Hot water boiler sizing question.

Threadcutter_3

I am a heating contractor in central Pa.  I am looking at a hot water boiler replacement in an old farm house.  The building is about 1400 square foot with blow wall and attic insulation.  It has a significant glass load. The heat loss calculation I did indicates a heat loss of 42,500 btus per hour. The connected radiation is standing cast iorn in all rooms but the kitchen, which is zoned seperate with a run of copper fin baseboard.

The conntected radiation is rated at 92,000 btu's.

Does one size the boiler to the connected radiation or to the heat loss calculation?

Thanks

Threadcutter

KBP&H_3

The building> The older people

I recently  talked to a boiler manufacturer tech gut about this and he said to go by the heat loss, my situtation was 75000 connected load and about a 50,000 heat loss.   so I downsized one size,, only to find out the homeowner keeps it 76-78 degrees in the house, back up one size,,,, 

I have recently ran across a few jobs where it is kept above 75* in the house> 7* - 10* more than usual, same as going 10* colder for design.

TonyS

Size your boiler to

your heat load then, im assuming you rated the baseboard at 180 or 170...then lower your high limit to the temperature that is needed to get your heat loss from that amount of radiation. If the boiler is condensing you can just keep dropping but if not you should keep at least 140 minimum.

lchmb

just to ask

This isnt steam with a hot water zone on it is it?

KBP&H_3

Many central pa boiler companies

There are many central / area PA boiler manuf's. Lancester, etc- call and ask them for tech services,give them the details-  you"ll get their answer, your putting in their equiptment.

JStar

RE

Did you convert your SqFt. radiation for steam or hot water?



If you did it for steam, then the conversion comes out to require about 110 Btuh per sq.ft. You would then only need to run your boiler at 145F maximum.

icesailor

Heating Loads:

Always go for the heat loss load. Not the connected load. But, you can play around with the connected load in relation to the heat loss.

Just correct your room temperature range. If you are heating hot water with the boiler, in my opinion, don't size the boiler to the heating load. The domestic hot water load is far greater. A 45,000 BTU boiler is hard to come by and gives you not a lot of hot water. Equal to a 30 to 50 gallon gas water heater.  Not a lot.

Right or wrong, in these low load houses, I consider the domestic potable water heating load the primary load. The heating load is secondary.

Jean-David Beyer

domestic hot water load is far greater

"If you are heating hot water with the boiler, in my opinion, don't size the boiler to the heating load. The domestic hot water load is far greater."



You are the professional, not me. So maybe my house and domestic hot water needs are the reverse of what you experience. At 0F outside, my heating load is 30,000 BTU/hr, though design temp is actually 14F. I have a "36" gallon indirect fired hot water heater. I ignored it when sizing the boiler, but the smallest boiler was so large that it does not matter much. The indirect runs at the highest priority and the two heating zones use the two lower priorities.



According the the indirect manufacturer, if I run 6.6 gallons/minute of 190F water through it, I can get 124 gallons per hour of 140F water for the first hour rating. I suppose I could run my shower very very hot for an hour and not use up the hot water. The boiler can burn 80,000 BTU/hr of natural gas if it feels like it.



Is it your experience that typical users use much more hot water than I do? I suppose (without evidence) that most indirect fired hot water heaters have similar efficiencies. Or do the typical houses have that much better insulation and less infiltration of cold air than I do so my heat loss is more than average?

Threadcutter_3

Hot water not steam.

The system is hot water not steam.  Does not now, but will later have an indirect hot water heater attached.

TonyS

Here is a chart

http://www.colonialsupply.com/resources/radiator.htm

I am assuming you figured about 541 EDR on the radiators.

To get 42500 out of your existing radiators you would need 78 btus per sq.ft.EDR which would leave you with a water temp of 135 on a design day. I would definitely use a condensing boiler.

icesailor

Hot Water:

Understand, you may or may not use more hot water than anyone else. But when you use it, you will be using it like any other.

If your house was designed to keep 70 degrees inside when it is zero outside, and the house looses 100,000 BTU's per hour when it is zero, it will only loose 50,000 BTU's when it is 35 degrees outside. If the heat emmiters are designed to use 180 degree water on the supply and 160 degrees on the return to the boiler, you are dealing with a 20 degree drop in tgemperature.

When it is domestic potable water, the incoming water to the tank will be 55 degrees and you may want to raise it to 135 degrees. That's an 80 degree rise in temperature. If you feel like doing the math, 1 BTU is the amount of heat energy required to raise one pound of water, one degree in an hour. A gallon of water weighs 8.33 pounds.  A shower head is supposed to be 2.25 GPM. That's 135 gallons in an hour. Do the math 

Your hair may be a smooth cut. Like mine. It doesn't take much to wash it. A washcloth will do. Not so with a wwife and teenagers. To run out of hot water with soap in the hair is considered a divorcable offense. Undersize your boiler and domestic hot water at your peril.

Jean-David Beyer

Hot water sizing.

Thank you for your reply. I think I now understand your point.



Let us use my numbers instead of yours.

My hot water boiler has 80,000 BTU/hr input and I would have gotten one half the size if they made one. It has three prioritized thermostat inputs. It also has outdoor reset. The highest priority is to the indirect. The middle priority is to my largest heating zone, which is the entire downstairs radiant slab at grade. The maximum input temperature is 120F to that, and I have never seen it do that because the design temperature is 14F around here. The temperature drop through the slab when it gets down to design temp is less than 10F, but I do not remember exactlly. The lowest priority zone is the smallest, and is heated with baseboard. The maximum input temperature is 135F because I put in extra to keep the return temperatures low. It might get 10F drop when it is 0F outside (the Taco 007 is probably too big for this zone, but that is what the contractor put in).



In any  case, it would need to deliver 22,761 BTU/hr if it went to 0F outdoors downstairs. The upstairs zone would need 6,473 BTU/hour. These numbers from the old Slant/Fin calculator program. If both zones were calling for heat, I would need about 30,000 BTU/hour for heat. The DOE rating of the boiler is 71,000 BTU, hour so if all the circulators ran at once, that would leave 41,000 BTU/hour for the indirect. Because the controls are arranged by priority, this cannot happen. If the indirect calls for heat, the rest of the house is cut off until the indirect is satisfied (or if 30 minutes have passed, whichever happens first). As a practical matter, it seems as though the indirect calls for heat two or three times a day, depending on what I do with hot water and runs for 10 minutes or less each time. No wives or teenage daughters, and I use mostly cold or warm water in the washing machine, never hot. My shower head is rated at 2 GPM, but I do not usually run it full open, and while I have never timed it, I am pretty sure my showers (run fairly hot) last less then 10 minutes. I imagine my hot water use is less, perhaps a lot less, than average.



So when the indirect is calling for heat, the house is cut off, and all 71,000 BTU/hour is available for heating the hot water. During this time, the house is not getting heat even if it is calling for it. This makes little difference for the large radiant zone because the slab does not cool off much during the 10 minutes or so that the indirect is reheating. It might make a little difference in the upstairs zone, but I have not noticed it during the two full winters I have had this boiler. This season so far, it has run only 4 hours and 14 minutes, so nothing can be concluded from that.



If I run 2 GPM and the supply water is 50F and I want 80F rise to get 130F hot water (assuming 100% efficiency for simpler math), I would need 80,000 BTU/hr for the hot water, leaving nothing for the house. That means I cannot take an hour long 130F shower while heating the house. But even if I did, the radiant slab zone would not lose too much temperature. The baseboard zone probably would lose enough to notice.



I infer that my low hot water use, the fact that the boiler is about twice the size I need, and that it never gets below design temperature for very long, makes my setup OK for my needs. That boiler was the smallest in the product line available.

icesailor

Boiler Sizing:

I'm not saying that your boiler is too big or too small. Just that the potable water heating will always remain the same, year around, whenever you use it. The heating load goes from nothing to maximum and back to nothing with the seasons of the year. Strategies have been developed to covet this with larger storage tanks or hot water mixing valves. But a 45,000 BTU boiler, heating a cold house with a small indirect at design day, could lead to a cold shower.

There is a place I do work at. They had a huge Geo-Thermal system done with water wells. There is no back up heating or cooling. It has not worked on hot summer days or cold winter days. Someone, somewhere with a laptop, is trying to control the output by changing pump speeds. No amount of pump speed and futzing around is going to fix this problem. Someone made a mistake. Getting someone to admit it is another story.

I put a variable speed Wilo pump on my FHW system. I set the pressure too low, I could tell right away that I had it too slow. The house was slow to recover. I turned up the speed a notch. I could tell immediately it was better.

Systems are sensitive.

TonyS

Not completely correct

You said water temp at 135 but you said volume was 2.25. Your not going to shower at 135, average shower is 107. So the math works but you plugged in the wrong numbers. 52 temp rise(for a shower) is less btus.

Jean-David Beyer

not going to shower at 135

Of course I will not shower at 135F. I used to have a chart on how long it would take to get first, second, and third degree burns at different hot water temperatures.. I lost it. This one isn't it, but it gives an idea: The shower head is max 2 GPM, not 2.25. But I very seldom run it full open.



.http://antiscald.com/prevention/general_info/table.php



At 131F, you can get a third degree burn in 30 seconds, and at 140F it takes only 5 seconds. I just picked these numbers to get an upper limit on how much hot water I could use. I also would not take an hour long shower at any temperature.



Right now, I really run my indirect at about 122F or so. Whatever it takes to get 120F out of the nearest hot water tap from the heater. I plan to raise this to 140F when the money tree blooms and I can put some thermometers and a thermostatically controlled mixing valve in there. Maybe in 2012 if the stock market cooperates. I would really like to put a clever two-stage pressure-balancing temperature controlled valve right in the shower, but that would require replacing all the tile in the shower (no access from the back), and that is just too much money. Right now, I cannot even replace what is there now: not code anymore. Three knobs : Hot, Cold, Up, Down. I have a termometer in the water line in my darkroom. In the summer, the cold water sometimes exceeds 75F, and in the winter it sometimes gets a little below 50F. So I really better have capability for 70F or more temperature rise, I suppose.

TonyS

JD

 I was just pointing out to Ice that the temperature rise would actually be 52 instead of 80 if you are going to use the shower head flow as part of the problem