Some minor conflicts.

Tom_133 · December 27

I am not much of a Geothermal guy. If anything I can take an engineered system, and install it to the exact drawing, and as long as the engineer nailed it, so did I.

Recently I was asked to put a wood boiler system on to an existing water to water system. The current heat pump works fine, but it was piped into an electric 50 gallon tank. They wanted the tank removed, and this is the final concept. There are actually 3 storage tanks, and a lot of other valves, and miscellaneous items but for you to understand here it is.

Now, the ONLY issue is when the heat pump comes on because a small zone is calling like a bathroom, the flow coming down the 11/4 primary loop is not enough, so the heat pump pulls a lot of its own heat back in through the closely spaced tees and it cycles. on off, on off, of off… so you may say put in a tank. The problem is the whole system is run on a control system that is 20 years old and they dont want to spend the big money to upgrade since its working, so we only have a couple sensors to make everything happy. One sensor sees the wood boiler temp and holds off the geo, and the other is placed at the closely spaced tees on primary where the wood boiler connects to fire the circ pump after mix valve.

I am probably not giving you enough info, but my question is: Would a new variable speed heat pump solve my issue? Would it ramp up slowly and not cycle? The bathroom loads are with in the spec that unit goes down to. But I am not sure how the heat pumps work. I know with a mod con I can ramp up after it initially fires, so it would be great, but I am limited in knowledge with heat pumps and cant find much info. They are looking at the GEOSMART NETZERO 22.

EdTheHeaterMan · December 27

Not looking at the specifications of the new HP I might be inclined to say yes, but it all depends on the turn down ratio. For example the HP may be able to produce 50,000 BTU of heat and if the turn down ratio is 10:1 then it will make 5000 BTU on the lowest output speed. If the small zone only requires 2000 BTU then the heat pump is still oversized and will cycle on and off during that small zone call for heat if no other zone is calling. Now if the pump that moves that heat from the hp to the zones is also variable speed then the cycles will not be as short. but there will be cycling.

You need to do the math for your system to see if it will work as you need it to.

EdTheHeaterMan · December 27

Your diagram for the wood stove needs a pump to move the heat from the stove to the buffer tank unless it is set up for very efficient thermocycling. That would mean larger sized pipes for gravith flow from the wood stove to the buffer tank. @hot_rod would have more info on that point. Also, there is a complete Idronics on Geothermal, #9… I believe, and another on wood stoves. You can get them online.

psb75 · December 27

There is no provision of thermal protection for the wood-boiler (i.e. Thermovar etc.) which needs its own circulator.

Jamie Hall · December 27

I'm not quite sure where to begin…

First, as said above,, there is no protection for the wood boiler. Those things can, and will, get carried away from time to time. It needs to have its veery own dedicated circulator to the main buffer tank and back, and it also needs to have a gravity circulation dump zone. The buffer tank could be the dump zone, if it is big enough and if but only if, it is arranged with enough elevation difference for gravity circulation.

The way that is arranged is flat out dangerous.

Second, the heat pump also needs to feed the buffer tank. There is no particular reason why it couldn't be controlled by the buffer tank temperature with a simple aquastat set for the best performance from the heat pump.

Then the rest of the system can pull from the buffer tank. Since the wood boiler is quite unpredictable, however, you will need thermal mising valves and individual pumps for the various zones so that they get the water temperature they need.

Tom_133 · December 27

As I mentioned. I left out ALOT of things in the schematic because I didnt think you would be looking for a ESBE Series 1-1/4" 3-Way Thermic Valve. Yes it's installed, Yes the pump is installed. Everything works great except the HP. The current HP is old and doesnt do variable speed. My lack of knowledge with the HP's is why i am asking if the pump on the heat load side also slows when it ramps down? Thats the number one question. If it doesnt, then it wont help my issue at the closely spaced tees.

We stole the sensor that would typically sit in the buffer tank and tell the heat pump when to come on, and put it in the storage tanks so the heat pump would know not to come on. It's an older control system and cant be added on to without an upgrade. Which is apparently very expensive.

EdTheHeaterMan · December 27

After some research, it appears that there are integrated variable speed circulator pumps, so that would make sense to handle different compressor outputs. The maximum capacity is somewhere around 75,000 BTUh with a turn down ratio of 4 to 1. So the lowest it will go is 18,750 BTUh output. That may be more than you need for the smallest zone only operation. That means there will still be cycling.

Tom_133 · December 27

https://forum.heatinghelp.com/discussion/comment/1879319#Comment_1879319

There are 900 gallons of storage that is typically stored at 180 degrees, I dont think that is what the HP what the heat pumps to do

hot_rod · December 27

why did they install the wood boiler if the HP cover the load just fine?

I think a fixed speed compressor like that will need a buffer

There is a rule of thumb, so many gallons per ton of hp capacity

hot_rod · December 27

https://www.caleffi.com/en-us/magazine/9-geo-hydronic-systems

EBEBRATT-Ed · December 27

Not a HP expert but I have done a lot of water chillers for both comfort cooling and for process cooling.

For comfort cooling where the load is more or less stable Carrier recommends a Minimum of 3 gallons of system water/ton of cooling.

For process cooling (which I would use for this job) they recommend MINIMUM 6 gallons of system water /ton of cooling.

I n this case your heating so I would take the heating capacity of the heat pump in BTUs and this would be the heat rejection and use that.

But in reality I agree with a fixed speed compressor you need a buffer tank to prevent short cycling the more water in the system the better.

Trane and others use a different method of calculation but if you work the math it comes out the same as Carrier

Tom_133 · 10:51AM

I agree with all of you. If we keep this HP then we need a buffer. Sadly that means upgrading the control system. So since the HP is close to 20 years old they wanted to swap that. Thats why I was curious about the ramping of a variable speed HP. I just dont know enough about them. I will conference call with the manufacturer on Monday and get to the bottom of this.

Thanks

hot_rod · 4:10PM

An outdoor reset control, sensor in the tank is one option to fire the HP on/off.

The buffer tank formula is similar to that of a heating buffer tank size.

Some minor conflicts.

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