geothermal with standing iron, near boiler (heat pump) piping

imatellerslie

I am planning to heat an old (1895) victorian farmhouse in maryland with geothermal heat pumps coupled with the old cast iron radiators (20 of them installed in 1947). I also want radiant under the kitchen floor, basement floor, and bathrooms. I intend to install TRV's on the radiators at some point, but not initially. The heatpumps would be staged using setback controllers tied to a sensor in the buffer tank. One concern I have is that without TRV's, the buffer tank will never backflow and be brought up to the output temperature of the heat pumps, which would make the buffer tank a waste of space. I don't want to decouple the primary & secondary completely through the buffer tank, because I don't want the heat pumps to have to heat the buffer tank before I see any heat at the radiators, and I want the heat pump to draw the coldest water from the system to give me the highest efficiency, so I've designed it so that the radiators and radiant zones draw water directly from the output of the heat pumps and the heat pumps draw the coldest water at the return. Any feedback would be appreciated.

Bryan_16

just my opinion

but is 50 gallons enough? I did 3000 s.f. last year ans ended up with 85gal. 3100 ft of 1/2 pex and 2 fan coil units close to the heat pump. I needed the 85 to have capacity on cold days.

Jeffrey Campbell

Heat pump.

First of all msot heat pumps are limited by supply output temps. around 125 degrees. Most cast iron radiantors are designed at 180 degrees. Can 125 degree supply temp produce the heat you need? Next, your drawing showed the buffer tank piped as a secondary circuit. I would recommend piping it in series with the primary system. Also, the buffer tank size depends on you total btu load. I would be more concerned about your high supply temps(lower efficiency) than return temps to the heat pump. Might want to size you radiator system around 120 degrees at design temps and add more radiator's to the system.
Finally, I am IGSPA(International Ground Source Heat pump Association) certified you might want to look into it youreslf. A wise investment. I am also a cetified RPA (Radiant Panel Association) designer/installer. If you are new to the industry and are looking to do the job right look into the RPA they are all about educating you! radiantpanelassociation.com
Good Luck Jeffrey Campbell

imatellerslie

Thanks for the reply Jeff.

>Can 125 degree supply temp produce the heat you need?<
The radiators were installed before the house was insulated, and are significantly oversized. Based on EDR, and a thorough heat loss, they can heat the house down to design temp without going over 120 degrees. I also planned to add the radiant zones, which should help liberate heat into the house and raise the efficiency.

>Next, your drawing showed the buffer tank piped as a secondary circuit. I would recommend piping it in series with the primary system.<
I didn't pipe the buffer tank as part of the primary, because I don't want the heat pump to have to heat the buffer tank before heat goes to the radiators. Also, to make it as easy as possible for the heat pump to move heat into the primary, I want the water going to the heat pump to be the coldest water in the system, so I piped it so that the heat pump will draw the return water from the radiant zones and radiators directly, rather than drawing water from the buffer tank. This is tricky, because in order for the buffer tank to function as it needs to, the volumetric flow rate through the heat pump(s) will have to exceed the volumetric flow rate through the radiators & radiant, so that the water "backflows" through the buffer tank, heating it up to the output temperature of the heat pump. If there's a better way to do this, I'd be grateful if someone enlightened me.

>Also, the buffer tank size depends on you total btu load.<
I thought the purpose of the buffer tank was to prevent the heat pumps from short cycling. I question the need for it at all with the huge thermal mass of the radiators, but intended to put it there since TRV's will be added in the future. Why would the buffer tank need to be sized to the BTU load?

>Finally, I am IGSPA(International Ground Source Heat pump Association) certified you might want to look into it youreslf.<
What does the certification require?

Jeff, thanks again for your response. I appreciate your feedback.

imatellerslie

Bryan,

Thanks for your response.

>I needed the 85 to have capacity on cold days.<
How does a larger buffer tank increase system output?

Jeffrey Campbell

GSHP

125 degree sounds great! What type of control are you using to limit this. and what type of control are you using for the system. I like Tekmar. They can control everything and you can use the outdoor reset to add to your effiecy. But you MUST limit the heat pump supply temp to 125. Did you get the desuper heater option for DHW? Great bang for your buck. I would install the buffer tank so the heat pump heat itfirst. Then using the ohter two taps(hot and cold) supply the heat system. this will insure proper flow through the heat pump and through the radiant
The reason for the buffer tank is to add mass as we both know heat pumps have no mass. Normally this means short cycling. Ask the heat pump manufacturer if they recommend a buffer even with your radiators. The buffer tank adds consistancy and effcinecy to your system. Buffer tank equals longer runzing the buffer tank. times. Then about sizing the buffer tank. I have a software that simulates the buffer tank in the system- the variables are loads, temps, and gallons. I would recommend atleast an 80 gallon buffer. Install the sensor on the supply to the heating system. IGSPHA certified was a week long class and then an extensive test at the end. Jeffrey Campbell

imatellerslie

>What type of control are you using to limit this. and what type of control are you using for the system. I like Tekmar.< I'll probably use Tekmar controls.

>I would install the buffer tank so the heat pump heat itfirst. Then using the ohter two taps(hot and cold) supply the heat system. this will insure proper flow through the heat pump and through the radiant.< If you look at the diagram, the buffer tank will provide a flow path for the heat pumps if the TRV's on the radiators are throttling significantly. With the system controlled using outdoor reset, I doubt that the TRV's will throttle much, and doubt that the buffer tank is necessary. Twenty cast iron radiators should provide sufficient thermal mass to keep the heat pumps from cycling when they're in heating mode. One thing I was unsure of was where to put the sensor for the Tekmar. If it's in the buffer tank, then in the unlikely event that the secondary and primary have exactly the same volumetric flow rate, there will be no flow through the buffer. So I appreciate your suggestion to put the sensor in the supply to the radiators.