Right-sizing a boiler for radiant heat

[MongoTom]

Garage specs: 620 sq ft, 2X6 walls, blown in foam insulation in walls and roof, 12 ceiling downstairs, no insulation in the ceiling/loft floor, 8" slab, 2" insulation below the slab, 30 mil of vapor barrier, 3 loops of PEX in a single zone, R18 garage door.

I got 3 quotes for a heating unit in the fall. I have to use electric and all 3 picked the same Electro EMS-S-7, a 24,000 BTU boiler. I knew I wouldn't be working in the garage this winter, so I decided to wait and install the boiler next year.

In the meantime, I had a small oil-filled radiator heater sitting around, so I put that in the garage on it's lowest setting with only one of two elements active. With the help of a heated car at the end of the work day, that little heater has kept the air temp at 40 degrees and we've had many nights here in single digits.

My question is why do I need a 7,000 watt boiler when a 700 watt heater has been working this well? I know it isn't good to oversize a boiler and you'd rather have it working than sitting all the time. My thought now is to use the EMB-S-5 boiler at 15,000 BTU and 4,500 watts. This has nothing to do with boiler cost as they're nearly the same price. Is this train of thought off on the wrong track?

 

[theoldwizard1]

My question is why do I need a 7,000 watt boiler when a 700 watt heater has been working this well?

What is your goal ?

If 40F is good enough, stick with what you have. If you want it warmer, say 60F, occasionally, buy a second heater which you could use upstairs when you want.

 

[MongoTom]

My goal is the ability to work in a tolerable environment nearly the year around. Tolerable to me is 50 most of the time, but if I have to take a month off or add a small supplementat heater during the month of January, I'm fine with that. I have the PEX in the floor and I will heat it. The question is, if I don't use a big enough boiler, what is the danger? That the boiler will burn out and I'll have to replace it with a bigger one? That it will somehow cost more to run, because it will be running all the time? I'm trying to understand how this works.

 

[MongoTom]

posted twice for some reason

 

[hidollartoys]

BTU's are BTU's. If it takes 24,000 BTU's to heat your garage on any given day then the boiler will run 100% of the time. If you install a 48,000 BTU boiler it will run 50% of the time on that same day.

 

[HoosierBuddy]

I'm just comparing sizes:

My garage is 50% bigger than yours.
My insulation and construction sounds similar (2X6, insulated doors)
I'm 200 miles south of you.
My boiler is 5 times as large as you are proposing.

Granted, that boiler not only heats the garage, but it heats the part of the house and the bonus room above the garage too, so lets say I'm only dedicating half my boiler to the garage, that still leaves me 2.5 times as much.

24,000 just seems low to me...especially after spending 5 winters in Flint, MI going to college. It get's f-in COLD up there!

Phil

 

[theoldwizard1]

How much has your electric bill gone up running that portable space heater ? It will go up by twice that if you are running 2 portable space heaters !

What is your cash flow situation right now ?

Last, you can not beat a warm floor !



As for "right sizing" you need to hire a reputable heating contractor with experience in radiant floor heating and have them run the numbers.


If there is now insulation on the sides of the slab, that will be a big loss.

 

[theoldwizard1]

24,000 just seems low to me...especially after spending 5 winters in Flint, MI going to college. It get's f-in COLD up there!

Ha !

Cold ? Try Michigan Tech (Houghton, MI) in Feb !!

 

[wedge40]

Ha !

Cold ? Try Michigan Tech (Houghton, MI) in Feb !!

BTDT. Freshman year was '78-'79. Its a dry cold though.

Wedge

 

[BadgerBoilerMN]

A Btu is a measure of heat. The proper sized boiler is critical in replacing the heat that is lost. Beyond this main goal is the efficiency with which the goal is accomplished. In fossil fuel fired boilers size matters since steady state efficiency is akin to highway vs. city mileage.

With electric boilers, particularly on a slab radiant system the sizing factor for efficiency is not as critical since efficiency is not effected by operating time or temperature. It is good to use an electric boiler as it is purpose-built for this type of duty. The only way to improve the real cost of operation with an electric boiler is to control the design water temperature with the addition of outdoor reset and to properly size circulators and specify PEX loop length and diameter to facilitate optimum pump selection.

If you have a slab radiant floor, the best strategy is to set and forget the thermostat setting. Over-sizing the boiler for greater "pickup" is usually not effective in terms of comfort or fuel savings, but an electric boiler sized within 150% of the calculated load is not a sin. My personal electric boiler is also an Electro with two stages.

 

[Mattlt]

Just throwing this out there, but check with your electric utility, you may qualify for "off peak" electric rates on the boiler. The boiler would run overnight and your utility would charge a lower rate. Some programs may actually shut you down during peak periods - maybe not so desirable, best to check exactly how the program works.

This is a common practice among smaller rural utility companies, not necessarily among the large electric utilities. Doesn't hurt to ask.

 

[Highbeam]

What does it mean when the specs for these boilers say "maximum fuse size" of 35 amps like this one:

http://ecomfort.com/emb-s-7-electro...nt-heating-boiler-24000btu-240v-7kw-3139.html

Does that mean you can use a 40 amp breaker on 8 gauge wire, or you have to try and find a 35 amp breaker for the 8 gauge? Is there another fuse somewhere?

The price of about 715$ makes the boiler route not terribly expensive. Do these boilers turn on with flow? They say they have a circulator relay but does that mean that the boiler turns on the pump or that the pump turns on the boiler?

I am considering heavily undersizing my boiler as well. I only want to keep the shop above 40 or so and with ground temps at 50 in my area I would like to think that this level of heating would allow an undersized boiler.

Running a 7kw heater 24 hours a day is just under 17$ a day with my 10 cent per kwh power cost. I do not want to spend 500$ a month to heat the shop. As such, installing a 10kw boiler would just create the potential for even higher heating bills.

The flipside is that the boiler may not be able to keep up if it gets really really cold outside for a little while.

Badger, you mention two stage and these electro boilers claim to be modulating. Do they have two elements?

 

[BadgerBoilerMN]

It relates to the potential load and safe upper limits. You do not want a breaker that "holds" while the conductor or appliance is drawing more energy than it is rated for.

Tank-less water heaters are typically energized with a flow switch. Purpose-made boilers are typically energized with a room thermostat and cycled, on and off, via high limit, outdoor reset or the thermostat's normal response. The pump relay will typically turn the boiler pump on with a call for heat.

The cost of operation using any reasonably sized boiler will be dictated by the load, not the size of the boiler. The load is determined by the difference in temperature from outside to in and the materials in between the two.

Given an insulated slab, walls and ceiling, the cost of operation is directly related to the indoor design temperature (where you keep the thermostat) and the outdoor design temperature (how cold it gets outside). Time of course is the last factor.

They do "modulate" in a sense, switching on and off to follow the control strategy employed. Some of the larger--(9 kW units) have two elements to facilitate finer control the delivered water temperature. This will effect "system" efficiency and the heat load, as cooler water drives a lower temperature differential and lower heat loads in general.

Less cost of operation.

 

[MongoTom]

So many questions and great information.

Yes, the slab is insulated on the sides and bottom.

I figure my little oil-filled radiator increased my monthly bill by $15. I expect far more with a boiler.

Let me restate that I know I'm considering using a boiler smaller than that recommended by 3 separate contractors. I'd still like to know the ramifications of that. Will it burn out faster? Be unable to keep up? Cost more to run? If I find later that it's cooler than I can stand, I could potentially replace it with a larger boiler.

 

[Highbeam]

The only risk is that it smaller unit will be unable to keep up. Replacement with a larger unit is expensive. It won't be any more expensive to run than the larger unit, possibly cheaper since your shop may loose temperature when a really cold weather system comes through.

 

[MongoTom]

I might have some cold days? If that's all I have to worry about, I'll go with the smaller boiler. I have a 30 amp outlet sitting idle right now for this purpose.

Thanks for the input!