sizing new boiler

Paul_31

I am intending to replace the 300 kBtu Crown Aruba natural gas boiler in my 100 year old two family house. While much has been done over the years to improve the energy efficiency of the structure, there is still some work to be done, but I want to make the boiler change now. It is an old gravity conversion (in fact it will still heat the house without any electric power in the event of a blackout!) and I can see it must be wildly ineffecient, in that it takes 30 minutes or so to bring the water up to temp. LOTS of volume! I calculated 75 gallons in the pipes alone, and there are 18 cast iron radiators.

From what I've read here on the Wall, I think a condensing boiler is the way to go. It's hard to do an accurate heat load estimate, since I don't know exactly what's in the walls, but I do know how much gas I used last winter. Using that info, i calc 180 kBtu for a 0 degree design day here in Nyack, NY. In that this included two families of hot water and cooking, the actual would be less. Since we had several night last January which were in fact 0 degrees, I know that the boiler never really ran much more than 50% of the time at that temp.
My question is: many on this site have indicated that going from what I've got to a condensing boiler + outdoor reset could potentially result in 35% or more in savings, how do you size the new boiler? I'm leaning towards a Weill Mclain Ultra (and thinking of the 155).

Any Wallies interested in doing the conversion? I must add that I really want to be closely involved in the process (possibly doing some of the plumbing myself) in that it is crucial to me that I totally understand the system once installed...

Mike T., Swampeast MO

Perhaps get the "one-time" version of HVAC-Calc? www.hvaccalc.com

Take accurate measurements and unless you're utterly certain of insulation, I'd assume the worst, e.g. none. You might have to make some assumptions regarding construction unless you've seen such by opening some outside wall.

Contractor will likely want to do their own heat loss calculation anyway, but your money and effort won't be wasted. You'll get a very good idea of how such is done AND you'll get a chance to see the effects of any insulation/weatherization changes you choose to make BEFORE installing the new boiler.

Paul_31

hvaccalc doesn't cut it here...

the type of construction used in my 100 year old house are not options in the program (i.e. 12" stone + 4" seaweed + 1" plaster). I think the guesstimate required for the wall R's is so great that the final answer will fall over such a large range as to be nearly meaningless. However, I do know how many Btus/hr at design temp I used last year (~165,000). Is it rational to expect that to be decreased substantially, in that the system controls are TERRIBLE??? By this I mean that the circulator (only one-- gravity conversion) only runs when the burner is firing. And that the aquastat has no setting below 180. getting a modulating WM Ultra somewhat addresses this, but it would still be nice not to oversize...

Paul

dconnors

radiators have ratings

this will bring you closer to the truth but may not answer all of the questions

Paul_31

I have Don's books, and could theoretically calculate the output of my radiators, the fact that I was able to turn up the thermostat last January when it was 0 degrees out, and soon be able to walk around in shorts and no shirt comfortably (actually, sweating) leads me to conclude that I am also overradiated. By a lot! This however, is good, in that I believe that I can run at MUCH lower temps and put out sufficient heat. Possibly low enough to run at condensing temps on even very cold days.

Paul

dconnors

from what you say....

I agree with you. Which is why i love condensing boilers so much.

ernie_3

couplea variables here

I would also do a full EDR on the connected load. Also calculate the total water volume to size the pumps accordingly. As you have a millivolt system existing it must be gravity as you have heat during power outages.
Condensing boilers are the way to go, but choose your contractor carefully. Tekmar offers incredible controls for outdoor reset and will modulate the boiler temps accordingly. You will realize a tremendous savings which I'm certain will accumulate your upfront costs in less than ten years. Good luck!

Robert Depew

Gravity

I ran into a system the owner also in the plan was to do some zone control. took out the boiler ( not that old but instaled wrong and condesed in the boiler) and all piping in the basement. bushed the big pipe to radiators down and ran each line back to the manifold with pex and zoned the house in to there zones. did primary and secondary. keeping water in boiler return 130 outlet at 160. a lot of work put confort levels now are livable.

Mike T., Swampeast MO

Remember that walls are actually a fairly small component of heat loss. Ceiling and windows/doors are both generally MUCH greater.

Here's some r-values that might help you. All are per inch of the material. Stone r-values are an average of what I could find.

Limestone: 0.9

Sandstone: 0.6

Granite: 0.6

Seaweed: 4.0 (yes, found "seaweed" specifically mentioned--note that "seaweed" is often a misnomer for other things but they have similar--quite good--insulation properties.)

Plaster: 0.9

So, if yours are limestone walls, you get an r-value of 27.8! Were this mine, I'd drop that a bit to say 24 to compensate for stone/insulation irregularities. Sounds to me as if heat loss through your walls is the last thing you have to worry about...

Can you have a blower door test conducted? If so, that will help you nail down the infiltration--something that is only estimated for new construction yet with good insulation makes up the greatest part of the heat loss.

Yours would seem an ideal candidate for a condensing or condensing/modulating boiler, reset and (try them--you'll love them) TRVs.

As energy prices rise, the cost-effictiveness of high quality, high efficiency defices grows rapidly. Now is likely a VERY good time to do whatever you can.

The last time we had rapidly rising energy prices we entered that crazy period of "stagflation" that so troubles economists. As the cost of energy ripples through the economy prices everwhere rise, yet such produces a corresponding drag on everything else (including wages and productivity). Get now while the getting is ripe!

jerry scharf_2

what I would do

Paul,

If you figure that the boiler was running less than 50% and it will be a less efficient boiler than what it will be replaced with, you may be well below 150MBH. The advantage of a modulating boiler will really help you out, and you seem like a great opportunity for a condensing boiler/outdoor reset system to save you money.

Like Mike said, is there any chance of putting TRVs on your radiators? That would make for the most comfortable system, and one that has the best opportunity for fuel savings. Just make sure the pipes are well insulated. If you find the room that has the worst load to radiation ratio, that will define the reset curve.

If you can't do that, then you have to figure out how to best control the system. You didn't mention anything about zoning. There are many ways to do this, and it would be hard to do this at a distance.

go for it.

jerry

Paul_31

TRV's

Right now, the two family house runs as a single zone with a single thermostat (not a good situation!). Somewhere about half the thermostats are candidates for TRV's, as the other half are those funky Honeywell "special" valves that either divert flow into the radiator or allow it to continue along. I do beleive, however, that none of the rads are piped in series-- all go back to the basement, although not necessarily the boiler room. Part of me hopes to avaoid replumbing everyhting in the basement, even though I know that doing so would allow me to zone the house properly (not to mention eliminating many gallons of volume from the system).
Does anyone have any experience in replacing those funky Honeywell valves with TRV's? My intuition is that even half the rads being on TRV's would (in conjunction with the outdoor reset) allow reasonable control of temperature.

By the way, I tried calling the nearest "Find a Pro" but their number is out of service. Does anyone have any recommendations? I'm in Nyack, NY (Rockland County, just over the Tappan Zee Bridge) I'd really prefer to deal with someone local on this...

Plumbob

ignore heat calc advice, you're doing fine

If you have used your gas bills (therms or BTU and degree-days) to correctly extrapolate to your heat load on a design day, YOU HAVE DONE BETTER THAN ANY HEAT LOAD CALC CAN DO. Heat load is for people (like contractors) who do not have access to historical usage information, and it involves a lot of assumptions and tends to greatly overestimate the heat load. Why should you use a theoretical calculation when you have actual experience?

Now your real question is, if you have calculated current consumption to be 180MBTU on a degree-day with your old boiler, what should the new boiler be? This is where it gets a bit hairy and it may be better not to push it. Numbers like 35% saving may not pan out in your particular case, and also they are OVERALL savings. Your saving on a design day will be less, because you probably won't be condensing on a design day, plus the advantage of reset is reduced on a design day because the water will be very hot even with reset. Plus, it can get colder than a design day occasionally; no big deal in heat load calcs because ther is such a huge huge cushion, but if you are cutting the boiler size to the bone, this is a concern.

So, a 155MBTU condensing boiler is pushing it a bit. If you take the risk, you may be OK, but it is a bit of a risk. Remember that the WM Ultra modulates its burners, so it will behave just like a much smaller boiler when needed.

jerry scharf_2

Disagree on the 155 being undersized

RPG,

That 180MBH is gross. When you account for the efficiency difference in the boilers (modulating/condensing w/ better burner) and the fact that an oversized boiler is also less efficient, that number has to drop. With complete changeover to TRVs, I would bet on still less energy use.

With all this, I still think the 155 is fine. The cost for the bigger unit can be put into TRVs with a much happier result.

Careful insulation of the pipes is something that people miss but also helps put the BTUs where you need them.

just my $.02.

jerry