Hello,

I am relatively new to the forum and I am looking for thoughts on the system I would like to install - we are located in central MA.  I'll aplogize now for the lengthy post, but I'd like to explain everything from the start.

The house is less than 1 year old, we began occupying it in November.  I have a Buderus oil burner with multiple zones of heat/cooling - all heating is hot water.  The zones include two air handlers, one for each floor.  The first floor AH is in the basement, the 2nd floor AH is in the attic and is zoned in the ductwork into two zones.  Each AH has its own 2-1/2 ton AC unit located outside.  First and second floor are approx. 2700 sq ft.  This comprises the part of the house that is heated to a 68-70 temp in the winter, all other spaces are 55-60.

Additionally, I have HW radiant heat in the 2 second floor bathroom floors, baseboard heat in 200 sq ft room above my garage, baseboard heat in approx 1000 sq ft of basement.  Before oil shot up we plumbed in and planned to heat the 2 car garage with a hot water blower coil and a 3-4 season porch with radiant in the floor.  These last 2 projects are currently on hold.

Finally, I have a 60 gallon indirect water heater/tank.


The house is insluated with blown cellulose in the walls and ceiling, and is extremely (probably too) air tight.

Right now, overall the heating system is oversized which obviously creates an efficiency problem.

I also have a Logmatic controller on the Buderus oil burner which adjusts the burner temp (high/low) based on the outside air temp.  When the furnace was first fired it was in manual and consumed an excessive amount of oil, keeping the boiler at 180 all the time.  My heating contactor recommended this as the AH's needed 180 water to work.  After a month or so of this excessive oil usage and 90+ temp in my basement mechanical room I decided to try automatic control.  Turns out it worked great and saved tons of oil.  The boiler temp would fluctuate between 95-160 and the house had plenty of heat all winter even on the coldest days.  My AHs are rated to 50% heat output at 130 degree water, which is about where I figure they averaged this past winter.  Obviously I had longer run times due to the reduced water temp to the AHs, just like a GSHP system would have had.

FINALLY...

Here is my idea - I would like to install a water-to-water GSHP that would heat a 60-80 gallon water tank to approx 130+ degrees.  Return water from all heating zones would flow into this HW tank to get preheated prior to returning to the boiler.  The preheated water would then go to the boiler and the current Logmatic controller would decided wheter the preheated water was warm enough or if it needed to be further heated.

I am looking to do heat only with this system.

Advantages I see here are a simplified GSHP installation, with very few control changes.  GSHP would be tied only to the HW tank aquastat for control, which is then tied to all zones.  Energy savings would vary between total GSHP produced heat and reduced delta T for the boiler to heat the water.  If the GSHP is incapable of keeping up with heating requirements, boiler takes supplements or takes over.  Failure of one of the two systems will not cause the house to loose heat.

A second part of this idea is to have a valve on the return line that would send the water directly back to the boiler if it's temp were greater than or equal to that of the geothermal water tank.  This could possibly be the case if the boiler were firing due to the geothermal not keeping up or the house requiring more than the temp of gothermal water tank.

Thoughts, ideas, experiences all appreciated.....

Also looking for recommendations on GSHP equipment for this system.

Thanks,
Matt

I think you're on the right track, with a couple of observations:

1. Very few hydronic heat pumps are capable of 130F output for any length of time without damaging the compressor. The only one I know of that claims to be able to do this is the Climatemaster THW series, which would work for you since you don't need cooling. I don't know anyhting about the cost or reliability of these, just that they claim to be capable of 145F. Figure an absolute max of 120F for most, and even lower than that for best efficiency/longevity.

2. The concept of using the boiler to supplement the temperature on cold days is generally a good one, however, hooking it up in series might not be the best way to go. A typical hydronic system might run about a 10-15 degree deltaT, so if your water is leaving teh Buderus at 145F, it will be coming back at 130-135 - too hot for the heat pump to contribute anything. You could lower the flow and thus increase the deltaT, but this is directionally wrong for best efficiency when the heat pump is operating on its own. A better method might be to have the heat pump and boiler each capable of heating the buffer tank independently which would give you more control options. There was a good discussion on this at heatinghelp.com a while ago if you search for it.

3. However you control it make sure you use a reset control on the buffer tank. The efficiency of a hydronic heat pump is strongly dependent on output temp - don't heat the water a degree more than you have to.

I strongly concur with Cyngeo on 1 and 2. The relatively high return temp / low Delta-T across hydronic heat exchangers would defeat any attempt to simultaneously operate the boiler and GSHP .

I'm not sure what a reset control means in this context but the efficiency observation is correct.

If you went ahead and did both heating and cooling with a water to air geo, you could both heat and cool with the present refrigerant aircoils (or suitable replacements) and if the water aircoils were downstream of the refrigerant aircoils you could fire the Buderus supplementally only, possibly using thermostat Y2 output(s)

Depending on design, the floor radiant heated rooms might be compatible with lower geo heated water temps.

Yes, I concur with the issues related to running the boiler simultaneously.  I would hope to only fire the boiler if absolutely needed.  By needed I mean that the water from the GSHP tank coming to the boiler is dropping in temperature.  And I would probably set my boiler controller to have a max temp of approx 140, so when it fired to max it would be stopping at a low enough temperature so that the return temp would hopefully still be low enough for the GSHP to provide additonal heat to the return water and hopefully the boiler would only fire momentarily (i.e. output water from GSHP tank is also dropping in temperature).

Also, the controller on my boiler is tied to my HW tank and knows the rate at which the tank is dropping in temperature, so if it stops dropping rapidly it will fire to provide max temp water to the HW tank.  This is really the only situation that I see the return water temp being to hot for the GSHP to add any energy because it is a very short loop.

The radiant heat will definetly work with the lower water temp, as I am currently using a mixing valve to cut down the temperature supplied to it by the boiler.

One of the reasons that I prefer a series application is that I don't like the idea of continuously letting the boiler drop to very low temps when off, because it could eventually cause the boiler seals to leak.

My boiler control has an hour meter built into it.  A little quick math based on the run time and the rated heat output of the boiler tells me that I used an average of approximately 25k BTU/hr for both heat and hot water for the 5 month period beginning Nov 15th.

So I guess that math tells me that maybe 1 40k BTU GSHP may be sufficient for this application even on the coldest days of the year.  I have looked at the Climate Master THW, and the largest unit is 40k. 


- I am also confused by the reset control Cnygeo talks about?
- Are there HW tanks available with 2 heat input/retrun sources for this application?
- I assume when you refer to Y2 on the thermostat this would basically be creating a 2 stage system?
- Also, could you elaborate on how I could set this system up for water-air heating and cooling using the current AH refrigerant lines?

Big picture - does this all make sense before I spend $$$ on the idea?

Thanks again for the info...

Honestly, properly swapping out a baseboard system with a geo system is gonna run into 5 figures and shouldn't be done based solely on internet forum advice no matter how good the source. There are a few right answers and many many potential foulups in a project this complex.

It is all about the knowledge, proficiency and experience of the installer.