boiler sizing

George Haselton

Want to throw this out there for discussion, especially to Dan because it could be seen as questioning his (and the industries in general) advice. Of course I am playing the devils advocate here, so bear with me.
It has to do with boiler sizing for replacement in a existing structure. We have always been told to start with a room by room heat loss. Now we all know the garbage in garbage out rule. The heat loss calc's are a theoretical model of the house based on assumptions. I know many old places that have some insulation in some walls, some windows are a certain value, some aren't. There are multiples of variations that can accumulate substancial errors.
So is it wrong to find out if on the design degree days, or below that, if the boiler runs for 100%, 50% (and so on) of the time? I've put a hours run meter on several boilers over the last two record breaking cold seasons. Rarely have I found one to run 80% or more of the time at design conditions. In a few cases they have run 100% and never caught up. One or two of these has been when a substancial addition was added to the existing boiler. I always ask about the comfort in terms of capacity. Then I find out if cold areas are from not enough radiation in that area or boiler not keeping up. There are also too warm areas, is that because too much radiation, improper location of radiation or a remodel that messed up the pre-eisting placement of radiation. Taking all this history is nice when you can, but we are a small company and really get to know our clients.
I know over sizing is rampanant in our trade. So I have pushed the limit and scared my employees often, just to shake things up. Haven't got in trouble yet in 20 plus years. I will tend to size the boiler at a minimum, but be more generous with radiation for reasons like outdoor reset, constant circulation etc.
I guess my main statement is, a heat loss of an old structure can't be as accurate as an assessment of past performance of old boiler and its ability to "drive" the existing radiation. I know all the given reason not to assume the old boiler is correct, but it can still be used as a benchmark if the past performance can be accuratley assessed.
I submit that this is as important to consider as the heat loss, and even more if your data into the load calc's are too many guesses, but you have firm knowledge of experience of existing system. I want to hear your opinions on this.
George Haselton

Wayco Wayne_2

George

Funny you should post this as it morrors my thoughts lately an a job I looked at 2 years ago and sized and never did because my Father took ill and I didn't have time to do it. I had done a heat load and thought I knew pretty much what needed to go in but this Feb. we had the longest cold snap in literally decades here in DC. The customer told me what her gas bill was and it was lower than I would have expected. I was thinking if I took her Therms used in Feb from her gas bill, could I get a more accurate idea of the real heat loss, assuming a 70% efficiency of her old cast iron boiler. What do you think? WW

To Learn More About This Professional, Click Here to Visit Their Ad in "Find A Professional"

Boilerpro_3

These methods have been discussed alot here at the Wall

And I use them regularly when sizing boilers. I often work backwards from degree days or just read the gas meter overnight at design conditions (helps eliminate Solar gain effecting the readings). Something you have to remember about heat load calcs is that they assume no one is ever in the structure (no internal gains from body heat, showers, cooking, etc), the sun never shines (no solar gains), and heat loads change instantly with the outdoor conditions (no thermal flywheel effect of the mass of the structure to even out daily highs and lows). Room by room calcs are almost always very conservative, as you have seen.

I had a long post recently describing my methodology using degree day data. It too requuires at lot of judgement calls, but I believe it is certainly another very useful tool.

Biolerpro

Gordy

I personally think that the heat load calcs. need to be done room by room especially in the older homes. Things change in older homes over the years along with changing owners.Example added insulation, radiation loops that have failed and were maybe never fixed. Just to give you my personal experiance,my house 1950's had initially 5" of vermiculite insulation in attic, 2" fiberglass batts in walls, the old boiler used to also supply heat to a detatched garage which was converted to a doctors office and had its own boiler installed. So the supply loop from the house to the garage was abbandoned. More insulation was installed in the attic in the house before new boiler was installed, DEsign temp back in the 50's may not have been the same value as used today, old boiler was not as efficient also. SO...... The new guy comes in in 1993 and puts a new boiler in after the old girl gave 43 years of good service. so he matches the new boiler to the old with out doing a heat load calc. what I have now is a 2300 sqft. house with a heatloss of 85000 btuh and a newer 172000 doe boiler 150,000 net IBR. Now I think designing a little over sized is a good thing but this is a bit extreme here. The only reason I know all this info. on the house is because my grandparents built it so, I know the background. Saying that some installers do devote alot of time to do the detective work on a heat calc. and has to do it on homes that he is unfamiliar with and as you said sometimes you are shooting from the hip still.
Simply matching the old boiler is not a good idea in my opinion.Also from what I have been reading around the web it seems that SOME designers are trying to get the amount of the heat emmiter down to the bear minimum to keep things affordable. I think that is another toe waiting to be shot off. JMHO

Mark Eatherton1

My thoughts on the subject...

We MUST do a room by room heat loss calculation for dilligence, if nothing else. I'd hate to be on the witness stand explaining to a jury that I hadn't performed a heatloss calc on an underperforming job. Might as well bring your own rope to the hanging...

That said, I've had the opportunity to drop in on many of the jobs we've performed using industry standard heat loss calcs and seen the boilers doing a 50% duty cycle when the home is at design condition. That tells me upfront that the system boiler is twice as big as it needs to be. However, as BP stated, there are MANY things that aren't taken into consideration on our calculations that SHOULD be an integral part of the formulations.

As for using exisitng utility consumption to size a boiler, I'd have no problem doing that but you need to make sure that you throw something in there to cover the unforseen.

What if the old HO kept his tstat turned down to 65, and the new HO wants the home to be at 72?

What if the old HO utilized solar gain as much as possible, and the new HO is allergic to the sun?

What if the old HO kept evey light in the house on and the new HO uses candles to light there way around the house.

Some of these examples may seem draconian, but they reflect real potentials. I thnk that if a person does a minimum of a skin loss on the building, and tempers that with a historical consumption calc that he's performing due dilligence that will keep him out of trouble with the law dawgs, sizing equipment efficiently to the end user and still being capable of delivering the end product we promise to deliver, comfort.


I think the DOE should do some checking of existing buildings and come up with the necessary factors that have a major influence on "real time" heat loss factors, and write a program that takes this information into consideration on the upfront load calcs.

Heres a picture of "R Value" in real time. It doesn't look ANYTHING like I'd picture it to be...

ME