FHA to HWB conversion project

Brewbeer

Hello all,

My force air system stopped functioning recently after 30+ years of service. Not being a fan of forced air, I am in the process of pulling together a design for a low temperature baseboard hot water system for my modest home located outside of Springfield Mass. The house was built in 1965 and utilized electric heat until 1983 (many of the radiators are still in the house). The house has four levels, one of which is slab-on grade. On the slab level, there is a family room and spare bedroom/den with carpet over the slab, and a bathroom with ceramic tile on the concrete slab. I have taken careful measurements and worked up a room-by-room heat loss calculation in a spreadsheet to help size the radiation.

Question: How do I calculate the heat lost to the concrete slab in these three rooms? I've searched the internet for answers but can't seem to find one. In my analysis (attached) I've assumed a slab temp of 50F and an R factor of 2.1 for the carpeted slab and 1.1 for the tiled slab, but I imagine the slab is much colder around the exterior perimeter.

Thoughts? Comments? Thank you. --=--Ed.

Brewbeer

Here's the heat loss calc.

Spence

Your load calculation program should have slab data included. And, if it is based upon ACCA Manual J, it should be quite accurate.

Brewbeer

Hi Spence, I created my own load calculation program using a spreadsheet. I've been using literature values for various R values, but the literature for concrete slab on grade is thin, and varies.

Thank you. --=--Ed.

Brewbeer

I am sizing the radiation for each room and looking at baseboard products. My plumber has recommended that I stay away from Slant Fin baseboards and to go with units manufactured by Sterling Hydronics.

I'm looking at the Heat Trim line for the upper level, and the Heat Trim Plus line for the lower two levels. Anyone have any comments on the Sterling products in general or the two specific product lines under consideration?

Anything else I should be looking at?

Thank you. --=--Ed.

Spence

Multiply the edge transmission coefficient by your design temperature difference. Then multiply that by running feet of exposed slab edge to arrive at your loss.

The coefficients are:
Light dry soil = .989
Heavy dry/light wet = 1.180
Heavy moist = 1.358

Spence

Questions:
1. Did you get the slab info?
2. For what reason is the anti-Slant/Fin comment from your plumber?

Brewbeer

Hi Spence, thank you for the info on the slab-on-grade heat loss. I've got a 70 degree design and light, dry soil. It doesn't change the heat loss enough to change the proposed length and location of the radiators. Revised calcs attached.

The plumber said something to the effect that "the Sterling baseboards are better". I took this to mean that he thought the Sterling products were of higher quality than Slant Fin. The fact that the Sterling products are manufactured in the next town over may also have had something to do with it.

I plan on installing the baseboards, piping, and thermostats, and and then have the plumber install the boiler. I originally offered the entire job to him but he declined. His specialty is boiler replacements (he did my last house). I was not surprised he declined the rest of the job.

Spence

The family room alone would be .989 x 70 x 90, which is twice what is shown on your load sheet as a loss.

Brewbeer

Thanks Spence,

The family room has 2 slab edges exposed to the building exterior. One edge 27 feet to the back patio, and another edge 15 feet to the (unheated) garage:

0.989 x 70 x (27+15) = 2,908

The other two walls in the family room are exposed to the interior of the building (bathroom and basement). Since these slab edges aren't exposed to the cold, I don't count these as "exposed slab edge", do I?

Again, thanks for your input. --=--Ed.

Spence

You're absolutely correct. The load sheet showed the 27' and the 15' as exposed. Since I didn't have a drawing I doubled them in error.

njtommy

The first problem is ur talking to a guy who said to you His specialty is boiler replacements. Meaning he doesn't look at the system as hole. Any contractor that doesn't do all the foot work your doing right now shouldn't even be looking into your heating project.

The next problem is Sterling baseboards are going to need to run at a High supply what temp like 180. Now this company here you can run at 120-140 SWT all day long and get the same if not more BTU out put.

http://smithsenvironmental.com/html/he.html
http://smithsenvironmental.com/ES_HeatEdge4cBro_SM_4.pdf

njtommy

What type of fuel and what type of boiler are you looking at? also how are you going to pipe the baseboard in? on loop or home runs to each from a manifold?

Brewbeer

njtommy, thanks for your replies.

In fairness to the plumber, he did come over and spend about 45 minutes looking at the project, figuring out piping and baseboard placement, and then called back several days later to say that he didn't want the job, because he is a two-man shop and that my project would require both of them to be here for longer than they typically stay on any particular job, which takes time away from all the other jobs they have. That's fine, I hope someday to be in the position to pick and chose the jobs I want.

Regarding the Sterling baseboards I am considering (Heat Trim and Heat Trip Plus), according to the company literature, the Heat Trim puts out 270 BTUs per foot and the Heat Trim Plus puts out 384 BTUs per foot at an average water temp of 130F and 4 GPM. See

Heat Trim: http://www.sterlingbaseboard.com/documents/HAC-8.pdf
Heat Trim Plus: http://www.sterlingbaseboard.com/documents/HTP-6_10-12.pdf

Thank you for point out the Smith Enviro baseboards, I will give this product a closer look, their product literature gives output values exceeding the Heat Trim Plus at equivalent temp and flows.

For my project, I am sizing the radiation using an average of 130 degree water (see spreadsheet posted above). Piping will be in the loop configuration, 3 zones, one for each floor. Natural gas is the fuel. I am hoping to use a modulating, condensing boiler.

Thanks again, njtommy, any other comments you have are appreciated.

njtommy

OK with the Mod con boiler and your SWTs along with your small zones I would highly recommend 2 things.
1st A buffer tank this makes sure your boiler will not short cycle. Due to your lowest firing rate on every mod con boiler being higher then your smallest btu requirements of your smallest zone. check out Boiler buddy buffer tanks. they have plenty of info on there web site.
2 pulling a supply line to each section of baseboard from a radiant manifold to insure you get that 130 swt to each section of baseboard. Please do not pipe this in one continuous loop you will never get the BTU out put you need for each section of baseboard.

How are you going to control your zones? pumps and zone valves or just pumps?

Brewbeer

Thanks njtommy. On the top floor I can't homerun to each baseboard without removing the ceiling in the family room. The house is a four level split, with each level 1/2 flight of stairs from the next. To get piping to the top floor, the ceiling in the lower level bathroom and hallway and part of a wall in the basement stairwell will need to be removed. My plan is to run this zone in a split loop configuration, probably with one inch pipe to the split due to distance from the boiler and the number of bends needed to get there resulting in a zone length exceeding 170 feet. The middle floor zone only has two lengths of baseboard, so that also seems like a good split-loop candidate. The lower level has good basement access so homeruns are possible with that zone.

I haven't decided on a boiler, as I am still focused on getting the radiation sorted out, but the load calcs would seem to suggest a 50K BTU or smaller boiler. I am also thinking of using an indirect tank for DHW so I can do away with the water heater. Haven't given any though to circs vs. zone valves, but I guess that decision is influenced by the head loss of each of the zones. I will need to do some research on the buffer tank. Any links to info you suggest?

Thanks again. --=--Ed.

njtommy

Well I think what your going to find is that your going to have to run your supply water temps higher on ur baseboard then you want, but it will be a little bit of a balnceing act to get it just right.

Check out
www.boilerbuddy.com.

njtommy

Also for DHW when looking at inderect tanks they have all the specs on them for minimum btu requirements most are up around the 100,000 btus or so.

If you have a smaller boiler you end up making hot water slower but all the calculations will be there too. Depending on brand.

Brewbeer

njtommy said:

Well I think what your going to find is that your going to have to run your supply water temps higher on ur baseboard then you want, but it will be a little bit of a balnceing act to get it just right.

I can see the concept that as the water flows through the baseboard, heat is given up, which reduces the water temp, and decreases the output of each unit of baseboard further down loop. Thus, heat output per unit of baseboard is a function of decreasing water temperatures as the distance the water flows through the baseboard increases. This function should be solvable using differential equations. Someone must have done this already. Does anyone who has this equation want to share it?

njtommy

I'm sure there is a way to figure out the math on this. Most fine tube baseboard is designed around a 20 degree delta T.

The end of your runs are where your heat will suffer the most. But if you could lay out the piping and basebard in such a way that you could have your smaller btu rooms be at the end of the loop.

The other way is you would have to calculate the supply water temp by the time it hits ur other runs. And recalculate how much base board u need for those rooms. You would be looking to add more linner feet to get more btus out put.

njtommy

The majority of the time this is not a problem do to the fact the most homes with baseboard have way more then they need and they are running 180 degree supply water through the baseboard.

Even if you have to bring ur supply water temp up to 140-145 mark you will still be condensing with return water at 125. You want no higher the 130 on ur return loop.

You seem like your a pretty smart guy. I'm sure you will have it figured out.

Brewbeer

Thanks again.

The Loop Hot Water Q&A states that at a 20 degree differential, 1 GPM delivers 10,000 BTUs. Given these, I'm assuming the following statements are also correct:

1) At a 20 degree differential, 4 GPM delivers 40,000 BTU/hr.
2) At a 20 degree differential, 2 GPM delivers 20,000 BTU/hr.
3) At a 10 degree differential, 2 GPM delivers 10,000 BTU/hr.
4) At a 1 degree differential, 2 GPM delivers 1,000 BTU/hr.

The last statement, if true, is useful for estimating baseboard temps (and heat output) as you get further down a length of baseboard.

Since my calculus skills are very weak, I used a spreadsheet to estimate decreasing BTU output per foot of baseboard as heat is given up and temps decrease. For the Heat Trim Plus baseboard, for an input temp of 130, the first 2.6 feet of baseboard puts out 1,000 BTUs/hr with a 1 degree temp drop. By the time the water passes the 10th foot of baseboard, the water temp is 126 degrees, and the next foot of baseboard puts out 354 BTUs/hr. By the 20th foot of baseboard, the temp is 123 degrees, and the baseboard output is 327 BTUs/hr per foot.

Using these principles, I ran the numbers and put together a table of output for each run of baseboard along with supply and return temps. Table attached.

All feedback is appreciated, I want to get this right before I purchase materials.

Thanks again.

Brewbeer

For those interested, I've attached an abbreviated version of the spreadsheet used to come up with the output calculations.

njtommy

Have you been able to figure out your total feet of piping for each loop yet?
This would be ur next move to really do the math for your feet of head per loop to pick out your zone pumps.

Brewbeer

The top floor zone's longer split loop is 60 feet of 3/4 and about 90 feet of 1 inch up and back from the boiler location, not including losses for elbows. The other two zones are shorter since they are closer to the boiler. I haven't thought about the pump yet since I'm not sure the radiation is correctly sized/calculated.

Anyone want to look at the math used to size the radiators? Am I in the ballpark or am I out to lunch?

hot_rod

Here is what I come up with for the upper zone.

SlantFin #30 baseboard 180°F supply, Grundfos 15-58 spd 1

Brewbeer

Hot rod, thank you for that info. I took a look at the ratings for the slant fin 30, and they are basically the same as the heat trim at low temps. Would you be willing to run another simulation using your program for 130 degree supply water temp and 2 GPM? That would give me a good check on my calculations.

Thanks again. --=--Ed.

Charlie from wmass

You are talking to the wrong plumber. Anyone know a good one near Springfield?

hot_rod

Here is the top floor with 130°F supply, 4 gpm. I started at the bedroom this time.

The warning at the bottom indicated low return temperature, for conventional, non condensing boilers.

The last board on the loop should be labeled bath, not m bed.

Brewbeer

Thanks much hot rod. Looks like my calcs don't agree with the output of your program. I'll need to go back and try to figure why my calcs are lower than yours.

Brewbeer

It would seem that baseboard manufacturer literature output tables include an extra 15% for "heating effect". In other words, the listed ratings are overstated by 15%. Also, finned length is 4.5 inches shorter than baseboard length. Two sources for errors identified so far.

Rich_49

Tommy's idea about buffer tank was warranted . You should look at this boiler , you can eliminate the buffer tank . Boiler Buddy Tanks are insulated by the manufacturer of this boiler also . How much do you suppose it costs to ship a steel tank from Ohio to Mass , have it insulated and ship back to OHIO ? Do you want to pay for that ? Do you want 2 pieces of equipment when 1 will suffice ?
Use the Smith's on the first floor in any piping configuration that works for you , the outputs vary . You're kinda at the mercy of the installed baseboard on the second floor and will have to make that work . You have already identified the MAGIC 15% HEF and Rod has sent you some good models using HDS so you should be fine . What Rod sent is accurate by the way . Maybe you are using some other type circ in which case maybe he would be kind enough to show you the system with whatever you were thinking of using , if in fact you were thinking of something other than the 15-58 .

Brewbeer

Hi Rich, thanks for the feedback. This is a completely new install, so nothing exists yet. I'm going to recalculate the heat output using the lower values to account for the HEF, and adjust the baseboard lengths where possible. I can add more baseboard by coming around room corners, or switch to a higher output baseboard if needed. The goal here is to keep the boiler operating in the condensing temp range.

Has anybody used the smith enviro baseboard? Its made of lighter gage steel than the sterling and I am wondering how they hold up. I have children.

Thanks again.

Charlie from wmass

They can use lighter steel as the fins fill they space behind it and stiffen it.

hot_rod

Here is another sim. I slowed down the flow, it did jump the baseboard lengths up a few feet per each board. So instead I upped the supply temperature a bit, and took some BB length out.

I'm assuming the piping detail from the boiler, I use 100 equivalent feet, estimating some ells, valves, check, etc.

Consider investing in the HDS program if you want to run a bunch of "what if" simulations to really optimize the design.

A few key strokes saves hours worth of long hand or data input into a spread sheet. And the info is correct

Brewbeer

Thanks hot rod, I appreciate it.

So, I've done more research and spent more hours with the spreadsheet and given more thought to this. Some key data points and info to share:

My gas bill for heating and hot water has been running about $1,200 per year, for an average 100 per month. Looking at winter and summer usage, we use about 15 units of gas in the summer (for hot water), and a max of about 125 units at the peak of the winter heating season, so about 20% of my yearly gas usage is hot water.

The (now-dead) forced air furnace in my house was direct-vented through PVC, and did have a condensate pump, so I am assuming this unit was reasonably efficient. For discussion purposes, let's estimate it's efficiency at about 85%.

Boilers start to condense at approximately 131F (depending on the hydrogen content of the natural gas input fuel). Above 131F, not much condensation is occurring. To get the highest possible efficiencies out of a condensing boiler, you need temps much lower than 131F (e.g., below 100F).

New, non-condensing boilers run at about 75% efficiency. A condensing boiler in my application would achieve an efficiency in the 85% to maybe 90% range (this is a guestimate).

Looking at these numbers, it would seem that the potential dollar savings of a condensing boiler over a non-condensing boiler is on the order of maybe $100 or $200 dollars per year at current natural gas prices. While I don't know how expensive boilers are, I am guessing that a condensing boiler for my house is probably $2,000 more to install than a non-condensing boiler. Not including the higher cost of the additional radiation needed for a condensing boiler, this gives me a pay-back period somewhere in the 10 to 20 year time frame, given current natural gas prices.

Which leaves me thinking, is a condensing boiler the right boiler for my application?

Thoughts? Thanks to all who have provided input. --=--Ed.

Brewbeer

A very interesting efficiency graph:
https://csemag.com/uploads/RTEmagicC_CSE1206_FHVAC_1.jpg.jpg

Based on this graphic, I am really questioning the pay-back of a condensing boiler for my application.

Additionally, I'm assuming that domestic hot water (as in a indirect) is made above condensing temps, correct?

hot_rod

Good thoughts. Remember Mod Con boilers are usually also modulating, so there are some energy savings there. Also it's the return temperature that puts or keeps the boiler condensing, so with 130° supply, you should still drive condensing conditions.

If you design for those temperatures at design days, most of the heating season you will be below those operating temperature requirements.

It is possible to generate DHW at condensing temperatures. If you are looking for 120- 125° DHW temperatures, which is what most codes require.

Perhaps the biggest unknown is future fuel costs, and ongoing maintenance costs for the various boiler choices.

Certainly there is a place for conventional, non condensing boilers. At a seminar I attended last week presented by ECR, (Dunkirk, Utica boilers) they indicated it is still about a 50- 50 mix of condensing to non-condensing sales.

Charlie from wmass

Non condensing boilers are 84 percent

SWEI

The actual fuel savings difference between 83-84% conventional boilers and 93-95% condensing boilers is much larger than the difference in AFUE would indicate. We see more like a 20% difference for new versus new if both are properly sized, installed, and commissioned (which is far less common than you might think.) When replacing an older, improperly sized, or badly neglected conventional boiler, the savings start around 30% -- and can easily break 50% depending on the degree of knuckleheading present. Find a good contractor.

Charlie from wmass

I have an idea of the level of knuckleheading that could be done in my area and there are jobs I could install a chimney vented cast iron boiler and cut the fuel bill compared to their condensing boiler installations.