Boiler sizing question

Redraptor · Oct 28, 2017

Been doing some reading here and havent found my particular situation yet. I just finished my garage, its 936 sq ft with 3 zones of pex tubing in 4 inches of concrete. I ran the number thru a calculator on supplyhouse or somewhere and came uo with a low btu compared to the more popular selling sizes. Would i be better off with a HWT for the size of the building? I plan on keeping the temp around 55.

yeldogt · Oct 28, 2017

There are smaller modulating gas boilers -- guess you are speaking of gas?

Output is the issue with using a water heater -- basic 40 gallon WH's have burners that have outputs under 30k and have not been designed to run 24/7 .. it's a question of intended use and longevity. If a building needs 25k the burner is never going to shut off. I have a neighbor who has been using one in his single garage size studio for years with no issues -- same with people who build small writing/ studios and want hot water -- it was common to use a WH. They make WH's with higher output burners .. actually bradford white makes a water heater with an additional coil inside and a 90k burner to run "space heat". I used one years ago to warm the floors in a kitchen and bath.

There are various ways you can proceed -- do you need hot water for a sink?

Jackfre · Oct 28, 2017

Did you insulate under and slab edge? If so I bet your heat loss number, which you didn't share is about 30kbtu. You can do it cheaply with a WH or you can invest in a boiler to do the job. The better more efficient boilers will fire down to about 12,000 and up to about 50-60,000 with the modulating burners. As well the better boilers will come with an outdoor reset controller to moderate the output temp of the boiler as the outdoor temps go up and down. That is the best and correct way to heat your slab.

Redraptor · Oct 29, 2017

I forgot to mention i was refering to gas. I also want to run a small laundry including sink. I insulated the slab but not the edges.

yeldogt · Oct 29, 2017

Redraptor said:
I forgot to mention i was refering to gas. I also want to run a small laundry including sink. I insulated the slab but not the edges.

Does the slab extend past the building or are you talking about -- inside along the foundation?

As I said above -- it comes down to longevity. I would not want to design a system that required the WH's full output most of the winter ... It's not designed for that .. and I'm not lucky. I would be looking at a small combination boiler .. do it once if you plan on being at the property for a long time.

Jackfre · Oct 29, 2017

Without slab edge insulation you are heating the atmosphere. I was just in a lady's home again Friday with her contractor. It is a beautiful 1400 sq ft single story home with a radiant slab. She is running a Triangle Tube 95% mod-con boiler on the slab and has had to abandon the radiant due to costs of $600-800/month, and that is in this moderate climate. Why? No slab edge insulation. We put together a detail for her to add the insulation, but all the landscaping and patios, etc would have added up to serious money, so she heats with wood and the Rinnai EX-38. It really is a shame, but with radiant, the devil is in the details.
I would suggest that if you are going to run the radiant, that you look into insulating the slab and counter flashing the insulation. If you choose to run it without, to see how it works, Get everything heated up and do a thermal image of the outside of the place. It will blow you away. I would also suggest that before throwing any serious money at this that you hook up a temporary set-up and see how it works.

Redraptor · Nov 1, 2017

yeldogt said:
Does the slab extend past the building or are you talking about -- inside along the foundation?
***
No extension further than 6 inches past the o/h doors. don't know how you get around that.***

As I said above -- it comes down to longevity. I would not want to design a system that required the WH's full output most of the winter ...

Do you think there's that much heat loss to cause that? You have volume to mitigate that. I have a neighbor that's running an open system to heat a living space a little bigger than mine and hasn't complained. Did mention there's no snow 8 inches beyond the walls.

Redraptor · Nov 1, 2017

Jackfre said:
I would also suggest that before throwing any serious money at this that you hook up a temporary set-up and see how it works.

Any suggestion about that? I'm committed to everything except source. Just wanted to see what makes the most sense for my situation. Thanks guys for your responses

Jackfre · Nov 1, 2017

Redraptor said:
Any suggestion about that? I'm committed to everything except source. Just wanted to see what makes the most sense for my situation. Thanks guys for your responses

The short money is a tank WH. Long term, given you desire for a laundry/sink I think you are looking for a mod con combi boiler.

Redraptor · Nov 3, 2017

You're right. This looks interesting. I don't know what the 10 to 1 turndown means but these seem like a cross between a WH and boiler.
https://www.homedepot.com/p/Westing...n-Recirculation-and-Pump-WGRGHNG150/301172582

Redraptor · Dec 1, 2017

Stopped at a local HVAC store the other day to see what they recommend. I bought a Hydra-Smart 199btu water heater, Taco pump and switching relay, slab t-stat, and the rest of the trim. Also picked up a copy of that book that yeldogt beats people over the head with. Good read! :beer:

yeldogt · Dec 2, 2017

I don't see where you will need that input for under 1ksf .. how much was the boiler? . do they say it can be used for heating?

Raed PA -- create the simple module. I would use a smaller input. Warming the slab is going to get you above 55 ... especially with that size boiler.

Redraptor · Dec 3, 2017

yeldogt said:
I don't see where you will need that input for under 1ksf .. how much was the boiler? . do they say it can be used for heating?

I showed him my diagram of my loops and how I insulated, both of which are not ideal. I was looking at a 120k btu heaters but he was insistent I needed more and with a 10 to 1 turndown I should be ok in milder weather. The unit was $1200. We shall see

red · Dec 3, 2017

Do some more research with how to treat the water you'll be using in your system. Remember hearing from my plumber that the type of water matters with mineral build up inside the units coil.

Another point make sure it's perfectly level. They don't run too well if NOT level, because the condensate is acidic and needs to drain properly.

mygarageone · Dec 3, 2017

I just finished a system for a couple , he got a internet package . He. Got sold a on demand water htr and a bunch of control . Told him to send it all back , independent water htr's not approved in mich for hearing.
I Put in a Challenger mod boiler and necessary controls . The internet company had no idea if mich would allow a water htr for heating , they. Never told the owner about perimeter insulation and a whole bunch of thing.
The net has it's place but this kind of thing is BS when he had to send it all back because they haven't a flipping clue about code requirements.

yeldogt · Dec 3, 2017

Redraptor said:
I showed him my diagram of my loops and how I insulated, both of which are not ideal. I was looking at a 120k btu heaters but he was insistent I needed more and with a 10 to 1 turndown I should be ok in milder weather. The unit was $1200. We shall see

Have you figured out the heat load ... I do 1700sf with under 35k.

That's going to make very hot water and cycle on an off -- the hotter the water - the less control. Mostt of those start high and come down

finn · Dec 3, 2017

Make sure the heater is rated for space heating. Some aren’t.

Temperature of the water out shouldn’t be a problem, even if the design is primarily for domestic hot water, as it has to have settings low enough to prevent scalding, by design, in order to be sold.

You will want to use a primary / secondary loop design, with a mixing valve, in my opinion.

Install plenty of temperature gauges to facilitate set up and tuning during the installation.

Yeldogt, what’s your location? 35k must be with R100, and no windows or doors, or else you live in a temperate climate, with low temperature rise requirements.

Jackfre · Dec 3, 2017

For what you are describing as your system size and the laundry I would have put you into a modulating combi boiler. The one I know best is a 13,500-50,000btu input and will make about 2.3 gpm at 75* rise. The absolute largest I would have gone in that space is a 75kbtu. Once you get your space up to temp your 199 will short cycle like crazy. I think you have triple the needed capacity. Does HTP authorize its use for heating?

Redraptor · Dec 9, 2017

Jackfre said:
The one I know best is a 13,500-50,000btu input and will make about 2.3 gpm at 75* rise. The absolute largest I would have gone in that space is a 75kbtu. Once you get your space up to temp your 199 will short cycle like crazy. I think you have triple the needed capacity. Does HTP authorize its use for heating?

I don't believe I've ran across a combi boiler under 100kbtu. Size doesn't seem to matter they are all expensive. I agree it's way more than I need but with heat loss around 30kbtu he felt I needed it. And one that size can be used for heating. With a 10 to 1 turndown and 10* rise it shouldn't short cycle should it?

Boiler sizing question

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