Getting Ready for Gas in Central CT

WalnutFarmer

Charlie, Rich-

I ran some new numbers with the scenarios you mentioned (68 indoor/-10 outdoor, 72/6, 68/35). See new table.


The one zone that calculates out to 803 BTU/hr/lf is an odd space. It is a mudroom and entry area where we don't spend much time. It is also open on 2 sides to other spaces which I'm sure lessens the load on the emitters there.

So, it seems the 110k boiler could keep the whole house at 68 down to -10. We use programmable tstats so it's rare we are heating the whole house at once.

Bigger concern I suppose is short cycling in the milder winter weather and shoulder seasons.

njtommy

Even at 35f odt your going to be short cycling with your smaller zones. Go up in out door air temp 10-20f. It will definitely be short cycling with even your largest zones

Either a different boiler that's has some water capacity or a buffer tank would work very well with the KHN boiler.

Charlie from wmass

@WalnutFarmer that is why I installed that one Buffer tank for @Brewbeer . I would not take the mud room into consideration when planning against short cycling. I would also like to know how much baseboard or heat emitter you have in each zone.

Brewbeer

@WalnutFarmer are the baseboards fin tube design? If we could nail down the output of the radiation per foot that would help with estimating supply water temps needed to meet the heatloss. Fin tube baseboards come in a few different sizes each having different outputs depending on things like the number of fins per foot and the surface area of the fins and the diameter of the pipe supplying the hot water.

In my system, 130 degree supply water provides a theoretical output of 330 btu per foot of emitter. If your emitters have a similar output, zones 2 and 3 appear to be large enough in relation to heat loss to get you into the condensing range for much of the heating seaon, an important consideration if you are considering a mod con boiler.

Brewbeer

@WalnutFarmer I also agree with @Tim McElwain about his comment regarding standard vs.high efficiency boiler. The life cycle costs of the high efficiency boiler is in my opinion probably going to be higher than the standard efficiency boiler due to the higher initial installation cost and higher service costs being more than the savings realized by slightly lower fuel consumption. If lowest life cycle cost is your primary driver, the standard efficiency boiler is probably a better choice. The high efficiency equipment becomes a more favorable choice if non-pecuniary considerations also factor into your decision.

Rich_49

Charlie from wmass said:

@WalnutFarmer that is why I installed that one Buffer tank for @Brewbeer . I would not take the mud room into consideration when planning against short cycling. I would also like to know how much baseboard or heat emitter you have in each zone.

He listed emitter lengths Charlie in the first post with a chart about 4 -6 post prior .

Charlie from wmass

Missed that. Has the brand and model of baseboard been determined? Actually I saw that and I saw the listing by what would be required per lineal foot what I mean is what is the rated output per lineal foot

WalnutFarmer

The baseboards are fin and tube. The pipe is 3/4" copper and the fines appear to be ~2 3/8" across x 2" high. I counted 27 fins in a 5 inch span, so roughly 5 fins per inch.

Charlie from wmass

So for 180 degree water I'm guessing that would be 610 BTUs per lineal foot. Can you post a photo

WalnutFarmer

Here's the baseboard. Time to get the vacuum out.

Brewbeer

Compress air works really well if you have an air compressor.

NY_Rob

If that's the Fine/Line 30 radiator, here's the ratings chart..

WalnutFarmer

I never answered some of the questions posed along the way. Here goes:

-Coldest it gets around here: Record low is -18F. Not uncommon to get down around 0F but rare for any extended time. (With our zones and setback tstats, it's also rare we need to keep more than half the house at 68 during a cold snap.)

-Hard to say how long we will be here. 51 now, and long term destination could depend on where kids settle after college. Probably better than 70% odds we'll be here until 70.

SWEI

Brewbeer said:

The life cycle costs of the high efficiency boiler is in my opinion probably going to be higher than the standard efficiency boiler due to the higher initial installation cost and higher service costs being more than the savings realized by slightly lower fuel consumption.

It may or may not, depending on the current and future cost of fuel.

WalnutFarmer

I am starting to appreciate how complex this process can be... Here is a simplified view that I'm interested in your views on.

My heat load at 68/6 is ~80k BTU/HR. At a gross level, an 85k BTU boiler *should* be able to handle this

If conditions are more extreme, say -10F, the load would be ~110k. This is beyond boiler capacity, but do I really need to keep the entire 3100 sf at 68F? Probably not. If I keep a few zones in the 60-65 range, that would potentially allow an 85k boiler to handle the load?

The KHN85 turns down to 8.5k btu/hr, which should allow for less short-cycling?

I guess the fundamental questions I am getting at are:

1. Is the KHN85 big enough for a severe cold snap?
2. Does the reduced size (vs. KHN110) make a buffer tank less critical?

WalnutFarmer

I may be getting carried away here....

One more thought for today before I have to get back to my day job.

See chart below. This captures my BTU/Hr load (based on heat loss) per LF of emitter in each zone... across the four indoor/outdoor temperature scenarios. The dotted lines represent the output per LF of emitter at different hot water temps to see what operating temp is needed to match the load. [Corrected chart]

Does this get us any closer?




Note that Zone 1 is the mudroom/entry area that is an anomaly... not core living space.

Charlie from wmass

It comes down to how much you want to push the envelope of efficiency. Are you willing to have parts of your house only at 60 degrees or less during a cold snap? If you utilize a buffer tank and the 110 you give yourself a much wider operating range and inability to keep the home comfortable even in negative 10 weather. I feel neither boiler would give you issues with domestic Hot Water Production except for obviously during -10 weather the 85 will have to sacrifice some of its BTUs it is capable of to domestic Hot Water Production.

NY_Rob

IDK if it's been mentioned to the OP... but for a mod-con you want the return water temp at or below 130F as much of the time as possible to be in the condensing zone. If RWT is above 130F the boiler efficiency drops down to standard atmospheric boiler range.

Charlie from wmass

It has been determined though that during the shoulder months systems reach condensing temperature as much of the time even with conventional sizing all the heat emitters

Brewbeer

I'd be surprised if the heat loss you estimated is really that high. I've got 2,000 sq.ft. of heated spaced in a 1965 model split, none of it below grade, with a heat loss of about 30,000 at 0F.

Rich_49

Man J and ASHRAE heat losses average 15 - 20% higher than actual .

WalnutFarmer

Brewbeer said:

I'd be surprised if the heat loss you estimated is really that high. I've got 2,000 sq.ft. of heated spaced in a 1965 model split, none of it below grade, with a heat loss of about 30,000 at 0F.

@Brewbeer How did you calculate your heat loss? I used the Slant FIn calculator and I am admittedly a novice, but I am fairly confident I entered accurate data for what it asked for.

Thanks.

Brewbeer

@WalnutFarmer, I measured up the interior of each of my rooms exposed to the building exterior or the unheated basement, estimated R-values by looking up literature values of the various structural assemblages in my house, and put the data into a spreadsheet I wrote to help me design my system. I've attached a PDF of the results.

I also did a heatloss using the HDD/gas consumption method, and that heatloss came in even lower than the spreadsheet method I just described.

One thing I've observed in my house is that the calculated heat loss does not need to be 100% met by the new heating equipment. Electrical loads, cooking, and even people, all put heat into the house, and it is substantial, probably accounting for 10-15% of the heat lost by my house on a design day (0F).

WalnutFarmer

@Brewbeer Thanks for sharing that. I may have to go down the same road.

If you'd consider sharing your spreadsheet template, I'd love to give it a go, but completely understand if you'd rather not. (Is the heat load formula something I could find online somewhere so I could build my own spreadsheet?)

I too suspect the slant fin calcs overestimate my load, and based on how we use the house, I doubt there would be more than a handful of hours per year when we would be needing to meet demand for the whole house at the same time. I'd hate to install a furnace designed to meet demand that will never need to be met.

Thanks again.

Brewbeer

@WalnutFarmer , send me your email address in a PM and I'll email it to you. It is an excel spreadsheet.

njtommy

its been mentioned on here before the slant fin app is off a bit for heat loss calc. Most peopl range to be off any where from 5-20% off. For my house the slant fin app said 45k for 70f at 5 odt. I did the heat loss by usage after installing new and it came out to about 40k.

WalnutFarmer

If that means 5-20% more emitter sales for Slant Fin, I can't say I blame them. Has to be some return on offering the free calculator

Charlie from wmass

@njtommy in the end you still would have the same boiler whether you used the apps number or your usage number though. Right?

njtommy

Yes Sir Charlie same size boiler.

In walnut farmers case I would go with the with 110 with a buffer tank and be completely comfortable on the extremely cold days. I would try to adjust the odr curve to maximize efficiency and possibly add a bit more baseboard in areas to allow lower supply water temps if at all possible

WalnutFarmer

@Brewbeer was nice enough to share his heat loss spreadsheet, which I have adapted for my own situation. I need to triple-check my inputs, but I was shocked at how much lower my heat load came out.

The slant fin model at 68/6 design temps gave me a total load of 84,643 BTU/HR.

Calculating the load myself in Excel for those same temps generated a result of 31,158 BTU/HR .

I'll play with the calculations a bit more to determine where the sensitivities seem to be, and to make sure I didn't screw up... but seems that @Brewbeers suspicions of my original numbers being to high are valid.



Even bumping the temp profile to 70/-15 only shows total load of 42k BTU.

WalnutFarmer

It seems a big source of the discrepancy between the numbers I got from the Slant Fin calculator vs. the spreadsheet @Brewbeer shared is the "infiltration factor." Slant Fin asks for a description of tightness of window construction and how many walls contain windows in a room. When I selected an infiltration factor of .027 (for weatherstripped and storm-sashed, with windows/doors on 3 sides), it nearly doubled my heat loss values compared to not using the infiltration factor.

I'd appreciate any thoughts you pros might have on this.

FYI... I have replaced all windows and doors in my 1975 house over the past 10 years, have added weather stripping, and have icynene foam in my attic which noticeably reduced drafts when installed. I suspect my infiltration is low, but no idea how to measure.

Thanks.

Rich_49

Please verify infiltration factor .an you enter ACH anywhere ?

WalnutFarmer

Slant Fin allows me to enter infiltration factor per their suggestions (which is how I got 0.027).

Brewbeer's spreadsheet does not seem to account for infiltration.

I am admittedly flying a bit blind. I've done some reading on ACH online, but don't really know where to start in getting a realistic estimate. One article indicated it was guesswork without a blower door test, and something that contributes to frequency of oversized boilers.

Brewbeer

@WalnutFarmer , correct, my spreadsheet does not include a factor to account for infiltration, which isn't a big problem for my house, which is very tight, since it was built with electric heat, and all exterior surfaces have a substantial polyethylene film built into the wall/ceiling assemblages. Check out this link to a method for calculating heatloss due to infiltration.

http://www.greenbuildingadvisor.com/blogs/dept/musings

WalnutFarmer

I ran some infiltration calculations using ACH values of 0.5, 1.0 and 1.5. (I suspect I am at 1.0 or less, but clearly that's a guess.)

Results are in chart below. The gray box is my thinking on the likely "design space" that should be considered for sizing a system.

At the high end of this design space, infiltration is adding ~50,000 BTU/Hr in heat loss. Adding this to my baseline heatloss of 40,000k BTU brings me to 90k, which brings us back to the KHN110 with buffer tank.

If my ACH is truly in the 1.0 or less range as I suspect, my total load is 75,000BTU or less, suggesting a KHN85 with buffer could handle the load.



Thanks for letting me noodle this out. Any feedback is appreciated.

WalnutFarmer

I was able to dig out the results of a blower door test I had done 4 years ago. (This was before I added 400 sf of tightly constructed space, and eliminated old, loose exterior french doors... so if anything things are tighter now.)

CFM50 was 2980, which translates to ACH50=7

Based on a few online articles, I believe it is reasonable to say this converts to ACH(nat) of around 0.50 or less. If I add mechanical ventilation (bath fans etc), I suspect I'm still under 1.0 ACH.

Am I making reasonable leaps here?

WalnutFarmer

At the risk of burying the information I posted above (for which I am still hoping for your feedback!), I came across one other important question:

As noted previously, I have 4 zones on baseboard heat and one zone running on a fan coil. I've been told that fan coils need to be run at higher water temps, otherwise the blown air can feel cool. If the coil needs 160-180F water (guessing here), does that toss out my hopes of running in condensing mode??

Paul48

Many of the mod/con nowadays can run multiple curves.

Zman

WalnutFarmer said:

At the risk of burying the information I posted above (for which I am still hoping for your feedback!), I came across one other important question:

As noted previously, I have 4 zones on baseboard heat and one zone running on a fan coil. I've been told that fan coils need to be run at higher water temps, otherwise the blown air can feel cool. If the coil needs 160-180F water (guessing here), does that toss out my hopes of running in condensing mode??

You can either run multiple curves, mixing or reconfigure the fan coil. If you either slow the air across he coil or increase the size of the coil, you can run it at lower temps.

njtommy

whos fan coil unit are you running? Depending on size you have now and what you need for btus you maybe fine running lower Swt. Also if you can change the fan speed that can help a little bit too. The manufacturer has charts and heating adjustments in directions/ install manual to check sizing and btu out puts for different water temps and entering air temps.

I run a fan coil unit for my garage with Max supply water temp of 150 and get plenty of hot air out of it.