AWHP - Plate heat exchanger or not?

I am having this same debate with myself. I have an AWHP system that currently services a portion of my house, with the eventual goal to expand the system to cover my whole house. I currently have the entire system filled with glycol. While this has worked OK for the last couple years, I am considering installing a heat exchanger and switching the indoor portion to water for the following reasons:

• I have had one leaky fitting on a fan coil in a bedroom. Thankfully it was minor, I caught it quickly, and the bedroom floor is vinyl so nothing was damaged. But its a lot easier to clean up water than glycol if a leak occurs. A catastrophic failure would result in not only a mess to clean but also require purchasing more expensive glycol
• I don't want to deal with draining and storing up to 100 gallons of glycol if/when I need to add or replace a component
• One of my fan coil emitters is pretty undersized, the reduced heat transfer of glycol is not helping this situation, especially in cooling season when the glycol becomes more difficult to pump due to low temp and I'm losing both flow rate and heat transfer

I'm not a professional, just a guy who has been living with this for a few years and wants to make my life as simple as possible to add, repair, or replace components as needed. My understanding is that if the heat exchanger between the inside and outside loops is sufficiently sized the efficiency loss in the exchanger will not be much different than the efficiency loss of using glycol throughout the whole system

Indirect is not the best idea with a modcon and ever worst with an AWHP. Go for a heat pump water heater, takes up the same space, higher efficiency and cheaper.

The only reason I run an AWHP with an indirect is that I already had it. The big issue is recovery time and efficiency in the deep freeze. Come winter, I will need to add a resistance heater so I do not loose house heat for an hour or more during larger draws.

I was also doing some gardening during the recent heat wave here when the unit went into DHW mode. The nice cold breeze was nice, but that bit of cooling should really have been going to cool the house not the outdoors.

My thinking on plate HX with AWHP has changed over time and I'm now less against the idea. As long as you select a large enough one to have low approach temp and make sure the secondary flow rate is always bellow the primary flow rate, it should work well. Not over-pumping is the important bit because if the flow rate is too high, you are essentially mixing down the supply water with the return water which is bad for COP. Efficiency of an AWHP is all about SWT, so your design should always prioritize getting that as low as possible.

With a bit of care you can also run plain water and avoid any glycol or plate HX. This is what I ended up going with as it is much simpler.

Read the warranty, I believe glycol or a couple freeze valves are required. I'm not sure freeze valves are a good option for cold climate however. Some units have the HX too low to use a freeze valve.

So glycol may be the safest for both freeze and warranty claims. I doubt a frozen HX would be a warranty item, and an expensive replacement.

Regardless of the DHW option with a buffer tank the glycol can be $$. A circ, valves and HX probably less than 500 bucks.

The newest A2WHP modulate pretty well and may not need more than 20 gallons of buffer. That makes the glycol option more reasonable.

@DCContrarian - "The heat pump is modulating the flow of its circulator to match the output so you want to have some way of matching the circulator on the other side"

I don't know the details of the brands the OP is considering but my Arctic heat pump, in factory form, runs the pump at a constant speed 100% of the time. It is supposed to modulate the compressor in response to return water temp but the pump does not modulate. In this scenario it is relatively simple to balance heat exchanger flow.

What are people's thoughts on using an indirect water heater as a buffer tank for the heat pump and a heat exchanger to separate glycol in HP loop from water in indoor loop? I'm wondering how the heat transfer efficiency of an indirect compares to a properly sized flat plate HX.

I think it depends also on the loads you are connecting to the HP. With zoned hydronics a buffer may be an advantage, although Enertech promotes a buffer tank less approach.

In cooling mode if you have a single air handler that is sized to the HP output, all is well. With hydronic heating that is zoned the HP may not ramp down low enough and you do not want it to short cycle.

As for HX, by far a FPHX outperforms a tank with coil. You can size a plate HX to a close approach design, A & B side within a few degrees. Using the FPHX design software is helpful if you go with a HX.

As always, pros and cons to glycol, both in SIM and HP. I like working with HX but you have some efficiency penalty and additional cost. So you need to look at the specific installation and weigh the pros and cons.

As for DHW, I would also want to know what is expected for temperature and capacity. DHW should be sized the same as a heat load is calculated. Some folks do fine with 40 even 30 gallons of DHW, yet others drain an 80 gallon tank.

When you use a HP to heat an efficient home and a HPWH you are cascading the HPS and the HPWH will operate at a bit lower COP than advertised.

I think a better way to think of a heat exchanger is a primary/secondary loop with a slight temperature drop. When you look at it this way, matching flows is not as critical except making sure that the secondary flow rate never exceeds the primary as I mentioned earlier. P/S piping does work with AWHP and the heat pump does pick up load properly and modulates accordingly (or at least mine does).

With a bit of careful design you can eliminate a buffer tank in most cases. The important bit is to have some zones that are always on or at least an aggressive outdoor reset. Defrost normally happens when there is a pretty decent heat load, so as long as you have the above, there will be some zones running. The heat for defrost would come from those zones. Since some zones will always be running, they will provide a decent volume of water and a base load to avoid cycling. This is the setup I run and I have had no issues with cycling except when I run radiant cooling, which is a story for another day.

If you do want a buffer, heat pumps do best with a series two port buffer. This doesn't have to be anything fancy, a simple small hot water tank with the element removed will do.

Trying to combine any type of indirect with AWHP sounds good in theory but it just costs you extra money and complexity. KISS is my moto.

Maybe do a heat load calculation first, get a better idea of how many btu you actually need.

a basic electric water heater is an option if. It. Is just two of you in the home?

I don’t read that you have any Hydronics currently? If your option is small fan coils that need or more, a heat pump may not be the best option.
are you grid tied, do you have a good deal with the power company, at par perhaps? If so it seems staying all electric might be your best option, skip the hydronic is retrofits unless you can add some low temperature radiant.

Invest in some thermal or battery storage to carry through the evening ifyou have a lot of PV. Or if your grid tie is a good option?

1500 bucks buys a simple 100% efficient modulating boiler, 10 Kw gives you 34,000btu/hr

You could be around 10 time that for a 3 ton HP and tank. Although the hp gives you a cooling option.

If you go with a hp, find a knowledgable installer\ service person. There is a lot of technology under the hood of the indoor and outdoor unit.

I would shop around a bit, air to water units are harder to find, but not that expensive if you can DIY.

I'm with @DCContrarian Install a simple direct to load setup with thermostats on the fans of the heaters. No buffer, no extra pumps, simple plumbing. You also will have very little volume so no need for a plate HX. I would still demo the ceiling of the basement a bit and at least install some floor heat for the bathrooms. These can be free running always ON zones, nobody complains if a bath is extra toasty. Too many issues with radiant cooling, stick to minisplits for that.

You can get 120F out of R32 units but they won't like it. 110F is a more reasonable number to shoot for, this will probably mean a couple of extra heater units with fans running at low speed. You won't be about to run much outdoor reset as nobody wants 90F air blowing at their feet.