Sizing heat exchangers

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Hearth Supporter
Mar 6, 2007
Among other things, I'm trying to shake out the details for my domestic hot water generation. First off, what is a good number (gallons per minute) to shoot for? We are installing an 80 gallon whirlpool, and have three showers in the house. Somewhere from 5 to 10 gallons per minute seems reasonable. It seems that 10 might be better with no DHW storage, and 5 would be fine with a buffer (like an indirect tank). I'd rather stay away from the expense of a tank if I can. The boiler will be a Tarm 40 or EKO 40 tbd in the next few days. Either way, I'm generating 140,000 BTUs/hr at full burn. I'll have a water storage system...1000 gallons of pressurized storage if I can find a used propane tank in time, or 800ish gallons of open vessel if not.

If I was to use a flat plate heat exchanger to retrieve DHW, how do I go about picking a size? Water on the hot side would be 180 if the boiler or backup (propane) boiler were on line. If the boiler was cold, water would vary from 110 to 180 depending on the storage tank temp. Can I take 50 degree well water up to 120 (or 100 if the tank is cold) with a heat exchanger? I assume I could also recirculate flow from a tempering valve on the exit side. Would I have an aquastat call for the propane boiler if the wood side can't get the DHW up to 120?
You wouldn't need much of a flat plate for DHW. Figure out how many btus per hour your tank is rated for, and size the hx accordingly. Or, you can build or buy a sidearm hx for about $100 which operates on the DHW side on gravity, so you eliminate an expensive bronze circulator. And yes, you can heat 50 degree water to 180 (or whatever) with a heat exchanger. I don't about running systems in parallel. I shut off my hot water heater and gas boiler when I fire up the gasifier in the spring.
HX manufacturers have websites with info on btu's, temp rise, gpm, etc. This will help you size your plate exchanger. Ebay prices can be pretty reasonable, and my tendency is to go a little larger than exactly "right." My boiler hx on a single pass brings 80F water to 160F at about 6 gpm with a 5 x 12 x 30 plate hx. Go for at least 1" inlets/outlets. Incidentally, the boiler input side of my hx in this same situation is about 165F in and 110F out. These are measured with inexpensive probe meat thermometers cable-tied to the copper pipes, so at best they are measuring only pipe surface temperature with losses to ambient air.
I might be jumping to conclusions here, but are you planning on heating your hot water 'on demand' from either the Tarm or the storage?

If so, I'd highly recommend an additional smaller high-temp storage tank - call it a water heater if you'd like. It's nice to be able to allow the storage tank to be cool and skip fires a day or two when it's not bitter cold outside. The limiting factor on firing intervals will be your DHW if you don't have independent storage for it.

I've struggled with the quest for the perfect design, and I think I'm close at this point. I've got a writeup on my site with the history and results of my experimentation.
I think it helps if you put a piece of insulation around the probes, Jim. I've found them to be pretty accurate this way.

If, as nofossil says in another thread, that the average home uses 60,000 btu per day for DHW, you don't need a very big hx to supply it. But I don't see anything wrong with oversizing it. However, I would seriously consider a sidearm, as I mentioned above. They're somewhat less responsive than a flat plate pumped on both sides, but not so's you'd notice--and that's with three people, one of whom is a teenage girl. As I said, you'll save money and complexity by eliminating the pump on the DHW side.
i have my plate heat exchanger piped into my cold dw going into the hw tank. the only pump for this is the one on my main loop that will run constantly. i do have it all valved to isolated if needed. will this work or am i wrong about needing more pumps im confused after reading on this topic.
You need pumped water flow on both sides of a flat plate, because it won't convect on the DHW side like a sidearm will. So I'd say you need another pump. Bear in mind that it will be pumping oxygenated water. Technically, you should use an (expensive) bronze circ. for that application, but if you can pay $30 for a new cast iron Taco 007 and have to replace it every couple of years, I think you come out ahead. Just watch for rust in your hot water, or your WAF will plummet.
my flat plate is on the cold side going into the hw tank. when the hot water is turned on in the house should that not flow into the exchanger and be preheated going into the tank only needing the one pump on the boiler water side. i know how the side arm ones operate i may put a homemade on on also since it would be free.
I know from experience that you need circulation to heat up a hot water tank with a heat exchanger. Simply passing the cold water through a hot heat exchanger on its way into the tank is not going to get the job done. Since you have it hooked up, give it a shot, but you won't be satisfied with the results. Hope I'm wrong. Be sure to shut off the water heater to get an accurate reading on how it's working. The beauty of a sidearm is that it constantly circulates as long as there is hot water flowing through the outer tube.
i was thinking originally that the water might get to hot coming out of the heat exchanger like 160 degrees. it was hard to find heat output numbers for the heat exchangers per plates. i think i have 20 plate exchanger ill check when i get home but i know its big. thanks for the info got me thinking. my brother-in-law has the sidearm and works good.
I think it might have been you in another thread asking if a flat plate could handle 50 degree water, and I apologize if I gave you the wrong impression. It can "handle" any temp or Delta T, but it won't get it up to the desired temp in one pass. All heat exchangers that I am aware of for heating large volumes of water need circulation to bring the temp up over time. They're more efficient with low input temps, but not nearly responsive enough to do it all in one shot.

I tried what you are suggesting with a tankless coil in a boiler once, and that didn't even work. The main problem was that you're only usually opening the tap for a short time, so while you may get hot water out of a tankless coil when the tap is turned on, you'd need to use 40 or 50 gallons of water in order to get the tank up to a decent temp. Short bursts are not enough to make any difference. It is a good way to preheat water, but why bother when you can heat it all with a relatively minor extra investment in a pump and some piping?
what your saying sounds correct ill have to see. it can easily be taken out and a sidearm installed. i can use the flat plate on another portion of the heating system. i know someone who makes heat exchangers that can make me a sidearm. thanks for the info.
My domestic water consists of the top half of an old burnham steel boiler. I cut off the combustion chamber and insulated the top half with 2 inch blue dow. The neat part is that is has all of the necessary tapings for stats and stuff. I run the boiler water thru it with a 007 taco, and I use a grudfus 3 speed pump to pump cold water thru the existing domestic coil to another water tank. It then will flow into the electric water heater before getting to the taps. I also switch it to run off a solar panel in the summer. The only drawback is that it takes up a good bit of space in the boiler rm.
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