Replacement for 1960's era N.G. boiler....

[NY_Rob]

^ Thnks John.
What's your square footage if you don't mind me asking?

When I mentioned to one burner mechanic (who has been to my house- but admitted he doesn't do mod/con installs) that by calculation of 20 BTU/sq-ft of heated area my 1550 sq ft area comes in at 31,000 BTU loss... he started shaking his head and said that was way off.... when I mentioned the 55k burner he looked at me like I hat two heads and said he'd go with at least 80k input for my house.

It seems the old school mentality of oversize the burner is still alive and well in 2016.

 

[John Molyneux]

My place is a 1940, 1400 sq/ft cape in southern Maine. It's pretty tight and well insulated (and we also get some decent solar gain on a nice day).

I came to the board a year ago to start my research. I was lucky enough to see the wisdom in doing a Dana-style heat loss before I started getting estimates. I also had the opportunity to have a professional Manual J done as part of an energy audit. The Manual J confirmed the DIY results so I was very confident in what I wanted before I started talking to installers.

My historic plumbing/heating contractor wouldn't install anything less than 110,000. I asked him why I'd want a modcon that couldn't even hardly modulate down to my design day. He used the typical 'pick up load' and 'DHW recovery' arguments. The next guy was a little more reasonable but still wanted to oversize it at 80,000. He said he wouldn't take the 'legal risk' of anything smaller. The contractor I ended up using didn't try to argue against my heat loss calcs and agreed to install the Greenstar 57. It was the first time they'd done a 57,000. They completed the installation by the book and with a high level of craftsmanship but then left everything at the factory default settings. Oh well. And it turned out they forgot to bleed two of my rads. Oops.

So yes ... old school is alive and well. We had gas come to our neighborhood this year and lots of us replaced boilers because of all the rebates, etc. that were out there. Most houses are about the same size as mine and most probably have 110,000 BTU combi's with factory default curves that probably don't spend much time in the condensing zone.

 

[Sponsor]

 

[NY_Rob]

^ Thanks John,
What was the size of the boiler you replaced with the Greenstar 57?

 

[John Molyneux]

140,000!!!

 

[Dana]

Yeah you didn't really need a 140K boiler, but you really should have installed at least 100-110K, just in case it gets down to -237F or something- because that can really happen in Maine, right?

On Long Island an 80K boiler would be about right for a 1550' tent with a window or door flap left open. It's a rare sub-code 1550' house would need more than about half the ~65K output of an 80K boiler, assuming it would still qualify for a certificate of occupancy (with luxuries such as glass in the windows, doors that actually close, etc.)

It's better to NOT use xxBTU per square foot as a rule of thumb, but keep it in mind for sanity-checking other estimation methods. For NY/NJ and southern New England for a 2x4 framed house 15 BTU/ft is somewhere near the middle of the bell curve, but its' a fat middle, and there are the exceptions that are truly north of 25 BTU/ft @ +15F, usually small houses with lots of window area, and an uninsulated foundation. When discussing it with contractors present the fuel-use calculated number first stating that its based on weather & fuel use data, then if they protest point out that it's still 20 BTU per foot, which is a typical real-world number for code min houses in Anchorage AK (at -9F!).

 

[NY_Rob]

140,000!!!

Wow, that's crazy!
You went from 140k to 57k and you still are comfortable.

Talk about over-sizing..
My house was only 1150 sq ft when built back in 1964 and it was furnished with a 100k input boiler.
In the mid 80's the original owners converted the garage and added a den which added an additional 400 sq ft to the heating space.. still using the original 1964 100k boiler. They simply tied the original upstairs and downstairs zones together on one circulator and put the new addition loop on a second zone with it's own circulator. And still the 100k boiler had plenty to give.

I'll be going from 100k to 54k and doing the math it looks like I should still be fine.

 

[NY_Rob]

In going over my measurements, I just noticed I left out the 18'x11.5' living room- so that brings the heated area total up to 1758' sq ft.
I didn't include the 18'x11.5' quarter basement (directly under the living room) which is unfinished in my calculations either.

 

[NY_Rob]

Using the now accurate 1758 sq ft heated space figure, and having burned 164 therms between 12/30 to 1/28 with 918 HHD and a real world efficiency estimate of 72.5% (I split the diff between 70% and 75%) for my 52yo boiler that puts my BTU/sq ft ratio at approx 15.35 BTU/sq ft which is pretty much where Dana had mentioned it should be for my house age/construction/location.
Calculated heating load now works out to be a tad under 27k.

 

[Leon82]

The 43 foot zone is going to cycle if you use a low water temp of say 110 degrees. If it runs on its own. How much is hard to say.

In my house my minumum of 110 water temp is good until about 35 degree outdoor temp

 

[NY_Rob]

One last question for tonight...
Considering that my new system will be direct vent/sealed combustion and I'll be sealing off the old chimney- will I get less heat loss in the house due to reduced infiltration?

 

[NY_Rob]

The 43 foot zone is going to cycle if you use a low water temp of say 110 degrees. If it runs on its own. How much is hard to say.

In my house my minumum of 110 water temp is good until about 35 degree outdoor temp

I'm sorry I really don't understand what that means, most of this is pretty new to me.
Regarding that 43' zone- the full loop length out from the boiler and back is almost 170', out of that 43' is actual finned tube baseboard.
The boiler I'm looking at will fire down to 8k BTU/HR.
Hope that helps.... and again, thanks for all your help!

 

[Leon82]

This chart is approximate btu output for baseboard.

So 43 ft x 140 btu per foot for 110 water temp is about 6k btu from the baseboard. So you have 2 k btu that is staying in the water not being emitted. I guess you could bump it up to 120 degrees and you would be OK.

The boiler would turn on and off while it was heating the small zone.

With all that pipe you may bleed off some excess but I don't know for sure.

 

[NY_Rob]

^ Thanks Leon,
Looks like at 120 deg I'll just be slightly above my minimum fire rate of 8K, I'm betting there's some loss in that long 170' run too.
Only the part of it (about 27') that's in the basement is insulated, the rest is most likely just run through interior walls and under the flooring so that's got to be lossy.

 

[Leon82]

With the Lochinvar you can block a new call for heat for 10 minutes after the end of a heat call.

You can also set 6 delays up to 10 min each limiting the fire rate. The last delay limits the firing rate for the rest of the heat call.

If you desired you could get 50 minutes of 20% output before it ramps up to the last limit.

 

[NY_Rob]

^ good info...
Thanks!

 

[NY_Rob]

My contractor came over and took a look at my house/system and calculations, so now I have additional questions.

I generally set the main house t-stat at 68 deg when we're awake then have it set back to 61deg at night for comfortable sleeping conditions. I have the heat kick back on at 68deg at 5:45am so the house is 68deg when we get up at 6:45am.
If I now go with 120deg return water (figure 140deg output water) isn't it going to take a lot longer for the house to get back up to 68deg from 61deg vs. the current 180deg out water coming from my old atmospheric cast iron boiler? If I go with 110 degree return water (figure 130deg output water) can I even set back the thermostat at night?

I've read that the return water needs to be at 130deg or below for condensing, so that would pretty much limit my output water to 150deg but it seems like that would heat the house faster than 130deg output water.

Is this where the outdoor reset comes into play?

I guess I'm mainly concerned with the length of time it will take to heat the 61deg house to 68deg on cold mornings with very low output temps.

Thanks for any replies....

 

[Leon82]

Yes it will take a long time to recover.

I removed my setbacks and have my tstat set to 69 and leave it there. The water temp just keeps up with the heatloss.

If you want a setback the boiler has settings for it. It will raise the temp to recover than go back to lower temps I presume, but I haven't played with those settings.

 

[NY_Rob]

^ so you just keep your house at 69 deg 24x7 even if you're not home?

 

[Leon82]

Yes. This months bill will be the first full one on the new boiler.
I compared my last one with one with a similar avg temp and it was lower sofar

I set my thermostat for a 2 degree diferential so it wont go back on until 67

 

[NY_Rob]

Is this the norm for houses with mod/con systems... leave the T-stat's at a constant temp 24/7 and don't set them back at night or when you go to work for 8-9hrrs, etc?