Trouble getting an Air-to-water heat pump for radiant retrofit in Virginia
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I wonder if the extra capacity on the Mitsubishi units comes from the higher delta T. I don't think any ATW heat pumps runs anywhere near the 30F-40F delta T you see on an air to air heat pump.
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My understanding is Mitsubishi expects a maximum delta of 36F, so leaving air temp of 106F with room temp of 70F. For all of the air-to-water heat pumps I used a leaving water temp of 113F (45C), because that was the only number I could find for the Arctic and I wanted it to be apples-to-apples. So the Mitsubishi does get a slight boost from having a lower output temperature.
A lot of manufacturers like to quote stats for leaving water at 95F/35C, because that moves both curves to the left by 18F. That would give a boost in COP to the water units. The Mitsubishi Hyperheat units are so good at keeping capacity at low temperatures that even if you move all the water units over they still dominate.
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I have finally found a start-up contractor interested in A2W heat pumps. He works for a large mechanical contractor and has a side business with ambitions to go on his own. We are now in search of the most capacity stable and robust-compressor cold climate A2W HPs. One candidate is the LG Therma V monobloc (which can be purchased online, BTW). Other suggestions for comparison?
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I think Arctic has the best cold-weather performance. Not sure what their distribution model is.
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I would suggest with any option purchasing through regular distribution if at all possible. Viessmann is now selling a split A2W unit in the US market. they sell them as a package with the indoor unit, outdoor unit, and a buffer tank
https://www.viessmann-us.com/en/products/heat-pumps/vitocal-100-aw.html
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Most HVAC equipment purchased online has No Warranty. The factory does not know who or the qualification of the installing contractor.
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I'll be installing the small Viessmann on my shop, here is there performance curve.
Bob "hot rod" Rohr
trainer for Caleffi NA
Living the hydronic dream0 -
@hot_rod , do you have an output curve? What really kills most air-to-water is not the COP declining when it gets cold but the output.
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I just pulled that from their website. They fo have the performance curve and altitude derate curve, and other data
At design of 5F, I can still cover the load. I tubed the slab 6” OC and held the tube into the slab. So my SWT is low in the best performance line
Bob "hot rod" Rohr
trainer for Caleffi NA
Living the hydronic dream1 -
Arctic heat pumps is a great company to buy from for a2w heat pumps. I installed a few my self here in Saskatchewan where it regularly drops below -15f. Arctic heat pumps work as advertised and their customer service is first class.
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A brief update and a few questions: Bought a ranch house (bricked) over 3/4 basement built in the mid 60s for our retirement years. Our retirement house now owns me, the over-extended retired engineer. Covid shut down all intents to hire help, so I spent the pandemic years digging drainage around much of the house to help dry up 2 damp/unsealed crawl spaces. (Found rotted rim joists in the vented crawl spaces deliberately hidden by the seller, but that's a sob story unrelated to this thread.). Also demoed a basement in-law apartment for electrical upgrades, plumbing changes and prep for HVAC replacement (Currently retrofit air-handler and ducts in the attic; 20 yr-old Carrier heat pump system). More recently got a residential energy audit from a well respected local design-build firm focused on energy efficiency. No surprises: inadequately insulated leaky house…. Got Manual J load calcs from a hydronic-savvy consultant in New England. Desiring hydronic heat and. cool all along.
Got 3 air-to-air quotes for HVAC replacement with new ductwork - all north of $30K. The last was for Mitsubishi Hyper Heat at well over $35k from a contractor recommended by the local energy audit/services company. This contractor informed that local building officials are adamant that overhead ductwork in the multi-use basement be a minimum 6'-8" clearance which is impossible (7'-3" slab to floor joists). They quoted two new air handlers and ductwork in the attic which is the opposite of my intent to seal and reinstate the attic. More determined than ever to go hydronic, even though my wife wishes I'd just get this HVAC thing done. Questions in next post related to fan coil units for hydronic heating and cooling…
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What I always look at with heat pumps is cold weather capacity, that's the only capacity that matters. If you look on the product page for the LG at :
There's a bunch of downloads, the last one is "engineering manuals." On starting on page 30 they have heating capacity tables. That series keeps 100% of its nominal heating capacity with 113F water down to 5F, and 90% down to -4F, and 85% of nominal capacity at -4F with 122F water. If you're designing from scratch I would design for the lower water temperature, you get better COP and it makes integrating with under-floor heat simpler if that's what you want.
That looks like a good choice.
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Fan coil units? My original plan was to have hidden small FCUs above closets, etc., with high-wall discharge for cooling and second stage heating (after radiant floor or baseboard heat) but that hit several snags, especially related to getting suitable smaller units for each room. We have moved on to low-wall-mounted FCUs with recommended models being Myson iVector and Jaga FCUs with other contenders. Arctic openly markets the Chinese Phnix models and others I suspect are rebranded Phnix FCUs. At this point, underfloor PEX heat is on hold except for a couple of bathrooms, so FCUs will need to do the job. We have pricing for Myson but are having problems sourcing Jaga. Polar Air was suggested but too expensive. So…
- Experience with the brands mentioned? Suggestions of others to look at?
- Since the house is still leaky and any tightening will be slow, dehumidification is critical. We often experience muggy summer days with high humidity and we run mutiple dehumidifiers. What is the best way to assure adequate latent heat removal via chilled water in any FCUs? Is published data sufficient to make this judgement call?
- If the cooling mode is continuously circulating chilled water and variable fan speed, is there any risk of subcooling the house on mild days when fans are mostly off?
- Finally, has anyone used the Therm Atlantic multi-port fan coil unit that has 5 duct connections and in-line fans (recommended to mount above a closet space and route ducts/fans in the attic buried in cellulose insulation)?? Potentially, since I already have ceiling discharge ports, this approach could save on first cost but future maintenance may be an issue. Fan speeds would have to be modulated for fine control of space temperature.
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By the way, the air-to-water heat pump is likely to be the largest LG R-32 monobloc. My new-found small contractor has gotten good response from his LG rep and corporate support folks.
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If it is single story, don't bother with anything in the closets or attic. Put one or two air handlers in the basement and run ducting to each room through the joist bays. Way easier install and much easier access.
I know people often think of the basement as outside the house, but no matter what you do a basement is part of your conditioned envelope, any heat loss there stays inside the house.
P.S. If this sounds close to a regular ducted heat pump/furnace. It is. 99% of the houses around me are heated and cooled this way. About the only reason for anything hydronic is if you want a lot of floor heat.
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As mentioned, town authorities won't allow a duct trunk below 6'-8" even in a bulkhead so that's not an option in the basement. Floor heat may happen in the long term but it's fan coils for now.
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Yes, I looked at those LG documents and capacity charts and asked my contractor to do the same. At first he didn't favor a monobloc but changed his mind. We will try to design for the lower heating water temperature. Cooling water temperature is a question in regard to necessary dehumidification capacity.
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I would start a separate thread to talk about air handlers specifically.
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Panel rads, properly sized are an option.
For me, radiant floors in bedrooms is a bit of a waste. So much of the floor is covered. A TRV controlled panel is ideal. In my last home I could reach the TRV knob and crank it before I got up. They respond within minutes.
I have just a few days on the Viessmann VitoCal brand, so far so good. It performs as the sales pitch and the online training suggests.
Bob "hot rod" Rohr
trainer for Caleffi NA
Living the hydronic dream0 -
I would find out what the actual restriction is. These are hyper local, so no universal answer, but I can tell you from experience there are lot of myths out there among trades about actual rules. A local favorite here is that heat pumps require a backup, which is not the case at all.
For example, our rules about height don't come into effect until the space is finished and even then you can have lower higher for a good percentage of the basement.
In low basements, what I have done is run the ducting along an outside wall perpendicular to the floor joists. The feeds to each room can now run inside the joist bay.
This means there are no low points anywhere inside.
You can even put the ducting on edge so it sits flat against the outside wall to take up even less space and frame your interior wall underneath it. Since it is part of the wall, it is not taking up any height.
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