combi tanks

zacmobile

anyone have any experience with combination DHW/heating tanks?



I will most likely be using one in my new personal installation. I'm in Canada and the only 2 that seem available here at all are the Buderus PL750 and the Latento XXL, the TiSUN tanks look interesting too and are apparently distributed by Lochinvar in Canada although I have yet to get a response from them. I like the idea of doing it all with one tank, after running a few scenarios in Viessmanns ESOP program the single combi tank gets about 1% more solar fraction & 2% higher efficiency than separate DHW & buffer, probably because there's less heatloss from a single tank & the parasitic power consumption is reduced by having fewer pumps & piping.



Just would like to know if anyones used them & pros-cons etc. etc.

radmix

combi tank

I have installed one of these tanks and its My opinion that a two tank system would be more efficient. If you use the same tank for your DHW and heating in my opinion you have to heat 250 gallons of water to 120* when there is no sun which to me does not seen efficient and also I could only use the tank to heat the house if the temps in the tank are over 125*.  If you have two tanks you could heat a smaller DHW tank say 60 gallons to provide you hot water needs and use a separate tank for the heating needs that you can run the temps down to 80*. To me you would also be running cooler water temps back to the panels increasing efficiency.

zacmobile

combi tanks

What brand tank did you use?



The thing I gather by using one of these tanks is that it's all about stratification: multiple tappings down one or several sides so you can take/return heating water at the right levels so as not to disturb the layers and the solar HX is always sitting in cool water at the bottom of the tank. By my calculation my DHW fraction for the year is at 76% (almost 50% in Dec!) so there wouldn't be much boiler heating of the DHW going on and when it does it will be when the system is running @ design conditions anyway (125 deg F) also because of the size of the heat exchange area of the inner tank on say the Buderus PL750 the boiler wouldn't need to run as hot as it does now to heat up the domestic (140 deg F) so I can't see there being much of an efficiency hit.



PS: on a side note for interests sake, the Canadian distributor for Lochinvar/TiSun just contacted me and said the TiSun tanks should be available in 2 months. (I would assume the same for the US too)

radmix

combi tank

This tank is a Buderus PL750 Its just my observation that on a day like today in NY where it has been cloudy for days that the boiler is running to maintain a tank  with a very large volume of water to 120*. I do understand the stratification of the tank but you will also have turbulence within the tank. Today the outside temp is about 50* The heating system is low temp radiant with a very insulated house. It is my estimate that I would only need about 80* water today to heat the house but because I can only run my tank down to a temp of about 125* I feel that I am losing efficiency on the tank.

hot_rod

140F

doesn't the code in Canada require DHW tanks to be maintained at 140F? Then a approved thermostatic valve on the tank to reduce the outlet temperature to 120F. I talked with the Bradford White folks at their booth in Toronto last week. I thought he mentioned something about a new CSA listing for solar and DHW tanks with that requirement.



The method BW and others are taking is burner and aquastat at the bottom and the temperature thermistor to operate the burner up near the top 1/3. The intent is to stack that 140F at the top part of the tank and allow the bottom to stay cold.



As long as the dip tube or cold water inlet is properly designed there should not be a lot of mixing in the tank, but layers of varying temperature.



I filmed a bare steel tank with an infrared camera once and it was clear that the tank loads and unloads evenly.



Various methods are being used to keep the tanks stratified, lances, webbing around the dip tune, a box like Lochinvar uses and one Italian tank with an active stratification device.



hr

radmix

combi tank

If you have to run your DHW temps @ 140* and if your max tank temp is 200* that would be a 50* difference. If you could drop that temp in the tank to 80* that would be a 120* difference. With the formula provided earlier of 8.33 x gallons x tank high-tank low it would be a significant difference in BTU output for the heating load or am I missing something. I could very well be wrong

Rich

zacmobile

combi tanks

I'm not sure about that 140F requirement but I will look into it, I have my domestic set @ 120F, my boiler has a sanitary function to nuke it once every 24 hrs but i've never used it. I think Mark Eatherton posted a little chart with bacteria survival rate @ various temperatures, i'll have to find that. Definitely something for me to ponder.



Here is an interesting read on combi tanks that I just found on the Solar Lexicon site:

http://www.solarserver.de/solarmagazin/anlagejanuar2002-e.html

their conclusion seems to be (a little difficult to process some of the Germglish that there is an efficiency loss on combi tanks compared to two separate tanks but the difference is so minimal that it's not enough to worry about.