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Old 10-18-14, 03:45 PM   #11
Ormston
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This should give you the idea, live monitoring of my ASHP (the graph is interactive so you can zoom in, scroll back in time etc.)
http://www.emoncms.org/ormston

Notice how when heating dhw the power consumption starts low and rises as the tank heats up.

Steve

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Old 10-18-14, 05:44 PM   #12
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Quote:
Originally Posted by Ormston View Post
For dhw heating the odr needs to be disabled as it would limit water temp.

For hydronic heating purposes the odr limits water temp to just hot enough to keep the house warm.
It saves energy due to the way refrigerant heat pumps power consumption is proportional to the output temp(and to a lesser extent the input temp). The higher the output temp the higher the power consumption.
My ASHP draws around 850w when heating my floor to 24C and 1300+w when heating my water tank to 50C.

Steve
An alternative is to add a thermostat to force the heat pump to run if the DHW tank is below a set limit. If you're on city water, the minimum DHW setpoint might be as low as 105F or so. You can also consider installing a desuperheater that will give very hot water (as high as 160F in some cases depending on system design) when the heat pump is running.
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Old 10-19-14, 11:40 AM   #13
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Thanks Steve,
Your data shows about a 2% loss of HP efficiency for each *C increase of load temp.

So keeping a load tank 15*C hotter to handle "design temp" winter night
vs. a mild Fall day,

would cost about 30% in HP efficiency.

Last edited by buffalobillpatrick; 10-19-14 at 01:20 PM..
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Old 10-19-14, 08:24 PM   #14
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Quote:
Originally Posted by AC_Hacker View Post
Mikesolar,

Are you stating this as applying to ALL heat pump situations (mini-splits are heat pumps, too) or are you referring to high-mass radiating systems?

Reason being, I am currently using an ASHP to heat my house, 9,000 BTU mini-split, with a built-in set back feature, and there is no problem what so ever.

-AC
A heat pump is, by its very nature, almost always undersized for the job (to some extent). This is why we have the backup heat installed.

The amount heat needed to raise a slab (it can be low mass as well) is considerably more than that needed to keep it a degree or so from the setpoint. We are wanting to have a system respond quickly when asked and the HP is not as capable of it compared to a gas boiler.

It is all about expectation. If you don't mind using the straight electric or gas backup to do the fast injection of heat, then you can use the setback but if not, I would not because it will take a long time to heat up with just the HP, especially when it is colder out and heat load is closer to the HP capacity.
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Old 10-19-14, 08:32 PM   #15
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Quote:
Originally Posted by buffalobillpatrick View Post
Thanks Steve,
Your data shows about a 2% loss of HP efficiency for each *C increase of load temp.

So keeping a load tank 15*C hotter to handle "design temp" winter night
vs. a mild Fall day,

would cost about 30% in HP efficiency.
I'm not sure about your numbers, it may be correct, but don't forget the time lag of a high mass floor. The ODR system is watching the temps and they don't usually fall or rise up that quickly. There will always be some under or over shoot. There are PID algorithms that can be written into the controller to account for the rate of change in outdoor temp. They would be a bit house specific, I think.

There are saving to be had for sure. With boilers, the industry sort of agrees on a 10% fuel savings. I don't see why an ODR motorized mix valve just for the floor loops would be an issue. The tank would be depleted more slowly so the HP would not come on as much.
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Old 10-19-14, 09:10 PM   #16
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That's my objective in this situation. For the new hack, I'm rigging up a small outdoor air source unit to a water cylinder, much like the diy water heater heat pump. The water cylinder will act as a buffer or heat store for space heating via passive or forced air radiators. Rather than having a bang-bang thermostat directly sensing the indoor air, the odr demands heat pump operation based on the tank and outdoor temperature. When the zone is warm enough or not in use, the flow can be cut off. Meanwhile, the tank will radiate some heat on its own to offset the natural loss.

The main goal is to have long outdoor unit cycling and more even heating of the zone.
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Old 10-19-14, 09:15 PM   #17
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Originally Posted by jeff5may View Post
That's my objective in this situation. For the new hack, I'm rigging up a small outdoor air source unit to a water cylinder, much like the diy water heater heat pump. The water cylinder will act as a buffer or heat store for space heating via passive or forced air radiators. Rather than having a bang-bang thermostat directly sensing the indoor air, the odr demands heat pump operation based on the tank and outdoor temperature. When the zone is warm enough or not in use, the flow can be cut off. Meanwhile, the tank will radiate some heat on its own to offset the natural loss.

The main goal is to have long outdoor unit cycling and more even heating of the zone.
Would you be using a Fujitsu or similar inverter HP or just an on/off one?
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Old 10-20-14, 03:22 PM   #18
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It looks like this one will be a 8 or 9kbtu rotary compressor (scavenged from a window ac unit) rigged to a large outdoor coil. I believe it is from a 2 ton air handler, maybe 2 1/2 ton. If the included txv doesn't hunt, it will be run stock. If it does, I can swap it out for a smaller one.

Defrost will be reverse cycle, run through a cap tube. I'm going to start off with the one from the ac unit and shorten it if I have to. With a hot water cylinder as a heat source, I'm not sure how much flow I will really need in order to defrost quickly.

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