my current boiler. That said, there are some reasons why my calculated result might be too low and that the best number is between the two results, but closer to mine.

First, I'm an energy miser. I use a programmable thermostat set to 63F at night but during the day have it at 69-70 about half the time. I'm retired but not always home. So my figure of 26.4 MBH might be a low estimate for the load on a modcon system that "wants" to keep the house at a steady temperature of 70.

Second, I should probably bump up my number to account for future addition of 400 sq ft. Yet it's hard to believe that adding 400 sq feet to my living area (assumed in the contractor's calculation, based on his square foot numbers) and keeping the indoor temp at 70 would double the heat loss. Oh, my actual usage includes gas-fired DHW. By how much should I bump up my heat loss estimate to account for a steady indoor temp and 40% living area increase?

Is it possible to choose a boiler such that it is not oversized but can still handle a future 40% increase (going from 1000 to 1400 total) in living area? No change in DHW needs.

To answer a previous question on this thread, I live in eastern MA about 20 miles from the coast and have baseboard heat.

Another factor is that I want my new boiler to handle an indirect DHW. How much would that add to the required MBH? There is no chance that I will share my house with teenagers; it's just me and often my boyfriend.

Thanks to all for the feedback so far!

RJMCTAFO

The indirect adds nothing to the sizing of the boiler. A competent installer is going to wire it properly and the boiler will prioritize its demand.

The average house in my area is about 1500 square feet. If you have any sort of insulation in the house the smallest boiler will still be too large.
As has been mentioned I would stay away from boilers that don't provide high turndown.

Zman

If you want one more way to look at this, add up the total lineal footage of baseboard heat and multiply it by 550. This will give you the maximum amount of heat that can be radiated to the indoor space.
You should never size the boiler this way, but it will give you an irrefutably maximum output. If you can't get rid of the heat, there is absolutely no reason to produce it....

Tinman

@jonny88 - If a customer tells me all they want, all they can afford is a Series 2 or another cast iron boiler and I can't change their mind, I give them what they want. A properly piped CI boiler w/ partial reset will give them 30 years of reliable, comfortable heat and a little bit better efficiency than they had. I'm not going to give up jobs.

jonny88

Good point Stephen thanks.

Robert O'Brien

Hatterasguy said:

Robert O'Brien said:

It's a tough situation, selecting on purely turndown and no other factors implies every boiler is equal other than that and we all know that is far from the case. What good is 10-1 turndown without a good rep, inventory at wholesalers and parts support? Not to mention overall quality of the unit in general.

Is there an implication that the two suggested 10:1 boilers mentioned above fail to meet the additional criterion?

If so, please state the facts without talking around the subject.

FWIW, any of the 5:1 turndown boilers are practically worthless without a huge buffer tank. They can't run at any outdoor temperature above 35F.

Hell, even the 10:1 boiler was limited today at 48F. It could just barely run when a single zone (60' baseboard) called.

The entire concept of mod-cons with 20K minimums is flawed.

Hatterasguy said:

Robert O'Brien said:

It's a tough situation, selecting on purely turndown and no other factors implies every boiler is equal other than that and we all know that is far from the case. What good is 10-1 turndown without a good rep, inventory at wholesalers and parts support? Not to mention overall quality of the unit in general.

Is there an implication that the two suggested 10:1 boilers mentioned above fail to meet the additional criterion?

If so, please state the facts without talking around the subject.

FWIW, any of the 5:1 turndown boilers are practically worthless without a huge buffer tank. They can't run at any outdoor temperature above 35F.

Hell, even the 10:1 boiler was limited today at 48F. It could just barely run when a single zone (60' baseboard) called.

The entire concept of mod-cons with 20K minimums is flawed.

The implication is that turndown is everything, and all else being equal, that may be true. All else isn't equal. There are approx 30 competitors in the US mod/con market, they're all the same quality? The statement stands by itself, I have no intention of bashing anyone on a public forum that doesn't deserve it and being unable to get wholesalers to stock your product or having lousy representation is not bash worthy, the products themselves may be great but due to these other factors, I'll never find out.

Chester

It's hard to imagine your heat loss is anything north of 30,000.

Definitely no need to upsize the boiler for DHW, especially in your situation. I have a 30 gal indirect and a 57,000 boiler and have never come close to running out of hot water.

As Aaron says, choose the contractor, not the boiler. Make sure they either understand how to calibrate the outdoor reset curve or are willing to make sure you know how to tweak it yourself.

Or, if you go with a Greenstar or Buderus I can definitely recommend taking a look at Bosch's new line of thermostats. They modulate the boiler based on indoor temp and seem to work as advertised.

bob eck

This Sage control that is designed for the Burnham Alpine and K2 boiler would that work better with either of these boilers for this job?

http://www.usboiler.net/control-system/sage2-1-boiler-control-system.html

Gordy

With the 10:1 modulation now hitting the market. Oversizing becomes more of a mute point. It actually gives the installer a little breathing room for those unexpected below design day events in the future.

However Btu requirements for DHW needs becomes ever more critical when low heat loss requirements are encountered IF you choose a small mod/con with a 7:1, or 5:1 TDR.

Example in this thread would be say the whn, or KHN 50. With outputs at high fire not much more than a standard 40 gal tank style water heater. Will it do the job? yes, but not like what an indirect can offer with a higher btu output from a larger boiler.

This where Loch shines because their 10:1 KHN 85 has a low end not much higher than the 7:1 KHN 50. So you get the low end for low loads in mild weather, and the top end for better DHW production. smart.

jonny88

I will stick with Hatt here UFT 80 30 gallon indirect.Great support which is critical.Lochinvar also has to be looked at Bob have you had luck with the K2 ?

Robert O'Brien

Now K2 experience but plenty of Alpine

Tim McElwain

I would suggest you get in touch with Rich Swatton of Mod Con Services in your area. I am not in my office but when I get in I will send you his phone number.

jonny88

Alpine through Nat Grid or by choice?Heard mixed reviews but again it comes down to install.What indirect are you using I ask because I am having an issue with 2 of the big brands with failure rates thanks.

njtommy

I will second the Lochinvar KHN boiler. Great boiler and low end btus.

John Mills_5

My house is 970 sq ft. I heat it at -15 with a 2 stage gas furnace locked on low so about 35K output. Obviously not heating water but seems like a mod con at max 80-100K input is huge for the house. Sure they can ramp down but is it worth it. If baseboard, are you really going to save enough to justify the mod con price over a good 84% atmospheric furnace? Look at how complex the mod cons are. 1 out of warranty repair could eat up any possible savings over the mod cons life.

Docfletcher

I think you are spot on John Mills. I'm wanting to replace my NG boiler and at first thought the Mod Con would be the way to go. After considering the service requirements and fairly short service life when compared with a good old standard cast iron boiler I feel I can't afford the Mod Con. I did have the lochinvar rep with his supplier of choice out to the house for a cost estimate and never heard back from either one of them or the installer they said would contact me. At 1st I was pissed but now consider myself lucky.

Jason_13

I use the Alpine a lot of times and limit the firing rate to the heat loss. So if they have two zones and the heat loss is 30k we can assume a single zone is around 15k. I realize this is at design temp. I can play with the control settings like extend min fire hold time, change the PID on the fan, drop the light-off rate, diff above and below, turn boost off and even disable DHW priority. I can run a tighter reset curve to extend circ run time. I find that I can possible get a longer run time and another zone calls before boiler shuts off. Of course this does not help much on the shoulder seasons except disabling DHW.

SWEI

John Mills said:

My house is 970 sq ft. I heat it at -15 with a 2 stage gas furnace locked on low so about 35K output. Obviously not heating water but seems like a mod con at max 80-100K input is huge for the house. Sure they can ramp down but is it worth it. If baseboard, are you really going to save enough to justify the mod con price over a good 84% atmospheric furnace?

It depends -- mostly on your fuel costs, but also on the amount of baseboard installed as compared with the design day heat loss. Around here, it would not make economic sense for an NG customer to choose the mod/con, though there are other reasons (venting, comfort, reduction of baseboard expansion noise) that might be important depending on the customer. For an LPG customer, the mod/con would recover the additional first cost in about two years.

JUGHNE

How about the Phoenix light duty.....probably still too big.......76,000 with 3:1 turndown......but 50 gal DHW "buffer tank" feeding a HEX feeding the BB.
Literature say's "aux tap for air handlers"...why would a HEX be different? Plenty of DHW available.

kcopp

Everyone has there favorite unit.
I like the IBC HC 13-50...
Simple and modulates down to 13,500.

Its most important that the installer knows whatever you chose very well.

john p_2

Very, very happy with my new Utica SSC boiler, HTP indirect, Design/Strategy by John & Rich at Langan's Plumbing, support from Utica and extremely happy with my "new" PSE&G bills for my 2800 sq. ft. home, so far $87.83 (avg. this year, Oct. - March)

john p_2

Oh, and forgot to add...that's a family of 4 or 5 depending if the oldest is at college or home.
So plenty of showers, two loads of wash & drying a day and cooking meals equals plenty of gas being used.

CMadatMe

While the preaching word is always do a heat loss, I never, me included see anyone asking whether anyone has actually sat down and calculate the DHW load vs using the, well a 40 always works for me, so on and so forth. We preach heat loss, heat loss, heat loss, but never preach about sizing DHW. Willing to bet 95% of installers out there don't even know how.

Which ever contractor quote the Viessmann, quoted the wrong boiler. Should have quoted a B2HA19 not a WB1B. I would always increase storage before I'd increase boiler size if that increased size is going to effect the low end of the modulation rate. If it's not then it's not a big deal, boiler is only going to go out to the high end of the modulation rate if the system demand needs it to.

jonny88

Honestly Chris I find nowadays especially with peoples large water demands there are a few of us who do account for it.I do get your point though.I know for a fact Rich does.A quickie,a customer insisted on having 4 showers running for 30 mins at the same time.Designer did it,a year later customer complained about high fuel bills but hey he had his water.

gennady

Viessmann Vitocrossal 300 CU3A.
Great boiler, ideal for small houses.

JUGHNE

On the subject of DHW, is there a sizing guide for commercial application other than the AO Smith sizer on line. Doing water heating replacement for hospital and nursing home.

SWEI

ASHRAE Fundamentals has the numbers. Older editions are cheap

gennady

I agree, this boiler is on the high side, but I do believe it is a better boiler. Viessmann did a fine job with increasing heat exchanger surface, allowing to extract more heat from products of combustion, increasing condensation rate, lowering short cycling. Along with lambda pro and increased water gallonage of heat exchanger it is good contender. I installed one and was impressed.