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# Modular Home

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Member Posts: 12
I recently purchased a modular home. It's a two-story, around 2100 ft², with 87 ft of baseboard broken up into two zones (44’ upstairs, 43’ main floor) run with ¾” ProPex. I'm trying to come up with an efficient, reliable set up that will run on well water. The daylight basement has in floor radiant, 3-zones with approximately 500ft of 5/8” oxygen barrier PEX. I also need domestic hot water. I’d like to use a propane boiler and a 40 gallon indirect water heater. Any Idea what size boiler I need? Also, what do I need for circulator pumps and zone valves?

• Member Posts: 23,448
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At the risk of sounding like a broken record... the first thing, the very first thing, you need to do is to figure out what the heat loss from the building is on a design day. There are some very good calculators on the web for that, and hopefully one of the other guys will chime in with a link -- which I don't have.

Then you can figure the boiler size and manifolds and pumps and valves -- but not until.
Br. Jamie, osb
Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
• Member Posts: 7,379
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Have you done a heat loss calc? There's no way to know how to size anything without knowing the load. The load calc is the foundation for sizing everything in the system.
Bob Boan
You can choose to do what you want, but you cannot choose the consequences.
• Member Posts: 12
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Sorry, according to my calculations I need 105000 BTUs for the house and 25000 BTUs for the basement.
• Member Posts: 7,573
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Which calculator are you using?
The maximum you will get out of the base board you have installed is approx 550 per lin foot. A radiant slab will max out at about 30 btu/ft.
Heat losses over 25 btu/ft are really unusual. Most houses built to code end up in the 15-20 range.
With a combination of high and low temp zones, you will need to pay careful attention to the design.
"If you can't explain it simply, you don't understand it well enough"
Albert Einstein
• Member Posts: 7,379
edited October 2015
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I agree with Zman: the numbers that you posted come out to 62 btus per square foot. I doubt if you'd need that much in the artic circle.

500' of tubing @ 9" O.C. would mean you have about 355 square feet of slab in the basement. I've never seen a basement slab that needed 30 btus per square foot, but if it did, you would have an output of about 10.6k btus. Floor coverings could reduce that. Obviously, that's far less than the 25k that you stated.

Also, attempting to split a slab that size into three micro zones is not a good idea and can cause major short cycling of the boiler while producing no apprecial benefit.

Please confirm your numbers so we can help get you where you need to be.

Bob Boan
You can choose to do what you want, but you cannot choose the consequences.
• Member Posts: 12
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I used the boiler BTU calculator on supplyhouse.com I'll check out SlantFin load calculator with my numbers.
• Member Posts: 7,379
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rparent said:

I used the boiler BTU calculator on supplyhouse.com I'll check out SlantFin load calculator with my numbers.

That thing is totally unscientific and usually calls for more than twice the size needed. It's not even a good "guesstimater".

Could it be that they want to sell the larger boiler because that means more \$\$ in their pockets?

Bob Boan
You can choose to do what you want, but you cannot choose the consequences.
• Member Posts: 12
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Thanks for all the quick feedback. So I just input my data into USBoiler heat loss calculator (didn't have a smart phone for the slantfin app). I came up with 73500 BTUs for the main house. That's with 100F temp differential, I live in north western Maine. Still not sure how to calculate the basement. I did double check the amount of pex in my basement floor, it's closer to 800 linear feet 12" oc. The storage areas of my basement do not have pex 12" oc, the basement is approximately 1000ft2.
• Member Posts: 7,573
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I still having trouble with the number. You have 87 feet of baseboard heat at 550 BTU's per foot, you could not possibly distribute more than 47,850 Btu/hr even with a million btu boiler. Your actual heat loss number is probably lower than that. I am not familiar with the calculators you are using but the numbers they are spitting out are too high.

Another detail you should be aware of is you will need create a 2 temperature system as the slabs require a lower temp than the baseboards. It should be controlled in a way that restricts the amount of heat the slabs can take from the system particularly at startup.

What types of fuel are available?
What boilers are you considering?
"If you can't explain it simply, you don't understand it well enough"
Albert Einstein
• Member Posts: 12
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I just heard back from my modular manufacturer. The heat loss for the first and second floors performed by them is 35522 BTUs. I was way off! If that is the case how large should my boiler be considering the radiant slab in the basement. I do not have a chimney and would like to heat the house with a high efficiency propane boiler. I've been looking at the Bosch Greenstar condensing boilers but now they all seem too large for my application. I'm definitely open to any recommendations. Also, what hardware will I need to create a high and low temp system?
• Member Posts: 7,573
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The basement will also need a heat loss calc. On average, basements come in somewhere around 15 btu/sq foot.
The bosch boiler has an aluminum heat exchanger. My preference would be a model with a stainless steel firetube exchanger.
Some models are capable controlling a mixing valve to handle the 2 temps, others require 3rd party controllers.
Who is installing this?
"If you can't explain it simply, you don't understand it well enough"
Albert Einstein
• Member Posts: 12
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Zman, thanks for your help. Can you recommend any boilers with stainless heat exchangers? The Weil-Mclain WM97+ 70 CT appears to fit my needs. I'd really like to install the system myself.
• Member Posts: 7,573
edited October 2015
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That model has my preferred heat exchanger.
The important part about brand selection is to find one that the installer is comfortable with and has good local support.
Your project really needs some professional help. I am sure there are parts of it you could do to save some money. Installing the boiler is not a very good DIY project.
"If you can't explain it simply, you don't understand it well enough"
Albert Einstein
• Member Posts: 12
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I feel pretty confident installing the system. I just need some design help.
• Member Posts: 7,573
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I sounds like you should sketch up what you have in mind and post it. The manufactures often have recommended piping layouts that are good starting point.
"If you can't explain it simply, you don't understand it well enough"
Albert Einstein
• Member Posts: 1,105
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Caleffi has a webinar on YouTube about piping I believe. @hot rod
• Member Posts: 12
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I've done a little research, here is a design I made. Feedback/suggestions welcome! The top two zones will supply hydronic heat to the top floors. The right/bottom zone supplies the in slab radiant.
• Member Posts: 7,573
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That is pretty darn close.
The primary/secondary piping where you have the pink line drawn needs some work. See the attached detail.
You will need a flow check valve on all the circulators including the one in the boiler cabinet.
Be sure to do your homework on the mixing valve selection. The ones made for domestic hot water have too high a resistance for radiant heat. Your system would perform better with a smart mixing valve like the taco I series.
You would use both ch1 and ch2 in the boiler then match the outdoor reset curve for the low temp zone in the boiler to the one on the mixing valve.

More homework...
"If you can't explain it simply, you don't understand it well enough"
Albert Einstein
• Member Posts: 12
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Here is a revised drawing w/ an additional check valve and an attempt at correcting the piping dimensions. I'm using 1" copper for the primary piping. Does this look any better? Thanks
• Member Posts: 7,573
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The primary/secondary should be piped like the drawing. 2 tees in a straight pipe...
"If you can't explain it simply, you don't understand it well enough"
Albert Einstein
• Member Posts: 884
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In a purely schematic sense you could consider your drawing to be correct, however getting the tees right is critical. They must be side by side in a straight length of pipe. You should revise your schematic to show that. In other words the only thing you need to do is leave the pink line in place, and then bring the supply header and return header going to the system, close together and coming off the pink line instead of being in line with the boiler supply and return as they are now. Like this (just turn it 90 degrees clockwise). Sorry if that's confusing.

Getting this right is crucial to making the system happy. Other than that your drawing looks good, as long as you use that boiler (triangle tube solo). It has a built in boiler pump, if you use a different boiler that does not have that feature you will have to add a pump on the boiler loop.

• Member Posts: 12
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Thank you everyone for your help with this! I apologize for my ignorance guys, I am having difficulty understanding Zman's primary secondary schematic. I modified the drawing again based on your input, would you mind taking another look?
• Member Posts: 7,573
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Ack! No not like that.
The idea is to have the 2 loops hydrolicly separated. The problem with the original drawing was thar it was easier
"If you can't explain it simply, you don't understand it well enough"
Albert Einstein
• Member Posts: 12
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Zman,
Here is a drawing w/ a secondary series piping arrangement. To me, this looks similar to your drawing.
• Member Posts: 7,573
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Try this

"If you can't explain it simply, you don't understand it well enough"
Albert Einstein
• Member Posts: 2,766
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Wanna save some cash and complexity and have a really good system ? look at HTP UFT80 boiler and a SuperStor indirect . Lowest low end modulation , No primary secondary required , less complexity and better seviceability of the indirect tank . You will really appreciate this set up and less work and upfront cost .
You didn't get what you didn't pay for and it will never be what you thought it would .
Langans Plumbing & Heating LLC
732-751-1560
Serving most of New Jersey, Eastern Pa .
Consultation, Design & Installation anywhere
Rich McGrath 732-581-3833
• Member Posts: 12
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Thanks a lot! I appreciate everyone's help and advice.
• Member Posts: 7,573
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No problem.
I should add that you need a way to purge the air from the system.
Hose bibs on the radiant manifolds will take care of that side. You will need something for the rest as well
"If you can't explain it simply, you don't understand it well enough"
Albert Einstein