Design Help

BB2024

Hi All,

I'm looking to get some advice on a proposed design. It's a pretty basic system to run hydronic heating/cooling (not at the same time) with Domestic Hot Water as well.

It'll be a single zone system with the heat emitters being ceiling based capillary tube mats which are PP-R, hence the HX.

The system should be able to perform cooling and DHW functions during the summer.

I'll be engaging a plumber for installation but am finding it hard to engage with local hydraunic experts without having to run the whole project through them. I.e. I'm sourcing the Heat Pump, buffer tank and DHW tank myself. Contractos only want to engage if supplying as well which is a shame. (Australia location).

Any assistance would be greatly appreciated

hot_rod

Some good info here and piping options. Will it be a glycol system? Or plain water?

https://idronics.caleffi.com/magazine/27-air-water-heat-pump-systems

Kaos

I couple of notes. You never run a mixing valve on anything air to water, the unit should supply water only hot enough for the emitters. You don't want a mix valve after the plate HX.

Most AWHP don't care about a bit of oxygen, pumps are plastic/stainless and rest of the bits copper. I would not use any steel bits in the rest of the pluming and avoid the plate HX. Adding delta T always reduces COP.

What is your 99% outdoor design temp down under? Some of the design changes a bit in cold climate.

Using the AWHP to make domestic hot water is almost never worth it. Do stand alone heat pump water heaters exist there? Around me, HPWH is cheaper BOM cost, much simpler install slightly better efficiency plus free cooling in the summer.

BB2024

Thanks for the quick responses. I've used that Caleffi site as a reference (amongst others), it's a great resource.

It'll be a plain water system. I'm in Melbourne, it's only ever been below 0c degrees (32f) once (back in 1869). Most of the pipework will be in enclosed spaces, crawl and attic (both of which are protected from external elements with some ventilation).

Outdoor design temp for winter is 5c deg (41f) and summer is 35c (95f). Dry-bulb for both.

Heating degree days at 15.5c (60f) is 752 and at 18c (64.4f) it's 1362.

I'll take note of the mixing valve, thanks for the info.

There are lots of DHW heat pumps available here now, (gov push to move away from gas), the good units (eco-cute split systems) are AUD$5k+ and the all-in-one units vary from AUD$1.5k to AUD$3k+. The problem with the all-in-ones are noise and low recovery times. The drawbacks of the eco-cute style HP's are space and cost, but the recovery is up to 100L/hr and they are whisper quite (~36dB).

I was hoping to include the DHW in the one system as the HP will only lightly be used, if even at all, for heating or cooling for weeks at time during the shoulder months.

I would have thought the effciency of the larger HP with the DHW tank would be pretty good? At least good enough to be a viable option against the drawbacks of having a standalone system (cost, space, additional maintenance)

That's interesting about not worrying about oxygen in the system. I was under the impression that you definately dont want any oxygen in a closed-circuit system that has an AWHP. But thats good to hear if not the case, although Im not sure I'll be able to avoid having the HX in place as the pipework between tank and manifold has already been run and it's pex-al-pex. I don't believe I can mix PP-R and pex piping together in the same circuit.

The HP, DWH and buffer tank will be located outside the building and the HX/manifold will be inside (it's very common to have these items outside in Australia).

Kaos

You are in a very mild climate so some of the math changes. If you have more CDD than HDD, an interior stand alone heat pump water heater provides some free space cooling (do make some noise though). Recovery is almost never an issue with these, most here have a resistance element as backup that will kick in if you have big draws. Otherwise the in your case using the AWHP for hot water is fine. Typically when the AWHP goes into domestic hot water heat mode, it will run at full power, so definitely not quiet.

If you have the piping in already what do you need designed? You generally want a 3 port buffer tank with a direct connection to load or a two port (volumizer) one. This way if the AWHP is running, the hot water can go directly to the plate HX. A 4 port buffer tank makes sense if you have a very large one with multiple heat source such as PV or wood stove. I would check what your AWHP manufacturer recommends for buffer and go for that. Nice part about the two port series tank is you don't need that extra pump on the AWHP side.

Make sure you size your pumps correctly. You never want the space heat pump to flow more than the AWHP side as this will effectively mix down the temps (similar as a mix valve), which you want to avoid.

I don't know what your summer dewpoints are like, but you want to ensure that the cooling temps are never bellow this to avoid condensation on your emitters. This is simple enough to program into the outdoor unit.

BB2024

Thans Kaos.

It's only the pipework (2 runs) between the outdoor services area to the location of the manifold/HX that's been installed as it's beneath the subfloor (suspended timber) and a decision had to be made so the subfloor could be closed in.

I've noted your comments about the AWHP running at full power for DHW cycle. That changes things a bit. Although rated dB(A) is 44 @ 1m which I assume is full power, which could be ok. I'll have to get the finer details.

The 3port buffer does make a little more sense, and I read an article by John Siegenthaler regarding the 3-port buffer as a preferred option in many scenarios. I was initially thinking a buffer may not even be required. Could that be an option?

I'll take note about the pump sizing. I guess this only applies when using the HX, which means it'll apply here.

Summer's are quite dry in Melbourne but I have planned for a dew point sensor to be integrated into the design (although I forgot to add it here).

From the https://ashrae-meteo.info/v2.0/ site -

edit: update db(A) figure.

BB2024

Updated design taking into account the feedback, thank you for your comments @Kaos

I think I'm going to stick with the DHW tank inside the system instead of standalone. I'm still awaiting additional information from the manufacture on noise levels, but for space heating with ambient at 7C and water temp 47/55 (in/out) the sound pressure level is 45dB(a) @ 1m which is great.

hot_rod

Entertech is promoting their systems without a buffer tank. This 4 part series gets into their concepts.

https://info.enertechusa.com/advantage-webinars

The buffer tanks are getting much smaller with the latest compressor technology. Zilmet was showing wall hung buffer tanks starting at 7gallons on up at AHR

IMG_9653.jpeg

Kaos

Looks good. I would reach out the your AWHP manufacturer and confirm they are ok with some oxygen.

Make sure all the bits are fine with oxygen in the water, so only stainless/brass/copper/plastic. No bare steel.

Kaos

P.S. Just looking at your AHRE numbers, you are in such a mild climate that I'm not sure if all this expensive equipment is worth it. Don't know how far you are along with your build, but with a bit more insulation and better glazing, you can probably heat/cool the whole place with a single wall mount minisplit.

DCContrarian

@Kaos has given a lot of good advice. I also agree that DHW is a poor fit for most air-to-water heat pumps.

One thing I'm going to disagree with is this: "I don't know what your summer dewpoints are like, but you want to ensure that the cooling temps are never below this to avoid condensation on your emitters. This is simple enough to program into the outdoor unit."

That's assuming you never need dehumidification. If you do need dehumidification you won't be able to get the cooling you need without it.

The problem that people run into in the design stage is that with traditional air conditioning you don't really have to think about how much dehumidification you need, if you run air over a cold coil the amount of moisture removed depend upon the humidity of the incoming air and it's essentially self-regulating. So there's no established methodology for sizing a system where the sensible cooling might be separate from the latent cooling.

You'll hear people say you should just run a dehumidifier and radiant cooling. Since a dehumidifier is an effective space heater, that's kind of criminal from an energy efficiency standpoint, it's like having your windows open in the winter so that your radiant floor can be toasty warm. A better solution is to figure out a way to work in some fan coil units running off of the chilled water, which will provide dehumidification.

If you don't need much dehumidification, what gets provided by a heat pump water heater might even be enough.

BB2024

Hi All, thanks for all the feedback, my apologies for not responding earlier.

@hot_rod thanks for that info. I really like the way John Siegenthaler presents information and challenging concepts so I'll go through those webinars.

@Kaos I've spoken to the manufacturer and they've advised that there is no ferrous material in the heat pump or buffer tank, so not having to put in an HX is great.

Mini splits are everywhere in Australia but I'm not the biggest fan of forced air (no pun intended) and the head units aren't great asthetically. We also have a lot of ducted Air-to-air splits around but these are expensive to put in and not as efficient as air-to-water. And still are forced air.

I'm repairing/retrofitting an old double brick Victorian building that is ~130yrs old, ~150sqm. Upgrading of the windows to double glaze will be a slow process post-move in. Suspended timber floors are being sealed in with Proclima membranes to make it air-tight, along with insulation. The roof is getting insultion above the ceiling (above the mats) but won't be sealed in, there's no sisaltion/membrane or anything and it's not something I'm looking at doing at this stage.

@DCContrarian - I think I'll need some form of dehumidification regardless, but I'm happy to do this post implementation. I've been looking into either a fan coil just for cooling or putting in an HVR for ventilation. I'm not sure how "leaky" the building will be once it's all put back together and whether an HVR will be necessary. But I'll be making an allowance for it in the "ceiling plan". The good thing is that it can serve two purposes, ventilation and dehumidification (without negatively impacting cooling performance too much).

I've pulled the trigger and have purchase the heat pump, a 3-phase 14.8Kw unit, along with a 3-port 100L buffer and 200L DHW tank. From a DHW standpoint, it can produce 283L/hr which is great, however it can be programmed to perform at lower power during a DHW call.

I've purchased all 3 units from the same manufacturer in China. It's a significant cost savings even with the elevated shipping costs.

I'll be happy to pass on any information/experiences once this system is up and running. Hopefully it's only a couple of months away!

Thanks for your comments and advice guys.

Kaos

Our summer are much hotter and more humid than yours, but here HRVs won't dehumidify in the summer since outdoor dew point is too high. Something to watch if that is what you are hoping to do.

Be careful mixing fan coils and radiant cooling. Fan coils need very cold water which will create condensation issues with anything radiant (also lot of condensaion on your plumbing if not fully insulated). You will need a mixing valve for the radiant or disable it when the fan coil runs.

Sounds like a fun project!

BB2024

Yeah I've had a look and if humidity was looking like becoming an issue we'd look at using an ERV.

I understood that ERV's are better in humid environment, i.e. up north in Australia. They're not suitable for down south unless humidity becomes an issue with the HRV. Are ERV's in use where you are?

Good tip on the fail coil, will keep that in mind.