Geo Radiant with Buffer & Mod-con backup..AND 2-stage ODR?

PhillipJones

Please see attached Siggy diagram from a 2020 P&M article. I'm doing this nearly exactly, and would like to know what control he's using for the outdoor reset control. It needs to be able to control the buffer temp with the ODR curve, turn on the GEO as stage 1, and the mod con as stage 2. I'm looking to use a Tekmar 261, but could not find any application drawings to give me the assurance that this is the correct control. I first tried a Taco PC-702-1, but it relies on a call for heat from the Taco zone panel. In my case, the buffer sensor needs to operate like an aquastat; I don't want boiler calls for heat just because one of my t-stats are calling. Thanks!

GSpear

I would try this

https://www.watts.com/products/hvac-hot-water-solutions/controls/boiler-and-mixing-controls/261

You may need to use a jumper on the demand terminals to simulate a heat call.

hot_rod

The heat pump may have some of that controlo logic onboard. Usually it will have a backup heat demand, to call on the boiler at a condition you set, like outdoor temperature. Also ODR for the tank temperature control.

If you want to pull the loads off the buffer as ODR control, then you need a tekmar or HBX.

The boiler will probably have ODR also, for when it is loading the tank.

PhillipJones

Thanks Hot Rod and GSpear. I circled back to the Taco PC702-1 2-stage controller I already have, and possibly have a solution. Let me know if this sounds plausible.

1. Jumper my spare zone TT on the Taco ZVC-EXP panel, so there is always a call for heat in the zone panel. No zone valve would be wired to it, so the system pump would not fire.
2. With a call for heat in the ZVC, the PC702 should begin to operate.
3. Install the universal sensor in the aquastat well of the 80 gallon buffer tank. The PC702 is looking to this sensor as boiler supply temp. Therefore, the setpoint of the buffer tank should be selected based on the ODR curve, which is what I want. Even though there is always a call for heat in the ZVC, stage 1 GEO and stage 2 LP mod-con should only fire based on the PC702 ODR reading.

I'll admit I still have a niggling feeling that I'm missing something without an actual aquastat in the mix. but If I do put an aquastat on my spare zone, I think I lose ODR on the buffer tank. Any feedback is helpful!

DCContrarian

I have to ask what you're trying to do.

I love Siggy, I've found his writing to be highly influential, but he has a tendency to go a little Rube Goldberg.

Usually the justification for a boiler backup with an air-source heat pump is that as the outdoor temperature goes down the COP goes down and the capacity goes down, and either the heat pump stops being cost-competitive or it stops having enough capacity to heat the house. But with a ground-source heat pump you don't get that effect.

If the boiler is there because the heat pump just doesn't have enough capacity, I think it's going to be simpler and cheaper just to install a heat pump with enough capacity. If you want the boiler in case the heat pump fails then you don't need the ability for both to operate at the same time, which is a simpler configuration.

If the situation is you have an already-installed heat pump that is undersized, then it might make sense to go down this path. But otherwise it seems like a lot of trouble for not a lot of gain.

hot_rod

Rube Goldberg??

From someone with a handle of Contrarian 😉