I have a Geo-comfort 5 ton combination unit that I am hooking up to heat buffer tanks that feed a 40 gal DHW tank heated by the DSH. The buffer tanks will also connect to my in floor heat system. The inlet/outlet that comes from the heat pump are 1". If I reduce this to 3/4" will I get enough GPM thru the system to transfer the heat away from the heat pump fast enough.The Geo-comfort unit operates at 2nd stage during hot water production. I could use 1" lines but the buffer tanks have 3/4" fittings anyways and are within 8 ft of the HP. What size of circulating pump would I need for the 3/4" setup to work. Thanks

It will work out to just about the smallest pump you can find. You need no more than 2 GPM for this purpose, and moving 2 GPM through short 3/4 inch lines requires very little pumping power.

Why 2 GPM?: Plan for about 10% diversion of heat via DSH. For a 5 ton machine that's 6000 Btuh. Transferring 6000 Btuh via a 2 GPM flow results in a delta-T across the DSH of 6*F. That's about right for efficient operation. This is not set in stone, flows of 1-3 GPM would work well, and I'd err toward the lower side.

The right way to do this is to calculate the total equivalent length, inclusive of all fittings. Then figure out the head loss at 2 GPM through that equivalent length (It'll be very small) Then select a pump where 2 GPM falls near the middle of the pump curve at the calculated head loss.

Are you certain the unit doesn't already have a circulating pump?

3/4" lines are way overkill for this application. My own system incorporates 1/2" CPVC, with an equivalent length of maybe 40' each way, and  braided washing machine fill hoses - these have an inside diameter of about 1/4". My geo HP is only 3 ton, I'd avoid the washing machine fill hoses on any unit larger than that.

I should clarify. The DSH circuit does have 1/2 lines and came with a pump installed in the cabinet. Its the water heating capacity of the unit that I'm trying to hook up. According to your formula would it then have to move about 90% of the capacity of the unit? Sorry I didn't explain it well. Terry

It will work, we will do it all the time. Make sure you run at least 1" lines all the way to the tank before you reduce in order to reduce pressure drop. We use a 26-99 for that kind of scenario.

More complex is your water heater. Keep in mind that you are going through a indirect heat exchanger, so feed that with the water from the buffer tank(s) might result in low temperatures for DHW and poor heat transfer. Your DSH, when running, will compete with the feeds from the buffer tank. You might even end up transferring heat from the DHW tank into your hydronic buffer tanks. I have never seen an indirect DHW working well of a hydronic buffer tank. It is possible to work of one heatpump but it takes a more complex design and and especially a complex controller design. Here is one example of a combo/hybrid unit feeding both, the hydronic buffer tank and the DHW tank.


http://welserver.com/WEL0267/

Why are you feeding a tank that is also fed by the Desuperheater?

Bergy

Very good question...I should have read TT's first post more carefully.

Upon reread, it suggests domestic hot water and radiant water are intermixed - don't do that!

To sum up: DSH and domestic water should physically isolated from radiant water for health reasons as well as thermally isolated for efficiency reasons DJ cited.

Thanks for the replies. My original intention was to use the DSH to make the water in the DHW tank as hot as possible and for the DHW tank to receive its water prewarmed from the buffer tanks(radiant system) as the domestic hot water is drawn off. I understand the concern of radiant water intermixing with domestic if the radiant water has remained stagnant over the summer months. When viewing many of the systems on the WELserver, it appears that they are intermixing, unless their tanks have internal exchangers not shown. Are there concerns with DSH coils failing and contaminating DHW with refrigerant or something else. Are DSH's always connected indirectly with DHW tanks. Terry

After re-reading engineer's reply I believe its says its OK for DSH to directly exchange with DHW. Sorry.

Posted By TTerry on 27 Dec 2010 10:39 AM 
I understand the concern of radiant water intermixing with domestic if the radiant water has remained stagnant over the summer months.

Many ways to isolate radiant from DHW and few reasons not to. It is a code requirement in many circumstances.
Little reason not to do it, including cost which can be quite small for a plate exchanger.
J

You simply have very few reasons to brew your coffee with the water which circulated in your radiators first for a while.....The DSH is a closed system, separated from your other Water-Load coil.To make a long story short, DSH output will be significantly scarified if you do not use a buffer tank for DHW. WEL graphics are made by the owners for purpose of monitoring, they usually do not contain design details. Buffer tank temperatures are to low to transfer enough BTU to the tank for quick recovery. You can design it that the full capacity of the HP goes into the tank on demand, full time, but you need a more complex design for piping, pumps, checkvalves (or zone valves), heatexchangers, and then the knowhow to automate the system with the appropriate controllers. We have done many of those systems, one thing that never worked is to run the single DHW tank of the hydronic buffer tank. Plus it takes away your ability to increase efficiency with an outdoor reset.

Thanks to all for advise. I will keep systems separate for safeties sake. Terry