The Homemade Heat Pump Manifesto - Page 148

stevehull · 04-11-13, 10:48 AM

Yup, the German Roth is very pricey. Look up WaterFurnace (made in Ft. Wayne IN) or ClimateMaster (made in Oklahoma City, OK) for comparable "026" water to water units. Their public websites have all the specification data just like Roth.

No point on paying duties, shipping for stuff made right here.

The buffer tank is absolutely necessary to prevent short cycling.

Steve

AC_Hacker · 04-11-13, 11:19 AM

Quote:

Originally Posted by michael

...Trying to decide between PEX and HDPE; you successfully talked me out of PVC...

OK...

The thermal transfer of PEX is the same as HDPE.

In fact, PEX begins life as HDPE, but is modified by some selected process to become PEX, due to the cross-linking (that is where the "X" comes from... "PE" is PolyEthylene).

HDPE is a thermoplastic, meaning that it can be welded.

PEX gains strength, particularly important is its high-heat strength, but it loses its thermoplastic properties... so it can't be welded. I am such a rebel that I tried to weld PEX, and it just will not weld... won't ever happen. This means that all underground connections done with PEX must be done with barbed metal connector and crimp rings which will eventually fail due to corrosion.

The acidity of your soil will determine the corrosion rate... it may be years, may be decades, may be hundreds of years... but it will fail.

Welded HDPE joints will not fail. Now, if you had an earthquake big enough to rip apart highways, maybe then it would fail... but barbed connectors would fail long before.

If you make the decision that all of your underground runs will not have any joints, there really is no difference between using PEX or HDPE. You can find plenty of PEX couplings that would make joining PEX to the rest of your system easy. These barbed couplings will work with HDPE, too.

Also, check out the price of PEX and the price of HDPE... HDPE is cheaper, but that is the poorest reason to choose it over PEX.

The skill to make strong welds is not difficult to acquire. You can rent tools, or buy them, or you could make your own like I did. For you, renting the tools might be the best. Vlad bought a good socket welder off of ebay, for about $250 as I recall. His plan was to sell it when he was done. That way, he could use the tool at his leisure... no rush. Smart guy, Vlad.

Best,

-AC

michael · 04-11-13, 09:37 PM

Quote:

Originally Posted by stevehull

The buffer tank is absolutely necessary to prevent short cycling. Steve

OK, I get it, but it just seems to me that if the circulation rate of the heated loop matched the circulation rate of the radiant floor, and the heat pump could raise the 85 d return water from the floor to 110 d in one pass, it ought to work without the dread rapid cycling. As long as the thermostat called for heat, the heat pump and the floor would run in synch unless there's some sort of modulation that goes on with the heated output loop from the heat pump, but even that may not make a difference. It's possible that the buffer is needed in complex radiant systems where one finds zones or mixed heating methods with some chance of slowing down the passage of heated water through the heat pump, but if that's not the case, I don't quite get why a buffer tank is necessary. It might be nice to have, but perhaps not necessary. Michael

AC_Hacker · 04-12-13, 12:48 AM

Quote:

Originally Posted by michael

OK, I get it, but it just seems to me that if the circulation rate of the heated loop matched the circulation rate of the radiant floor, and the heat pump could raise the 85 d return water from the floor to 110 d in one pass, it ought to work without the dread rapid cycling. As long as the thermostat called for heat, the heat pump and the floor would run in synch unless there's some sort of modulation that goes on with the heated output loop from the heat pump, but even that may not make a difference. It's possible that the buffer is needed in complex radiant systems where one finds zones or mixed heating methods with some chance of slowing down the passage of heated water through the heat pump, but if that's not the case, I don't quite get why a buffer tank is necessary. It might be nice to have, but perhaps not necessary. Michael

> The buffer tank is absolutely necessary to prevent short cycling. [S.H.]

I agree with you michael. It depends on several variables, most notably the rate at which the heat pump can supply heat and also the thermal mass of the floor.

If your thermal mass of your floor is very large, compared to the heat output of your heat pump, no buffer tank is required.

If the thermal mass of your floor is small and your heat pump output is large, then you would get short cycling, so a buffer tank would be required... because the buffer tank is all about adding thermal mass to your system.

> ...circulation rate of the heated loop matched the circulation rate of the radiant floor... [m]

I'm afraid I don't see any validity to the physics of this concept.

-AC

randen · 04-12-13, 05:43 AM

From my experience a buffer tank is not an absolute. With my installation we have a buffer tank 60 gal but it is not nessisary for the purpose of preventing short cycling of the heat pump. In fact to raise the temp in the tank from 80 Deg F to 110 Deg F would only take about 4 min. so by this it would not prevent short cycle.

The purpose of our tank is 4 fold. (1) It has an internal copper heat exchange coil that exchanges the heat from the solar hot water panels. (2) It has 6 electrical heating elements for future load diversion for future wind turbine. (3) It preheats our domesitc hot water. (4) It receives and distributes the heat from all the above including the heat-pump. IT WILL NOT PREVENT SHORT CYCLE.

The heat load of your hydronic floor will do this unless you made a grossly huge mistake in the size of the heat-pump. For our installation 1700 sq ft concrete slab 3 T heat-pump to raise the slab 2 deg. F can take 4 hours of solid run time. When its snapping cold with wind it runs almost non stop. The floor itself is the buffer!! You will always have a Delta T of about 10 Degrees and if the temp limit for your heat pump is regulated by the floor temp window of 2 degrees. You can place the limit temp switch on the return from the floor but the limit temp will never be obtained unless the floor approached the 110 Deg. F. The heat-pump will run until the floor is at temp and will never over shoot because of the floors Delta T of 10 Deg. The FLOOR IS A HUGE BUFFER.

Randen

AC_Hacker · 04-12-13, 08:05 AM

Quote:

Originally Posted by randen

From my experience a buffer tank is not an absolute... The FLOOR IS A HUGE BUFFER...

That's pretty much what I was trying to say, too.

I wish my thermodynamics was more on tap... this certainly would be the moment for it.

When we talk of short cycling, we're describing excess temperature overshoot.

I have a cheap Chinese PID controller, which has an interesting 'auto-tune' feature. What the auto-tune feature does is to switch on the heat source and count the miliseconds until the thermal mass reaches the set-point temperature, and then it cuts power. Then it waits (while counting miliseconds) until the temperature of the thermal mass returns from its over-shoot and again crosses the set-point temperature on the way down.

It doesn't 'know' what quantity of heat is being applied, and it doesn't 'know' what the characteristics are of the thermal mass.

But by being able to measure the time between applying energy "X" to thermal mass "Y" and and measuring the time to get to temp and then return from overshoot, it is able to calculate the rest of the PID formula. From then on, when the demand is made for the thermal mass to be heated to the set-point temperature, the PID controller will do a fast ramp-up toward the temperature, and then as it approaches the set-point, it progressively feathers off the power (by gradually reducing the duty cycle) until the set-point temperature is reached without overshoot... and it holds that temperature by supplying heat energy in a pre-calculated stream of low duty cycle pulses.

Very elegant.

For our heat pump radiant heating situation, we don't have the advantage of being able to smoothly feather off the heat output (duty cycles) as we approach the set point (unless we have a variable frequency compressor). So we live with some amount of overshoot cycling. We try to limit the amount of overshoot with a proper thermostat (as SH described), and reduce to an acceptable level, the frequency of heat pulses, or ON-cycles by properly matching our heat source (heat pump output capacity), or if the capacity is too big, by using additional thermal mass (AKA: 'buffer tank').

And, randen, "(unless we have a variable frequency compressor)" this is why I am so interested in your VFD controlled three-phase compressor... because I'm sure that some kind of PID control could be arranged for your system. That's really an important project you have going on there.

-AC

stevehull · 04-12-13, 08:28 AM

Here is why a buffer tank is necessary. Almost all thermal systems have a series of loops. Typically, the loops on the periphery deal with conduction infiltration and have a large water flow and are busy fighting cold. But smaller loops, say in an interior bedroom, do not have to deal with that cold.

The thermal loops I have worked on not only have manual modulating valves to adjust water flow (and thus heat), but off - on valves as well connected to floor thermistors. About 3-4 of these in each floor system.

The problem occurs when the fixed speed pump (and they are almost always fixed) comes on and directs water flow into one of the smaller loops. That loop heats up quickly and the system shuts off - quickly. Then another loop call for heat and the same thing happens. Only with a huge load does the system act as a huge thermal mass. And a huge load is not the typical.

Steve

randen · 04-12-13, 09:04 AM

Yes Steve you are right those conditions of small heating loops with low thermal mass would cause a problem expecially in number. But even a buffer tank of 120 gal would that be enough to buffer the short cycle?? Would that buffer tank require a huge temp range for longer cycle??

I just ran a quick test Our 3 T 60 gal. tank 41 Deg C to 45 Deg C 4min run Thats a bit of a short cycle. A small floor loop with a small thermal mass still wouldn't be a benifit the short cycle situation much with a tank of similar size. Albeit better than no tank.

Randen

randen · 04-12-13, 10:46 AM

AC_Hacker Wrote:

And, randen, "(unless we have a variable frequency compressor)" this is why I am so interested in your VFD controlled three-phase compressor... because I'm sure that some kind of PID control could be arranged for your system. That's really an important project you have going on there.

Yes the control of the 3 phase is truly fantastic with the VFD. As I didn't have too much time to prove it out and I'm still concerned about the compressor. It was producing more noise than I'm comfortable with. A bit of metal on metal noise from with-in.(maybe thats why it was cheap) It had oil, I checked and it was turning the right way.

I quickly swapped it out with the 2 Ton in-case it quit at the worst time. Freezing floor tubes would not be a good thing.

When the weather changes I will work again on the 5Ton. There maybe a summer coming where air-conditioning is required.

Randen

Mikesolar · 04-20-13, 08:43 PM

The buffer is an absolute necessity. The European blokes I know (who only do rads and floors, typically) tell me that 30L/kw (25G/ton?) is the right size to prevent issues. Those issues can be based on multiple zones and therefore multiple flow rates (which can cause freeze ups in the HP), but are as much an issue of trying to maintain the closest dT across the HP to keep the running current down. If the suction and discharge stays at a close ratio, the unit will last longer and have less start/stops. ALL my discussions with the dudes over there come to this conclusion, and their COPs are better because of it.

04-11-13, 10:48 AM	#1471
stevehull Steve Hull Join Date: Dec 2012 Location: hilly, tree covered Arcadia, OK USA Posts: 826 Thanks: 241 Thanked 165 Times in 123 Posts	Yup, the German Roth is very pricey. Look up WaterFurnace (made in Ft. Wayne IN) or ClimateMaster (made in Oklahoma City, OK) for comparable "026" water to water units. Their public websites have all the specification data just like Roth. No point on paying duties, shipping for stuff made right here. The buffer tank is absolutely necessary to prevent short cycling. Steve __________________ consulting on geothermal heating/cooling & rational energy use since 1990 Last edited by stevehull; 04-11-13 at 01:51 PM..

04-12-13, 08:28 AM	#1477
stevehull Steve Hull Join Date: Dec 2012 Location: hilly, tree covered Arcadia, OK USA Posts: 826 Thanks: 241 Thanked 165 Times in 123 Posts	Here is why a buffer tank is necessary. Almost all thermal systems have a series of loops. Typically, the loops on the periphery deal with conduction infiltration and have a large water flow and are busy fighting cold. But smaller loops, say in an interior bedroom, do not have to deal with that cold. The thermal loops I have worked on not only have manual modulating valves to adjust water flow (and thus heat), but off - on valves as well connected to floor thermistors. About 3-4 of these in each floor system. The problem occurs when the fixed speed pump (and they are almost always fixed) comes on and directs water flow into one of the smaller loops. That loop heats up quickly and the system shuts off - quickly. Then another loop call for heat and the same thing happens. Only with a huge load does the system act as a huge thermal mass. And a huge load is not the typical. Steve __________________ consulting on geothermal heating/cooling & rational energy use since 1990

04-12-13, 10:46 AM	#1479
randen Uber EcoRenovator Join Date: Apr 2011 Location: Strathroy Ontario Canada Posts: 657 Thanks: 9 Thanked 191 Times in 129 Posts	5 Ton GSHP With Verible Frequency Drive AC_Hacker Wrote: And, randen, "(unless we have a variable frequency compressor)" this is why I am so interested in your VFD controlled three-phase compressor... because I'm sure that some kind of PID control could be arranged for your system. That's really an important project you have going on there. Yes the control of the 3 phase is truly fantastic with the VFD. As I didn't have too much time to prove it out and I'm still concerned about the compressor. It was producing more noise than I'm comfortable with. A bit of metal on metal noise from with-in.(maybe thats why it was cheap) It had oil, I checked and it was turning the right way. I quickly swapped it out with the 2 Ton in-case it quit at the worst time. Freezing floor tubes would not be a good thing. When the weather changes I will work again on the 5Ton. There maybe a summer coming where air-conditioning is required. Randen

04-12-13, 05:43 AM	#1475
randen Uber EcoRenovator Join Date: Apr 2011 Location: Strathroy Ontario Canada Posts: 657 Thanks: 9 Thanked 191 Times in 129 Posts	From my experience a buffer tank is not an absolute. With my installation we have a buffer tank 60 gal but it is not nessisary for the purpose of preventing short cycling of the heat pump. In fact to raise the temp in the tank from 80 Deg F to 110 Deg F would only take about 4 min. so by this it would not prevent short cycle. The purpose of our tank is 4 fold. (1) It has an internal copper heat exchange coil that exchanges the heat from the solar hot water panels. (2) It has 6 electrical heating elements for future load diversion for future wind turbine. (3) It preheats our domesitc hot water. (4) It receives and distributes the heat from all the above including the heat-pump. IT WILL NOT PREVENT SHORT CYCLE. The heat load of your hydronic floor will do this unless you made a grossly huge mistake in the size of the heat-pump. For our installation 1700 sq ft concrete slab 3 T heat-pump to raise the slab 2 deg. F can take 4 hours of solid run time. When its snapping cold with wind it runs almost non stop. The floor itself is the buffer!! You will always have a Delta T of about 10 Degrees and if the temp limit for your heat pump is regulated by the floor temp window of 2 degrees. You can place the limit temp switch on the return from the floor but the limit temp will never be obtained unless the floor approached the 110 Deg. F. The heat-pump will run until the floor is at temp and will never over shoot because of the floors Delta T of 10 Deg. The FLOOR IS A HUGE BUFFER. Randen

04-12-13, 09:04 AM	#1478
randen Uber EcoRenovator Join Date: Apr 2011 Location: Strathroy Ontario Canada Posts: 657 Thanks: 9 Thanked 191 Times in 129 Posts	Yes Steve you are right those conditions of small heating loops with low thermal mass would cause a problem expecially in number. But even a buffer tank of 120 gal would that be enough to buffer the short cycle?? Would that buffer tank require a huge temp range for longer cycle?? I just ran a quick test Our 3 T 60 gal. tank 41 Deg C to 45 Deg C 4min run Thats a bit of a short cycle. A small floor loop with a small thermal mass still wouldn't be a benifit the short cycle situation much with a tank of similar size. Albeit better than no tank. Randen

04-20-13, 08:43 PM	#1480
Mikesolar Master EcoRenovator Join Date: Aug 2012 Location: Toronto Posts: 958 Thanks: 40 Thanked 158 Times in 150 Posts	The buffer is an absolute necessity. The European blokes I know (who only do rads and floors, typically) tell me that 30L/kw (25G/ton?) is the right size to prevent issues. Those issues can be based on multiple zones and therefore multiple flow rates (which can cause freeze ups in the HP), but are as much an issue of trying to maintain the closest dT across the HP to keep the running current down. If the suction and discharge stays at a close ratio, the unit will last longer and have less start/stops. ALL my discussions with the dudes over there come to this conclusion, and their COPs are better because of it.