Hi,

I posted several questions here and I appreciate all the feedback.

I am working with a radiant supplier but they are not recommending a particular brand of hot water tank for my radiant and DHW use. Can you recommend a brand that will not kill my budget? I will need three tanks - one for DHW, one for radiant buffer tank, one for low rate 3.4 cents/kwh hot water usage. Is the extra $$$ worth it for Marathon and others? I also need a heat exchanger between the low rate unit and the radiant unit. My supplier said I need to look for ones with a "thermal pool" to easily attach a tekmar control.

It been an strange project considering I am doing most of the leg work like heat loss calcs, loop design, run determination, brand selection, and all manual labor. My supplier is designing the controls including an outdoor reset, and supplying all materials except the heat pump, plumbing, and hot water tanks.

Any suggestions?

I don't know your design particulars, but you may try a dual coil tank.
Sounds like you have a water to water heat pump, have that go into one coil, when you need the back up from a boiler, that could go into the upper coil, from the tappings on top (or side depending on brand) pull your radiant and dhw.
I know viessman has their vitocell 300 dual coil tanks, If your budget allows, otherwise I'm sure you can find other manufacturers, I even remember seeing a triple coil tank some place!

The water to water heat pump (waterfurnace Synergy3) has its own built in heat exchanger. I need another heat exchanger to pull heat from the lower rate unit running at a higher temperature unit to the heat pump buffer tank and the DHW tank

the water furnance heat exchange is between the ground loop and itself, correct?
On the house side of the water furnace have that dump into one coil, the other coils can pick up other sources. See this link, to get the idea.

http://www.solarthermal.com/downloads/application6print.asp

Great diagram! That is exactly what I need. Only problem - my radiant system needs to be a closed system so I need a 2nd hot water tank. The DHW tank could have a heat exchanger with the radiant unit separating DHW with the radiant fluid

I'm not really a fan of piping arrangements that heat storage tanks with less efficient/green sources... that further reduces the contribute such source can make, as instead of giving them a chance to get their own tanks up to temperature, you jack up the tank temperature via another means, lowering heat transfer from the greener/more efficient source.

I much prefer bypassing the tank entirely if it is not up to a high enough temperature to meet a demand by itself, and let it do what it can, when it can.

This does result in more complicated piping arrangements in some cases, but a few really elegant examples are in Siggy's Modern Hydronic Heating, 2nd ed (available through the RPA bookstore, though it's not cheap, it's a fantastic resource). With a simple piping arrangement the water heater or the geo unit could bypass each other entirely.. trick is, using a controller than can determine when to use which.

That can be pricey.. the Tekmar 363 can poll between two sources of water and compare to its computed supply water temperature to determine which to use, which does the trick for us, but it does carry a price tag. Brings a lot to the table too though... mixing, resets, choosing the source, etc etc.. good stuff.

Why are you tying in your DHW? Or will that be a seperate system entirely? do you have a limited amount of low rate electricty?

My geo system does not have a desuperheater so I am trying to figure out a way for it to efficiently heat my DHW. One way I am thinking of configuring the system is to set the DHW tank as another zone within my radiant pump control system. The DHW would have a heat exchanger tied to the radiant pump control system. In the summer months my radiant zones would not demand hot water so I would have plenty for my DHW. In the winter months I could reduce or shut off the DHW zone entirely to support only my radiant zones.

I still need to learn details about the waterfurnace Synergy3. It is both a water to water and water to air unit. In the summer I hope it can both heat water for my DHW and run AC.

I understand that any system design is great on paper - design the control is this critical. I appreciate your concept to keep systems simple to increase ROI. It may be worth simply purchasing a high performance hot water tank for my DHW without connecting it to my geo system. My hvac is not experienced in radiant system and my radiant supplier is unfamilar with Synergy3. I need to run the PEX tubing next week to stay on schedule. So can I pay you to design the system and build the control?

downtown,
the diagram shows a close loop radiant system.
I'm not all that familiar with the water furnance make, but without the desuperheater I doubt you can cool and make dhw. It's best to ask the manufacturer.
A buffer tank is essential for a water to water geo unit, it will cycle like mad if you tie it in directly. Of course the COP efficiencies drop off above 120* So, if you need to go above that then a second heat source , such as a boiler will back you up.
I have to agree , why the need for multi sources, the geo alone can't handle the radiant? Is there high temp heat also. Controling does not have to be elaborate, an aquastat off the tank may be all you need, again the manufacturer of the geo should be able to help you, to tie into their control boards.

Thanks again!

I have a call into waterfurnance concerning the cooling and heating at the same time. My geo covers 2/3 of the potential heat loss so I need to plan for a backup source on cold days. I think I rather go with an electric hot water tank instead of propane. I am already planning for a buffer tank tied to my geo and radiant system. I was just looking for additional DHW heating as well. Thanks!

Any reason why I could not attach my DHW as a seperate zone on my radiant system? I would not use a manifold just connected it to my pump control system. The water would circulate through a HX coil in my water heater to heat the DHW all season long. The thermosat for the zone would be a aquastat controling the DHW tank.

If you are talking about using a water heater or a geo system to heat an indirect water heater, you won't have the water temperatures you need for good transfer, and so unless you have a huge amount of storage and a low domestic usage, you'll probably run out of hot water a lot.

Generally indirects are heated with high temp water because you need maximum transfer to keep up with what is a pretty high heat draw.

I just learned the that Synergy3 can provide both AC and hot water together. That is pretty cool.

Waterfurnace can help me develop a control that will honor the AC request first, then produce hot water for the buffer tank. A DHW tank with a coil is attached to the buffer tank. Question is if the buffer tank is set to 110 degrees will it be an effective heat exchange source?

You mean when you have a DHW draw, it will draw 110 degree water into its coil?

No, that won't be effective. Geo units that provide domestic use a "desuperheater". I don't really know what that IS, but I know that's what they use

As usual great advice. But as I learn I only have more questions

Back to my original question. Please recommend a an electric buffer tank that 1) provides backup water heating 40-50kBTU/hr 2) no ferrous 3) easy to control aquastat capability 4) internal heat exchanger (optional - external heat exchanger ok)

someone else may be able to do that better than I, though I would generally prefer an external HE. No reason to have to replace both when either one fails..