Is this boiler actually condensing?
The neighbor’s house is almost ready for move-in day. She has a Navien NCB-E 180 that can run at 95% efficiency. Here’s the question – although I already know the answers are probably going to be shots in the dark due to lack of detail on my part. There was no heat-load calculation, and the building is about 800 sf with a volume of about 7,300 cf. Insulation is FG batts (R27 and R49, two inches of R4 under the floor. Installation wasn’t too bad, but air leakage is most likely about 4ach/50.
The boiler runs and there is always steam visible, but it is January in zone 5. I do recall last fall seeing steam as well. I doubt strongly that the return water temperature was ever checked at installation.
I know a condensing boiler should make for a low-humidity exhaust, but I don’t recall ever reading what absolute humidity might be expected in the stack. I don’t have access to the house so can’t check the condensate, if there is any.
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Replies
Worried about the neighbours furnace condensing, there has to be a joke in there.
On a more serious note, combustion exhaust is VERY high humidity, on a properly running modcon the exhaust should be 100% RH. The difference between a well set up one is the temperature of the exhaust. The colder the exhaust the more condensing the unit is doing thus higher efficiency.
If you feel it with your hand, it should be luke warm (90F to 100F). If the exhaust is too hot to touch (above 140F) than there is not much condensing going on and you are loosing efficiency.
There is a good probability the system is not set up well, less of chance with a new build, I've seen modcons over-pumped and configured to run at 170F. At that point it is at best mid 80s efficiency.
With such small space and the min fire of 18k BTU on the unit, there is a good chance that it is well oversized for the load and would be doing a lot of cycling especially on days a bit above design temperature. Probably loosing a lot of efficiency form that as well.
Well, thanks! I make significant gains on the learning curve every time I submit a question to the GBA Q&A. I am so grateful for the thoughtful answers that come back. And yes - there does have to be a joke in there. I'm sure it will come to me in the middle of the night!
Akos, Nobody ever mentioned the RH of the stack gasses, but now that you explained it, it is clear. Of course the RH is 100%. Now I have to explain this to my sister, who, thank my lucky stars, is curious enough to listen. Damn, I'm lucky!
Thanks again!