sizing water to air heat exch for wide temp swing?

[pybyr]

Posted this over at Heating Help's the Wall also, but figure the more minds thinking and responding, the better-

I'm installing a wood gasifier boiler (Econoburn 150) (arriving next week) along with a really big (1350 gallon) (under construction) stainless steel water tank so that I can run the boiler hot and fast (and efficient), heat up the big tank, and then the house can draw the heat back out of the tank while I'm away at the day job, etc.

Someday I may go with radiant, but the house (an 1830 VT farmhouse, now mostly insulated, at least for an old house) has an existing forced warm air system (125kBtu ThermoPride) with all the ducts in place.

The wood boiler/ tank, etc., is doing to be sufficiently pricey that for the short haul, that my plan is to put a water-to-air heat exchanger fin-coil in the main trunk of the ducts right over the oil furnace, and use the oil furnace's air blower to move the air.

From what I've read so far, I know that these hybrid "hydro/air" systems are sometimes used, although I also know that people's opinion's of them vary- in any event, it's what I need to do for now.

So here comes the question-- with "normal" hydronics, I know that you design and select components around a "delta T" and an assumed, and probably pretty predictable water temp, to size things like radiators and heat exchangers.

In my situation, the variables start to get a lot fuzzier and wider ranging. The max that the "big tank" will probably hit when fully charged, will be about 175-180 F. The lower I can draw the temp in the big tank and still throw BTUS at the house, the better (I know that someday things like radiant floors or Euro-panel radiators open up a lot in that direction, but I just can't affort those in the foreseeable future)

If I can size/find/buy a water-air HX that will fit in my main duct above the oil furnace (which happens to be 22x22 inches) that will still add some useful BTUs into the airflow down to the _lowest_ possible temperature, then the effectiveness and capacity of my heat storage will be much better than if I use a "regular" HX that just assumes something more normal like 180F in/ 160 F out.

And for purposes of my question, forget about worrying about boiler return temp protection- I am dealing with that separately.

For now, please just help me pick, or figure out how to pick, a water-air HX that'll fit in a 22x22 duct and that will put out as much heat as possible as the water temp sags as low as possible (it would be great if I could still be getting heat into the ducts when the big tank is down around 120).

Thanks very much Trevor

 

[Redox]

Heat exchangers are rated at a specific flow rate on both sides of the heat exchanger for comparison purposes. If you increase the airflow or water flow, you will increase the heat transfer rate, but burn up more fan and pump horsepower. Oversizing the heat exchanger will help, but there comes a point where it all becomes excessive. Putting a pair of coils in series will help, but putting them in parallel may help more. Is it possible to install two coils arranged in an A shape to get them in the duct next to each other (much like an A coil for an air conditioner)? Half the air and water will be going through each coil, but it will be moving slower and may give you a better approach temperature. You could build a bigger coil box to hold them and still have room to get it on top the furnace.

Chris

 

[pybyr]

Thanks Chris- I guess next thing I should do ('been on my to do list for this project anyways) is to call my oil furnace manufacturer and find out the nominal CFMs that I should assume are moving through that 22x22 outlet from the oil furnace. I like your idea of the "bigger box" to hold more coils, but it has to end up rising up, turning a corner, and then transitioning to an 8x24 main trunk, all with limited headroom in an old farmhouse basement, so it's a little puzzling to figure out how to squeeze all that in.. but please keep the suggestions coming!

 

[Redox]

How big is the furnace in BTUs? How many horsepower is the blower? Is there an AC unit? How big? I could take a swing at it and get you in the ballpark.

What about the return air duct? You might be able to squeeze more coil into the return and still recover heat from the tank as the furnace is running to make up the difference. It's admittedly not as efficient overall, but workable.

Chris

 

[pybyr]

Chris- thanks! the furnace is a ThermoPride OL16, rated 125,000 BTU actual output (net, after accounting for efficiency) "at the bonnet" (it's about 156k gross, prior to losses up the stack)

the blower has a 1/2 HP 1725 rpm motor with an approx 3 inch pulley driving a 6 inch pulley on a big squirrel cage fan with an outside housing that's roughly 16 x 16 inches (the inside actual blower rotor is of course smaller but it's awkward for me to get in there to measure it). I'll try to call Thermopride today to see if I can get "official" word on the CFMS, but hopefully, maybe, the other stuff I just listed about the blower will let you ballpark in revese

when you say "extra coil in the return" are you referring to one coil in the return (which is 18x22) and one in the output? that's an interesting concept, especially if I staggered them in a counterflow arrangement, where the one on the furnace inlet was in series (hydraulically) with the one on the furnace outlet, so that even low-temp water coming out of the "main" coil would still help pre-heat the coolest air as it comes in via the return duct. I guess overall it'll be handiest if I can shoehorn "enough coil" into the return coming out of the furnace, but overall, it also makes sense to do what works best

I also realized that at least in concept, that location where I'll need to go round the corner and transition from the 22x22 furnace plenum to the 8x24 main duct ??might?? be a place I could have a wider section of duct if a main coil bigger than 22x22 ends up making most sense

Thanks

 

[Redox]

I can't find anything on the Web that gives the airflow range of that specific furnace, but it is probably in the 1200-1600 CFM range. The actual air delivery will depend on external pressure and the speed of the blower. The motor sheave is probably adjustable to allow you to change the blower speed. You don't mention A/C, but most furnaces in the last 20 years or so had blowers that would support air conditioning, which means that you can add some external resistance without causing a problem. It does sound like yours is big enough.

Actually, I hadn't thought of 2 coils on supply and return, but this has interesting possibilities as well as potential problems. What I was trying to say was that the return is usually larger and may give you more room to put a bigger coil in the system. The good part about putting the coil in the return is that the furnace can still run the oil burner with the heat recovery on whereas on the discharge, you will lose heat to the circulating water and this would be counterproductive. The problem with a return coil is it will collect dirt unless you move the filter to its inlet. There is also the possibility of overheating the fan motor as most motors are rated for 104F maximum ambient. Your coils are probably going to be putting out air a lot hotter than 100F, we hope!

I think it would be better to keep the coil(s) on the furnace outlet, but valving in a return coil to add surface area when the water is cool has possibilities. You are going to need something wider than 22", probably something on the order of 30" wide by 24" high just to make capacity with 180 F water, more if you want to go with cooler water. Maybe you already answered your own question and adding a second coil in the return will allow you to go down to 120 F? I see no reason why it wouldn't work, 'cept for the dirt issue. This is going to increase the pressure drop a fair amount and you will probably want to increase the blower speed some to compensate. Check the motor amp draw and see if it's got any HP to spare.

I hope some of this is making sense! :smirk:

Chris

 

[pybyr]

Hi Chris- thanks for the further follow-up--

work yesterday got in the way of my calling ThermoPride, but I was able to call them today, and they said that basically my existing oil warm air furnace is specified to push 1600CFM of air out the bonnet into an assumed typical 0.2 static pressure from the hot ducts. The guy who did my original duct install was not a corner-cutter, so I am going to assume I don't have any atypically high back pressure. Maybe I will turn on the blower by itself and stick my Bacharach draft gauge in a hole in the return just for curiousity; I know that vacuum in the return won't correlate precisely with back pressure in the hot side, but I assume they're vaguely related?

more head-scratching to do. I know that apparently there are some people who use plenum-coils above existing oil warm air furnaces for tying in OWBs, and I even heard of one person who used a coil in plenum above an electric resistance warm air furnace as a way to switch from electric to natural gas using a tankless water heater, so there must be a way to do this-

I just don't want to take a total blind guess and get it too far wrong, on either the overkill/expense or under-performing ends of things

 

[BrownianHeatingTech]

A nice big coil will let you draw usable heat down to about 140-degree water. Below that, and you will feel "cool" air from the ducts. It will still heat the house, but it will not be comfortable.

Another option to get a larger coil into a given duct is simply to install it at an angle (or even build an A-coil out of two coils).

Joe

 

[pybyr]

I appreciate all the suggestions people have offered so far; I want to finalize what I am doing for the coil and order it, especially because it looks like there may be lead time.

Since people seemed to suggest that I'd likely need a bigger than average coil, I looked for coil manufacturers, and found “Precision Coils” [ www.precision-coils.com ]. They have some form of software that allows modelling of all sorts of combinations of coils and how much heat they can move, and the various airflows and waterflows and losses of each. They apparently do not sell direct to end users, but the fellow I spoke to there at Precision Coils referred me to one of the various people who sells for them, and who they said would be very able and willing to help me, using their sizing software-- Ross at Nationwide Coils [ www.nationwidecoils.com ]. Ross was indeed very helpful,

I explained to Ross how I want to try to extract from water to air as close as possible to 125,000 btu/Hr output (the net rating of my oil furnace assuming 80% efficiency) at 1600 CFM of air (what the furnace's blower will push), at the wider than average water temperature range of a high of 180 (boiler running, or tank fully charged) down to 120 (tank nearly depleted).

Ross steered me to four possible candidate coils- basically a choice between two standard coil sizes-- 21x22 or 24x24, and then the additional choice of 2 rows of water tube vs 4 rows of water tube in either size. Dimensions are for the actual coil itself, with an inch wide flange around all sides. Headers for the piping can be any size I pick (any advantage to going bigger than 1”??? ).

Regardless of width, the 4 row will have an air resistance of 0.2 inches of water column, and the 2 row 0.1 inches of water column, either of which (from my checking with the manufacturer of my furnace) will be OK for the very big low speed 1600CFM belt driven blower in my oil furnace that'll be blowing air through the water-to-air HX. So the 2-row vs 4 row seems like “a wash” when it comes to how much air it will move.

The 4 row will have 16.8 inches of water column back pressure/ head, regardless of which size, and the 2 row will have 4.9. That seems like a pretty significant proportional difference, but maybe (I hope) only in terms of something I'll need to be aware of when it comes to picking the circulator for the secondary loop that'll feed this coil.

So- on to the biggest variables- ability to move BTUS, and cost to buy

21x22 2 row can move 109,000 btu with 180 degree water at 11.2 gpm flow
21x22 2 row can move 47,000 btu with 120 degree water at 4.8 gpm flow
It would cost $330

21x22 4 row can move 163,000 btu with 180 degree water at 16.7 gpm flow
21x22 4 row can move 79,000 btu with 180 degree water at 8 gpm flow
It would cost $773

24x24 2 row can move 114,000 btu with 180 degree water at 11.8 gpm flow
24x24 2 row can move 79,000 btu with 120 degree water at 5.3 gpm flow
it would cost $653

24x24 4 row can move 179,000 btu with 180 degree water at 17.7 gpm flow
24x24 4 row can move 81,000 btu with 120 degree water at 8.2 gpm flow
it would cost $870

at first I was worried about maybe needing to do some sort of variable-speed circulator, then I re-read Siegenthaler's point that it's most important to make sure you have enough flow for the max-flow-need situation, and that there's little lost (other than a bit of electricity) if you are pushing water faster than needed through the coil under other circumstances. So I guess whatever coil I get, I just need a circ big enough to push the max flow through- or, if this all falls OK in the ranges/ curves, maybe one of those nifty newer “3 speed” universal circulators, so that I can try different settings.

I know enough to know that the above variables of BTUs and flows all definitely matter, but don't have the experience that some of the rest of you do to really try to “nail” the right balance of moving lots of BTUs without overkill on either cost or margin for error.

I have no desire to spend more than I need to on a coil, but I also would much rather spend a bit extra now than end up with a sub-satisfactory system that'll cost me more, in the long run, to operate or get working properly. I may transition to radiant floors someday, but the more I look at my old house's original 2inch thick random-width spruce plank floors, the more I realize that they won't transmit heat from underfloor radiant all that well, and they're too nice to mess with. So if I can get a good fan/coil that works well, it may very well be my permanent solution-- so I definitely want to do it once and do it right, and not end up with regrets about having cut corners.

The 24x24 4 row is definitely the spendy-est- but it also looks very much as if it will have no trouble moving substantial amounts of BTUs, in the general range of what I need (recall that my oil furnace is 125,000 btu/hr (output, assuming 80% efficiency) over a pretty wide water temperature range; the 21 x 22 4 row looks like a close runner up for about $100 less. The 2-rows are looking doubtful, in that the 21x22 2 row really won't move the sort of BTUs that I need, and the 24x24 2 row is nearly as costly as the 4-rows in either size, but with noticeably poorer performance. That is, unless some of you tell me that the 2 row 24x24 coil is going to allow me to use a circulator that will be substantially less $$ to buy and operate, and that I would not be able to use with the 4 row coils, and also that you think the 24x24 2 row will be safely “close enough” on BTU capability

Finally- these coils are definitely more expensive than some I have seen advertised for hydronic/air retrofits- for example, at
(broken link removed)
but it does definitely look as if the Precision-coils coils will have less air back pressure and move more BTUs- so I am assuming that the price difference reflects the “get what you pay for” factor.

What are the reactions and suggestions of those of you who really know your way around these types of design options and choices?

Suggestions on circulator and secondary loop pipe sizing are welcome too, in case that factors into best overall coil choic

 

[Redox]

I think you are going to have to rule out the 2 row coils as they won't even make capacity with 180 degree water. The price/performance ratio in the 4 row coils isn't enough to make a real difference in my book, so it all comes down to how much coil can you fit in the duct? You really can't go too big on a hot water coil.

I'll leave the water side of the equation to the hydronics experts. My specialty is in airflow...

Chris

 

[mtfallsmikey]

On these coils you mentioned pybyr, did the mfr. rate these at a certain EDB (entering dry bulb temperature) coming into the coil, and give you the LWB (leaving dry bulb temp.)?

 

[pybyr]

On these coils you mentioned pybyr, did the mfr. rate these at a certain EDB (entering dry bulb temperature) coming into the coil, and give you the LWB (leaving dry bulb temp.)?

no, although I could probably call him back and get that if it is a critical factor to enter into the decision. is it? I told him that return air to the coil from the house would probably average between 55-60 degrees

thanks

 

[BrownianHeatingTech]

The 4 row will have 16.8 inches of water column back pressure/ head, regardless of which size, and the 2 row will have 4.9. That seems like a pretty significant proportional difference, but maybe (I hope) only in terms of something I'll need to be aware of when it comes to picking the circulator for the secondary loop that'll feed this coil.

16.8"wc is a lot of head loss.

Putting two 2-row coils in parallel (physically installed in a A-coil shape, or in two separate ducts, if you have two main trunk lines) would be better. No additional head loss, and the delta-T on the water would be lower.

What is the actual heat loss of the house? Don't go by the existing furnace, since it's very likely to be oversized. Start with heated square feet. Real houses "almost never" exceed 40 btuh/sqft. That will give you an upper cap. If you currently heat strictly on oil, then knowing how many gallons of oil you use per heating season would be another way to figure it. Finally, doing up a print of the house and then calculating the heat loss from that would help nail it down.

How many btus you actually need to deliver is the big question.

The second question is the actual layout of your ductwork. If you have a single main trunk, then one big coil (or an A-coil) would be the way to go. If you have two trunks (or more), then multiple coils would be better.

Joe

 

[pybyr]

it's a one trunk, one zone duct system.

I'll try to double-check with the guy on the head loss through the coil tomorrow.

The house (depending on what you consider finished vs someday-finished)(when I bought it, there were puddles inside on the second floor when it rained hard, and it's been a rennovate as I live here project, which of course takes longer than planned) is about 2600sf, maybe someday (if I finish an aimed-for additional upstairs master bedroom that was a 19th century hired man's quarters- now un(never)plastered lath) it might be pushing 3000 sf total- so at "almost never exceed" 40 btuh/sqft (and in a fairly cold pocket of mid-northern VT) the 120,000 figure doesn't seem off the chart

 

[BrownianHeatingTech]

The house (depending on what you consider finished vs someday-finished)(when I bought it, there were puddles inside on the second floor when it rained hard, and it's been a rennovate as I live here project, which of course takes longer than planned) is about 2600sf, maybe someday (if I finish an aimed-for additional upstairs master bedroom that was a 19th century hired man's quarters- now un(never)plastered lath) it might be pushing 3000 sf total- so at "almost never exceed" 40 btuh/sqft (and in a fairly cold pocket of mid-northern VT) the 120,000 figure doesn't seem off the chart

Most houses are actually in the 25-35 range.

Do a heat loss. It won't take all that long, and stands to save you a good chunk of money on a coil, if the existing system is significantly oversized.

I wouldn't plan on heating the future "addition" with the existing system, if it were me. Plan on using a separate system (radiators, or a dedicated blower unit). That avoids investing in an over-large coil now, and lets you make the master bedroom into its own zone when you eventually do extend heat to it. An extra Taco twin-tee in your loop is a small investment, versus an oversized coil now...

Joe