Thoughts on my plan for DHW

[rkusek]

Taxidermist,

Did you implement your plate heat exhanger? I have a 20 plate 4" x 12" HX (Brazetek) installed on the cold side of my DHW heater. I have it set up to feed hot water into the tank. I have the intention of leaving the propane on should the temp ever drop, but that should be unlikely.

The circ pump for the boiler water on this line is governed by a flow switch, however the flow switch is not sensitive enough to turn on when I am running "warm water," or even when my kitchen faucet is running full bore on hot. It was a waste of $95. I was thinking about installing a surface mount aquastat on the heat exchanger. I could also use an immersion type aquastat, but I could avoid the extra plumbing with the surface mount type. I was curious if you found a solution that worked, and if you thought the aquastat would work.

I was also concerned about water being too hot with this method, so I installed a thermostatic mixing valve on the outgoing hot water pipe. I haven't had any scalding hot water yet...

Boomerj

I'm still looking for the best DHW setup to implement myself so I'm no expert. It would appear that with it installed on the cold side of your water heater it is more of a pre-heater and the propane is still going to do much of the work heating the water and maintaining the temperature. If I'm going to spend the money on a plate exchanger, a mixing valve, and an aquastat I would want it to heat the water exclusively at least when my boiler or storage is hot. I'm wondering if a plate exchanger was plumbed similar to a sidearm on a tank water heater and a potatable type circ was used to circulate the DHW side. An surface or well type aquastat could be inserted in the piping coming out of the bottom on the the tank before the potable circ. My setup is Primary/Secondary, so that aquastat would turn on both the circ to the plate HX and the potable circ. The tank would probably stratify (ie. hottter at the top, colder at the bottom) so a mixing valve would be necessary and should probably always be used anyway. I think 120 degree water in my storage would still give me almost 120 degree water in my 80 gal tank and probably still be able to handle showers, laundry, etc. If I set the electric to the lowest setting and set the aquastat to keep the tank 20 to 30 degrees higher, I would think the electric would only come on when either the storage is below 120 degrees or if there is a huge DHW water demand when the storage is at it's lower usable temps. My initial thought was a sidearm plumbed this way would do the job but I haven't heard anyone doing that. An OWB with a plate HX will work without a tank because the OWB is always hot (ie. 160 degrees probably).

 

[SmokeEater]

Boomerj
Saw your post to Taxidermist and tried to access his suggested webpage <http://slantfin.com/documents/755.pdf>, but couldn't get it to work. I have a similar plan to use a small buffer tank to cut down on short cycling of my pellet boiler. Although I'm just beginning to assemble my system, my plan is to use the buffer for potable DHW, so I'm installing a 60 plate 4" x 12" HX to the tank to pull supply heat off that tank only. I'll put a delta T circulator on the pellet boiler and set it for the tank supply and return temps I want. Don't know how it will work, but the "theory" sounds ok.

 

[in hot water]

Here is a low flow switch that is used on many boilers to build their instant DHW set up. I have a Laars mod con (Baxi 330) that has this switch. It trips with just a small hand sink flow and has worked for 4 years now.

hr

 

[taxidermist]

Well today I went on line with my plan. Tanks were at 135* and I had hot water at my taps. I hope it works good in the am when I take a shower.


Rob