I'm trying to educate myself before talking with a radiant installer, so be kind. We have a geo heat pump that uses a plate exchanger to heat a standby tank. The standby tank supplies DHW and radiant. We'd like to change that so that we can separate radiant water from DHW, and to improve the efficiency (current plate is developing a sediment problem that causes geo pump to shut down). So, we have 1 boiler (a waterfurnce synergy 3) and 2 demands (radiant and DHW). The installer has recommended an indirect water heater, and I'm hoping we can have a single indirect to support both demands and isolate the 2 water supplies. Would a triangle tube smart be a solution? It seems that the DHW could be pulled from the inner DHW tank, and the radiant hot supply pull from the boiler cold return (coming from the outer tank in the triangle tube), and the radiant cold return could T back into the boiler hot supply to the outer tank. There would be check valves to prevent the boiler loop from reversing. In this way the radiant would become a secondary circuit to the DHW, and the outer tank of the triangle tube would have more volume to buffer the geo and prevent short cycling. I've seen schematics for indirect systems, but the radiant always Ts off of the boiler hot side before it reaches the indirect tank, and returns to the boiler cold side. This obviously works, but it doesn't permit much buffering and requires the geo to run if heat is called for.

The smart tank I doubt will have enough volume to ensure proper bufferring of your geothermal heat pump. Maybe, but usually buffer requirements are pretty high.

Something more like the Turbomax would make more sense... then the volume is almost all heating water/buffer capacity.

However, having run the numbers on this kind of a setup, I can say that I doubt the upcharge for doing domestic in this way makes any sense at all. If you are doing cooling, use a desuperheater in the summer but otherwise, DHW from Geo is rarely cost effective. You're better off with a separate water heater.

If you had a cheap reverse indirect maybe it would make *some* sense, but I can also say this: if the idea is to maintain the tank at DHW temperatures, that does NOT make any sense. Your vastly overriding energy use is for heating in almost all cases compared to DHW, and you *should* be able to run heating at lower temps than DHW most of the time, which would greatly improve your efficiency. You should never considering lowering your heating efficiency to pick up DHW.

If I had a cheap tank available I would, at best, use the tank as a preheater, and use a boost heater to finish off the DHW, and just "get what you get" from the tank which would be controlled entirely for heating, with outdoor reset at least.

There is some sense in having geo heat pumps that temporarily switch to higher temperature mode to heat water as a zone (with a full size heat exchanger) - then switch back to more efficient, lower temperature operation.

We're stuck with the waterfurnace synergy and a desuper isn't available. We've been using the Synergy and an indirect setup to supply DHW, and up until the flat plate heat exchanger developed a scale problem we had plenty of hot water for showers, etc and the cost was pretty low. The turbomax looks interesting as it has more volume on the boiler side. I think the triangle tubes have between 10 & 20 gallons, depending on model. Most important for us is to isolate DHW and heating water to avoid contamination/bacteria. I don't mind running geo to keep an indirect tank at 110 to 120. This is pretty efficient for the geo. For DHW, we can run the 110 hot out of the indirect tank straight into an electric tankless to boost it 10 or 15 degrees - there are certain tankless systems that will work well for this, and it guarantees enough DHW when the radiant load is high.

I didn't say the cost wouldn't be low as is, I have no idea what you are running at now or what kind of energy usage you have. I DID say that not maintaining at a tank at DHW temperature for the geo would make it much more efficient: that is, lower than it has been. Significantly.

On the heating side, if you need a max of 120 on design day, you would be under 100 with outdoor reset most of the winter. I bet you could improve your COP by 1.5 or 2.0 over the course of a heating season by changing your strategy. And that should much more than pay for any additional energy you use in DHW... many times over, using the tankless heater. On the order of a 30% reduction or so, guesstimate but that's the kind of order of magnitude I would expect.

there is nothing particularly efficient about geo at 120. You should mind this. 110 is ok. Reset is way, way better.

I've run numbers on this and the additional cost of the turbomax (or any other solution you are likely to consider) is not likely to be justified in any energy savings in DHW you'll get from it. But if economic justification is not your thing, cool. It's fun and neat and even just using the tank at heating temp and bumping up from there with the electric tankless, you should be able to easily meet 1/2 to 3/4 of your winter domestic load through the tank at geo COP's without maintaining anything beyond heating system temperature requirements. It's just that the amount of energy you are saving there is small, and has a very, very long payback on the tank and tankless upcharges. You'd be ahead of the game getting a heat pump water heater or just doing domestic separately... economically speaking.

failed to quote, sorry, this is for jonr

I have never seen a geo heat pump that can do this particularly well. Unless you perfectly match your DHW exchanger to the output of the geo, or the geo unit is a modulating unit (something I also haven't seen) you'd be likely to be cycling pretty hard on DHW calls with most indirects.

maybe, with something like the smart which has a lot of exchange area, or with a well sized plate exchanger, you could make that strategy work with domestic priority and a higher temp aquastat though, you're right.

I appreciate the advice - I'm new to this and trying to understand terminology and practices. For us, we have to make *some* investment, because the current plate heat exchanger has almost stopped working and we want to separate DHW from water circulating through radiant tubes. Given that we have to make some investment, the ROI isn't as important, but we ought to consider a new layout that reduces energy usage. I gather from your advice that the geo pump is better suited to radiant temps than DHW. Ours is a warmboard system, so I believe it needs lower temps, anyways - 90 to 100 degrees is probably more than hot enough to supply the radiant system. Would a dedicated indirect tank connected to the geo pump hot water output be the best solution to supply the radiant system? And that means DHW is handled separately from some other system? Maybe a combination heat pump tank, or tankless? Thanks

You need a buffer tank for the radiant, regardless. No idea how big, no idea what size heat pump or load you're working with. but usually at least 50+ gallons. but no coils or indirects are needed, just a tank.

Running your radiant at 100 degrees, or with an outdoor reset temperature controller is great. really great.

Personally, I'd skip trying to do DHW with geo entirely, I don't think it makes much sense, I'd just get a separate water heater. But if it did work before, and you're set on doing it again, I would replace the plate heat exchanger you're losing either with another heat exchanger, and find/fix the water quality issues you have, or with an indirect fired water heater like the smart tank with a lot of exchange area, doing DHW only. I would never run the heat pump over 110 degrees (arbitrary, but good for efficiency), and an electric tankless would finish the job for domestic.

Heat exchange is nice though since if it goes bad, like it has here, you're only replacing the heat exchanger and not the whole tank. I would try and figure out why it went bad though and address that issue... electrolysis, water quality, sediment, etc. Maybe regular maintenance would preserve it.