Controlling the system

[Kevince52]

Hi,

I just had a geothermal system installed. It was a retrofit but on a home that is brand new. All in-floor poured slabs on 3 floors ICF construction. It's a DX system. I have a 50 gl buffer tank and a 50 gl preheat for DHW. The preheat tank has a loop in it to act as an extra zone along with the desuperheater so we can use the HP in the summer for DHW. I have 7 zones (not including the loop in the preheat tank) One of which I don't use, garage. The system works by using wall thermostats in each zone. They signal a zone valve for each zone and a common circulator pump. The HP is triggered when an 2 stage aquastat on it drops below 90°F. It will run until the zone is satisfied and the buffer tank reaches 110°F. This is the only control that turns the HP on and off.

What I'm wondering is is there a better way to control this system? The thermostats (Aube TH140-28) have a proportional adaptive (PA) mode and a conventional mode (CM). None of the literature I have on them tells me how to set them for a geothermal system. No surprise.

The proportional adaptive mode seems to learn during each cycle and adapt based on heat loss but a lot of the time that could mean that the zone comes on for a very short period of time, 120 seconds or less. I imagine that works well for a hot air system but I'm worried that these short cycles could cause undue wear on the HP and may not be the most efficient use of it. Not to mention the chorus of thermostats. Read loud clicking noises going off all around the house all the time especially at night.

The conventional mode doesn't do this. The cycles are much longer. It does though mean that sometimes a zone could almost be satisfied when another zone calls. The 70° water from the new call mixes with the other zone (80° - 110°) effectively cooling that zone until both zones are satisfied. If 3, 4, 5 or even 6 zones come on at staggered times the time for satisfying all zones is considerable.

Should I have some other system controlling this setup? Does it make sense to prioritize each zone. In other words tell the system not to heat any new zones until the current one is satisfied? Or cycle from one zone to another in say 15, 20 or 30 minute intervals until all zones are satisfied? Or am I just over thinking this and I should put it on one mode and leave it?

Any insight or advice would be greatly appreciated.

Thanks,

Kevin

[beenthere]

Should have floor sensors.

That 2 minute cycling will KILL your compressor.

Get your contractor back out, to set it up right.

[Kevince52]

Thanks for the reply.

My contractor has setup a 2 stage aquastat on the buffer tank. Originally they had jumpered the heat pump and had a single stage aquastat so that it only came on at stage 2. They changed that now with the idea that it would keep the heat pump running longer. On average I'd say the heat pump runs for 2 - 4 hours at a time. Usually 2 to 3 times a day.

I don't have slab sensors. The original builder did not put them in and I was not smart enough to ask for them.

This type of heating seems so different from traditional heating, low and slow, that traditional thermostats aren't the right control system. Especially these "smart" ones. I imagine that the algorithm that the thermostat uses to intelligently predict how long a cycle should be is based on having a fairly constant heat source. Since the geothermal system is so slow to react and the temperature could be anywhere between 80°F and 110°F at any given time is it possible for these algorithms to be correct/accurate?

My contractor has suggested using the proportional adaptive mode on the thermostats. So I am. They seem to be holding the heat to within a 10th of a degree even when it's 0°F outside. (rare but it happens) I have no supplemental heat so this seems good to me.

So what I'm really interested in is this the normal way to control a geothermal system? Is the consensus keep it simple? Is there a better way? Would anyone suggest I add slab sensors?

[beenthere]

At what water temp does it go to second stage.

[Kevince52]

It goes to second stage at 85°F. First stage is 110° with a 20° differential.

[beenthere]

Would probably be better off using a control that holds in second stage until satisfied, once its called for.

And floor sensors would help also.

[Kevince52]

Hmm. That's almost what it was set to before. The contractor said that the HP was jumpered only to use the second stage. At that point the aquastat was set to 110° with a 10° differential. I will mention this to them and see if what they think.

I'm not trying to second guess them but this is only the second geothermal system they've done as a company. They are a well respected HVAC company and did hire a guy with 20 years experience with geothermal. They have educated themselves and are certified as installers by the manufacturer. They are responsive and seem sympathetic to my need to understand the system. I do trust their judgement and am very pleased overall. I just wondered if the thermostats that the builder supplied, the ones that were here before the geothermal system, were working against the efficiency of the HP.

Would the normal control for a geothermal system be an aquastat on the buffer tank to control the heat pump, floor sensors and non-programable thermostats to open the zones? Is all this proportional adaptive stuff a waste of time?

[beenthere]

First. You are not suppose to use programable thermostats on radiant heat.
Takes to long to heat up the radiant mass.
Thermostats, should be set and forget.

Slab sensors, to prevent over shoot.

[Kevince52]

Great. Thanks. I did actually write non-programable. As of yet overshoot hasn't been a problem. I guess it might now that we're getting into the shoulder months. We only commissioned the system in early February.

BTW this is a great resource. I understand it's real intention is to help professionals but I'm glad that I am able to ask a question or two. The more we understand the way these systems work the more likely we are to buy them.