Heat Loss Calc. For Forced Hot Water Replacement Boiler?

Joseph_4

Can someone please explain why they say by a forced hot water system that you always should do a heat loss calculation when replacing a boiler. I just finished a hydronics course and still don't see it. If the house was heated well before and the owner was satisfied, then if I just add  all the feet of baseboard and assume it gives off 550 btu per foot. then that is my total btu required. add 15% for pipe loss (pick up factor) and thats it. I have my boiler size.  Why is this not correct?

     I don't see why I have to measure rooms etc.. if the owner doesn't plan to add anymore baseboard to any room.

Thanks

Joe 

billtwocase

heat loss

That has become the norm Joe,  The exact or close to size. Personally I prefer slight larger for future zoning, or an indirect in the future. Some home sizes or loads may never change, so best not to grossly oversize, as was done in the past. But i tell you, those were the most comfy homes in the dead of winter with big old cast iron boilers and cast radiators, just cost a fortune to run nowadays

EricAune

One class? It's a good start

The push for a heat loss calculation is the drive to use less energy.  Most older homes have their walls lined with baseboard (as you use in your example), this having more to do with the fact that a lot of plumbers did not take the time to do a heat loss calc and the fear of a call back....not so much because they enjoyed crawling around on their knees.



#Reason #1 (of many): More baseboard than needed lends opportunity to utilize lower water temperatures and/or outdoor reset. The math is simple, heck you can even have most of it done for you via computer programs, take the time to do it right.



Rating plate sizing is not the way to go.  Just lowering your labor rate or job cost will not save your customer money over the life of the new boiler.  There are many more reasons, and I'm sure others will join in.

billtwocase

Eric is right

as is anyone else who pushes heat loss. One thing on the sizing for older homes was poor to no insulation, drafty windows, and so on. All those, if not upgraded, are taken into consideration on sizing. Best if the customer makes that investment first so energy conservation can truly be achieved. 

CC.Rob

do it for yourself

Your business, that is.



Heat loss calcs are a) the right way to size a boiler, b) increasingly required by some jurisdictions, c) often required to qualify for utility rebates.



Doing a heat loss on your jobs distinguishes you from nearly all of your competition. It shows the customer that you have their best long-term interests in mind. Proper sizing leads to better performance and long-term energy savings. Tell that to the customer.



Get familiar with some heat loss software. With practice, it only takes an hour to do one. It's ok to charge for the service. Credit the fee toward the install if you get the job.



As others have noted, length of baseboard has essentially nothing to do with boiler sizing. Maybe it was sized originally, maybe not.



The past couple years have also seen a huge increase in people doing weatherization/insulation/air-sealing upgrades to their homes and businesses. In many cases this drastically reduces the heat loss, so you can downsize the boiler, or go with a modcon to take advantage of the excess baseboard capacity by using lower water temperatures and outdoor reset. Tell your customer about the economics and comfort. Show them that their work to reduce fuel costs by insulating has a real benefit by reducing boiler size or going with much more efficient modcon.



Make them feel good about your professional approach, and your interest in providing a quality service that ultimately saves them money.

Jean-David Beyer

you always should do a heat loss calculation when replacing a boiler.

I am not a professional, but when I replaced my old GE oil-fired hot water boiler with a gas mod|con, the gas company guy asked me how many square feet my house was. I did not know, but it was easy to measure. A small Cape Cod with a little over 700 square feet down stairs and around 400 square feet up stairs. I asked why that mattered. He said to determine the size of the pipe from the street to the house and the size of the meter. I guess that was good enough for that.



When the salesman from the contractor came, we talked about what I wanted, and he paced around the outside of the house. I asked what that was about, and he said he had to determine what size boiler to design into the system. This seemed pretty vague. By then I figured out I wanted a mod|con and indirect fired hot water heater (to replace an electric that had slight evidence of leaking that could probably be fixed around where the elements went into the tank, but it was old and probably needed replacement anyway). It was then that I got John Seigenthaler's book and read it.



http://www.heatinghelp.com/products/Books/5/96/Modern-Hydronic-Heating-Second-Edition-br-by-John-Siegenthaler



It was a revelation. I got a chart from the boiler manufacturer and figured out the heat loss. Came to 36,250 BTU/hr at 0F outside. 14F design day around here. The smallest boiler in the product line recommended by the contractor was 80,000 BTU/hr input. I tried to ascertain the actual heat load by looking at the burner on the oil boiler. 70,000 BTU/hr with the nozzle (1/2 gallon/hr) in there. I then used Slant/Fin's computer program and got 29,324 BTU/hr.



So when the contractor proposed the 105,000 BTU/hr model, to be on the safe side, I refused. He said it would modulate down to 21,000 BTU/hr so it would be OK. By then I figured out that he may have been a salesman, but not a good design engineer. As it is, the 80K unit goes down to 16K and I wish it would go down to 6K.



Meanwhile, I wanted maximum condensing, so I had two little "radiators" upstairs replaced with wall-to-wall pieces of Slant/Fin, 14 feet in each of two rooms. The radiators were metal boxes, open at the bottom, with slots near the top. Inside was a piece of fin-tube three feet long. It was always cold upstairs because the existing system ran the same temperature water everywhere, and even though the floor got a little too hot, it was not enough for upstairs.



I had the contractor make the system a two-zone one so I could control the water temperature upstairs independently from the in-slab radiant downstairs.



The present system has outdoor reset and the water temperature downstairs varies between 75F and 120F in the radiant zone (downstairs), and between 110F and 135F upstairs. So I do not get anywhere near 550 BTU/hr/ft from the baseboard up there. And do not need it. Actually, I doubt it goes over 130F upstairs very often. And these are supply temperatures. The return temperatures are not a whole lot lower than the supply temperatures, especially upstairs, because the contractor used Taco 007-IFC circulators everywhere, even though I am sure an 005 would be enough upstairs. I do not get water rushing sounds, so I guess I need not replace the 007 for up there. And since the two baseboards are in series, this way they both run at about the same temperature.



So I always get condensing downstairs, and it has to go to design day or colder up stairs to stop condensing upstairs. IIRC, it dfd that only once last winter and for only a few hours.



I would have put even more baseboard upstairs, but that would have been a pain as it would have to go around corners, and there were things in the way. But it would have permitted even lower water temperatures up there, and more condensing.