View Full Version : multi-zones effect on sizing a boiler
03-09-2005, 03:39 PM
(some of you will recognize me from a recent post, but now that we're starting over, let's start from the beginning.) I'm planning to install a new boiler, and I'm getting conflicting information on the "right" size. I believe the old boiler is too big. Here's what I know:
Old boiler: Bryant 185 MBTUH input, 65% eff., 50yrs old.
Zone1, 60’ of bb (x600BTUH max radiated capacity = 36000), 120’ of pipe excluding bb, serving 850sqft to be heated
Zone2, 60’ bb (36000 max radiated), 90’ pipe, 600sqft
Zone3, 70’ bb (42000 max radiated), 50’ pipe, 400sqft
Zone4, 30’ bb (18000 max radiated), 90’ pipe, 330sqft
Zone5, 30’ bb (18000 max radiated), 50’ pipe, 220sqft
House Totals: 250’bb (150000 max radiated), 400’ pipe, 2400sqft, approx 23 gal in pipe & radiators plus 6 gal in boiler.
2 heat-loss models calculate loss to be about 60MBTUH. Rule-of-Thumb says about 96MBTUH.
Some contractors say, "size the boiler to match the installed radiation, other wise the bb at the end of the zone might be cold. Also, it's nice to know the capacity is there if you need it on the really cold days."
Dept of Energy says, "slightly undersizing a boiler ensures that it won't short cycle, and will always reach peak efficiency, it won't condense, it will last longer."
I live in SE PA. My gas bill during heating season averages $350/month. What size boiler do I need?
Anxiously waiting your replies.
03-09-2005, 05:47 PM
You size the boiler to the heatloss of the home for the coldest day of the year.
You size the baseboard for the heatloss of the room its in
and the water temps the baseboard will be seeing on its inlet side.
Yes Mark we remember you as the guy that has to much baseboard and very little flow,not to mention with all that
active baseboard a boiler that is seeing very low return temps.
03-09-2005, 06:02 PM
Refresh my memory, SE PA, where.
You should size the boiler to the load of the house, and the circ to the head of the zones.
I prefer just a tad of extra radiation for the rooms, but not much.
03-09-2005, 06:36 PM
Beenthere: I live near Philadelphia.
Simpleman: are you suggesting that I pay to have the bb reduced in some of the rooms/zones? I have considered clipping a foot or so of fins in the room that gets really hot.
03-09-2005, 06:47 PM
We're in lancaster.
03-09-2005, 07:20 PM
He means that you need to measure up and determine exactly WHAT your consequences would be.
Extra baseboard or radiation allows lower temperatures to be used; which spells higher efficiencies. Trouble is, if you go below a certain threshold, which is the best efficiency, you can destroy the chimney and boiler with flue gas condensation.
Don't do anything rash, just make a drawing up with loop lengths, diameters, and baseboard lengths on each loop. Design for the return temperature, and you'll be fine.
As you know, size the boiler to the heat loss. Don't pad it.
Extra baseboard, already installed, is a plus, rather than a negative. Use it wisely.
03-09-2005, 07:27 PM
A question or two.
How are the circulator(s) set up? Are they set up as one circ per zone or one circulator for the entire system?
Does the house stay warm on the coldest days?
What is the aquastat set at?
I would not clip fins off the BB. That may not be a good idea when it gets cold out.
You may want to take this to the Wall...
03-09-2005, 07:51 PM
only 1 circ pushing into the boiler, TACO 007-F5, 1/25hp, 3250rpm, pushing into the boiler. . (I understand that this may be small.)
House stays warm on coldest days. One room in Zone2 is too hot. I believe it's simply over radiated compared to the other rooms in that zone. Zone4 is only one room, and it heats-up very quickly. It's a little uncomfortable, but only overshoots the t-stat by 1 degree.
Aquastat is set a 190. FYI, burners on the old boiler only run for 4-6 minutes.
03-09-2005, 08:02 PM
Okay, it's starting to make sense.
First off, I'd lower the setting on the boiler's aquastat a bit, to about 175-180. That will increase the run time a little of the circulator.
If you are going to replace the boiler, I'd install one circ per zone. Yes, it's a little more, but the comfort level will increase. If the zone 2 loop is running by itself, that's probably when it overheats. I'd put a smaller circ on that zone and on the zone 4.
As long as the boiler stays above 140 degrees, you won't have a boiler condensing problem.
Did you do a room-by-room heat loss?
If you are planning on replacing the boiler, why don't you use a P/S piping system. Then put a mixing valve in the zones that are to hot. You can also put in an outdoor reset and get more eff. out of your system.
03-09-2005, 09:14 PM
Its been my exp. that the old tale that BB needs to be supplied with 180* temps to convect heat is simply noy true. On design day temp. 180* may very well be needed and if designed properly should be met. However design day temps are not the norm, so supplying design day supply temps to convectors is simply wasting fuel and putting more stress on the system than is needed.Temp at 150* or even lower perhaps 10-15* lower would work. The problem occurs with lower water temps (120-130*) creating condensation in the flue pipe causing eventual HX failure due to deteroiration of flue pipe. Primary/secondary would solve that problem or go with a condensing boiler. IMHO.
03-09-2005, 09:25 PM
I am going to use a crystal ball here, and tell you your home needs about 60K btu all told for heat. But please, do a room by room heat loss to confirm this. Further, my ball tells me you need about 100' of bb all told, maybe less. Again, do the calcs :)
If you are feeling ambitious, shorten the bb where needed-just for aesthetics and reduced dust collection alone! If not, could always put some aluminum foil/metal plates over the extra fins to keep the delta up..
The guys are right..size the boiler for the structure's heat loss. Then size the bb for each room's requirements.
Then..to soup up the system, you can do outdoor reset to tailor the bb's water temps to the temp outside.
03-10-2005, 12:43 PM
Thank you for your replies. Lots of good information. I’d like to nail-down some of the general info first, and then move to the details. I think I have questions on every response, so please bear with me.
First, I am replacing the boiler. That’s a given.
I did a room-by-room heat loss using software from a well-known bb mfr. (Thanks, Noel.) I have more than enough baseboard, so where appropriate I’ll use Hydronicsman’s idea of placing foil over the extra fins or close shutters where available. (The old cast iron stuff doesn’t have a shutter.)
So if I plan for heat loss of 60MBTUH, do I select the boiler by its Output Rating or the IBR?
Beenthere: what outside air temp do you design for in Lancaster? (It’s a little colder in Lancaster than it is in Philadelphia, but it should be close enough.)
03-10-2005, 05:14 PM
0 with a 10 MPH wind.
03-10-2005, 05:58 PM
Ah, but here's the catch..what are you doing for hot water Mark?
03-10-2005, 06:40 PM
give you an exact heat loss on your home without knowing anything about it's age, construction, r-values, u-factors etc..
Ashrae winter design temp for Philly is 14. Personally, I would design for a 70 indoor temp at 0 outside with a 15 mph wind.
I can tell you that 60k sounds a little low unless home was built with newer style construction, r19 ext insul and decent windows (better windows if they are large)or upgrade a great deal if an older home. Heat loss programs are JIJO, so don't make too many over assumptions when inputting info. A good rule of thumb that I have use successfully is this:
well insulated, modern home- 18-25 btu's/sq ft
avg to poorly insulated- 25-35 btu's/sq ft
poorly to not insulated- 35-45 btu's/sq ft
barn- 45-60 btu's/sq ft
your home at 60k rings out to 25/ft based on your info. remember the net output of the boiler needs to meet or slightly exceed the heat loss, not the input
curious about the answer to another post- does this boiler need to make domestic hot water such as with tankless coil?
03-10-2005, 08:11 PM
I have a stand alone gas-fired hot water heater. My gas bills in the summer almost go to zero. I have no plans to use an indirect fired hot water heater.
03-10-2005, 08:30 PM
I thought 60K was low, too, but that wasn't the first pass. I had to scale-back on some of my facts. I believe that I made very few assumptions. For example:
house was built in 1955.
I know there's R-11 in the walls and attic. Nothing in the crawl space.
The 2 new rooms that I just added: R-15 in the walls; R-30 in floors and ceilings.
Recently replaced all windows with double-pane, argon filled vinyl.
Re-sided the entire house with R-4 foam under vinyl.
I used 13 degrees outdoor temp, 68 degrees indoor.
The only assumption I made was "infiltration". I had no idea what to use, so I used "average".
!!!CORRECTION!!! The second program I used makes infiltration very easy. So, again, I believe that I made very few assumptions.
[Edited by mark115 on 03-10-2005 at 08:52 PM]
03-10-2005, 08:54 PM
ok, sounds like you've made some improvements to the house. not telling you 60k is not enough- I assumed your house was built in the 50's or earlier by the boiler age and have never seen exterior wall insul in a house that was built then ,unless it was gutted later and added. good luck
03-11-2005, 08:45 AM
I've decided on a boiler: 103 MBTUH input. And, I like the idea of an outdoor reset. Let's just leave that alone for a little while. Of course, if there are any major objections, please reply.
Now, onto circulators. Several people have recommended 1 circ per zone. Sounds like a good idea. What exactly is "head-feet"?
Dof3 suggested, "If you are planning on replacing the boiler, why don't you use a P/S piping system. Then put a mixing valve in the zones that are too hot."
I'm not exactly sure what is meant by a Preliminary/Secondary system. Is that the same as what someone described to me as a "two-pipe system", where there is 1 loop for each zone, and the radiators are Teed-off of it?
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