Boiler downfiring - kevin Coppinger

Duncan_2

You definitely need to calculate your hot water requirements.

The thing is, Kevin never told us his requirements! John, I don't doubt your knowledge or experience for one millisecond.... you and I have compared notes over the years, and I know you know your stuff inside and out.

Consider this....

1.6 gallons of 120°F hot water per minute is a LOT of hot water if you're only taking one shower,

Assuming a standard 2.5 gpm showerhead, 2/3 hot water and 1/3 cold water, I could take a 100°F shower all day long and not run out of water.

If you're used to a standard 30 gallon water heater at 30,000 btu/hr INPUT and 60% efficiency, that's 30,000 x 0.6 = 18,000 btu/hr of hot water delivered (output).

Looking at a 60,000 btu/hr input from the boiler with a 90% efficient heat exchanger, that's 54,000 btu/hr of hot water delivered, orTHREE TIMES as much as that standard water heater.

But if you're filling an eighty gallon tub, and doing dishes and doing laundry, maybe 60,000 btus/hr boiler output isn't gonna cut it. Maybe an 11 square foot surface area heat exchanger isn't gonna cut it.

I've found that the average 2,400 sq.ft. household with 1.6 kids is easily handled with a 60,000 btu/hr input. I can't see oversizing a boiler when all the btu's in the world are limited in delivered hot water by a certain sized heat exchanger.

Kevin, what are your hot water needs,?

That's why I like indirect water heater manufacturers that list outputs based on different sized boiler inputs, it makes figuring hot water outputs a lot easier

kevin

Down firing (oil)

I am to put a new heating system in my house that I just purchaced. (I am doing the world a favor by ripping out the Scorched air furnace) The heat loss comes to just over 65,000 btu.(w/o the indirect,80 gal SS Contender) I would like to put in a Buderus 115 ser. boiler however the 115-21 nets out @ 64,000. The 28 model seems a bit too big and I do not want to oversize. Can I down fire the nozzle? what or where do I get the info to do so. I have heard both positive and negative thoughts on this matter? What do you guys think?

John (jcg)

With a Buderus

You would be much better off over-firing the '21 than down firing a '28. Buderus does not have a problem with over-firing 10 to 15% but if you try to down fire you will run into big problems with the exit gas temperatures condensing.

But with the 80 gallon indirect I think I would opt for the G115-28 and not worry about it.

jcg


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Duncan

Run the numbers, consider water use, use manufacturer's specs

Eighty gallons of water at 8.3 pounds per gallon is 664 pounds of water.

If your cold water comes in at 50°F, you need to raise it 70°F to get it to 120°F.

80 x 8.3 x 70 = 46,480 btu/hr needed to heat 80 gallons of water in one hour.

64,000 btu/hr (boiler output) should handle about 110 gallons of water an hour.

I think there might be some lag time for a cold start boiler to catch up with the initial hot water drain unless you put in a setpoint controller that can be adjusted to your use patterns, something with an adjustable on/off differential.

I'd go with the smaller boiler because I'm an energy miser. That's IF it can meet your DHW needs.

Make sure the heat exchanger in the indirect is pumped per manufacturers specs, that it's getting enough hot water flow through the coil. If you can't keep the flow up to keep the HX hot, it doesn't matter what size boiler you use. The indirect literature should have some specific information about Hx flow and head, and about indirect output for different boiler sizes. Your choice should match your hot needs.

jcg may know something I don't know about Buderus boilers, though.

I would size according to the home heat loss, since the water heater only runs for short periods of time.

Or do you have HUGE hot water demands?

John (jcg)

But

While this the 64,000 BTU boiler should heat the 80 gallon tank in 45 min with no draw. It will not increase the tank temperature at all when the flow rate is over 1.6 gal/min (which isn't much). [(64,000 BTU/hr) / 60 min = 1067 BTU/min (1067 BTU/min) / (664 BTU/gal 80° rise = 1.6 Gal/min]

Nothing makes a customer more irate than running out of hot water.

Check your HW requirements and factor them in if necessary.

jcg


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Kevin Coppinger

hw water requir...

I have 2 bathrooms and will be adding a 3rd. at sometime down the road. no whirlpools , i just want/need the ability to take three showers at the same time and not run out. My concern is that the heat loss is +/-over 1000 btu the boiler 's IBR net ratings. I don't want to have a problem in the dead of winter or have condensing problems with too big a load(from the heat load and the water heater) on the boiler. I im going to line the chimney w/ a SS liner.Thanks for all your imput guys! kevin

Duncan_2

A thousand btu/hour shy.

That's usually not much of an issue, Kevin A thousand btus compared to 64,000 btus is an error of about 1.6%.. Each non-active person in the home adds about 200 btu/hour of heat. Then there's lighting, stove, refrigerator, television, and other appliances that all add btu's to the building envelope.

It used to concern me when the heat loss was that close. When that happened, I'd double check my figures and make sure I calculated the heat loss correctly. If the numbers checked out within 5%, I'd go with it. If the boiler was oversized, I'd downfire it to fine tune (I'm talking gas boilers - listen to jcg on oil). There is lots of cushion built into the heat loss numbers. I know, it feels like a bold step installing a boiler that's even a little undersized. If it really concerns you, there's probably not too much of an energy penalty for a boiler that's ten percent oversized.

It's just that over the years, I've seen way too many boilers that are way oversized for their application, and they beat themselves up short cycling.

Consult the manufacturer of the indirect and make sure they can deliver the hot water you need with the size boiler you select. It's not uncommon for the domestic hot water load to exceed the home heating load, especially in a well-insulated home with significant hot water needs.

Once you get above a certain boiler input to the indirect, the limiting factors are the indirect's HX (heat exchanger) area and flow rate (they add up to heat exchanger capacity). It doesn't matter how many btus you put in if you can only get so many out of the HX. First hour rating of the indirect and flow rate of shower heads will tell the story for three showers at once.

John (jcg)

Of course youre right

I just can't equate an 80 gallon water heater to low demands.

As to whether the small boiler is enough with little hot water usage I would say YES. Especially if the heat-loss was done using the IBR method (Slant-Fin) as this method, while very good, usually gives results 5% higher that manual J based programs.

jcg


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kevin

Thanks

Thank you all for the imput, I will re think the water heater sizing.And re-check the heat loss. (It is a older house 1800 ad ) and go from there.kevin C.
P.s.- What is the loss of heat output over the years ie loss of efficiency? Will the boiler that nets 64K now stll net 64K in 15 yrs. from now?

David Sutton_2

Great debate.......

. i got a ton of great information just from reading these pages . you guys are great teachers thank you and keep up the good work .....David

ritebair