Hydronic heat or reverse AC

[Hippocampus]

Wondering about people’s thoughts.
Any way to figure out at what outside air and water temperature it’s more efficient to switch from reverse AC to hydronic heat if you have both?
Also figuring in you need to run a typical genset and it’s fuel use if using HVAC. Also figure a 40-50’ boat with 3 AC units and 3 zones for each type of heat source.

 

[rslifkin]

If on shore power and water temp of 40+, I'd probably use reverse cycle unless electric rates are high. Depending on vent placement, you may need to keep a fan running to avoid hot ceiling / cold floor issues. Away from shore power, I'd use hydronic.

 

[w8n4sun]

We have both and prefer diesel hydronic heating. Much quieter, stays warmer more evenly, can use without genset (if on the hook), etc.

We've run our reverse cycle when air temps got down to 30s (more to see if it would work?) and with our water temps being in the 40s it worked fine. Still prefer diesel hydronic though.

 

[IRENE]

We have both and prefer hydronic also. Unfortunately, our Webasto is rather thirsty...making it NOT economical for living aboard. So we used the reverse cycle last winter while on the dock. Our water temps were in the 40s.

 

[Lepke]

Depends on the cost of diesel or electricity. Hydronic is probably more efficient. I don't like reverse cycle because you are using your AC in a less than optimal way. And you're wearing out your AC using it year round instead of just the hot days.

 

[Hippocampus]

Thanks for the thoughts. Good to hear confirmation of my thinking. Our exception is when we need to run the genset anyway for the batteries. Then figure we need the load for genset health. Like a good load on the genset. We don’t typically get to float with the genset. Do at least one/two times a week during the day off the solar. Like to start the night at 80% SOC if possible. Does that sound reasonable?

 

[O C Diver]

My guess is that hydronic is likely more cost effective compared to the all in cost or reverse cycle AC when powered by a generator. Calculate an all in cost on a generator with maybe a 10,000 hour life, and you might choke on the cost of generator power.

Attached to dock power with 40+ degree water temperatures and reverse cycle will probably always be cheaper, and lower risk.

It important to understand that reverse cycle valves need to be cycled periodically to maintain function. They do fail, get stuck from lack of use. IMO, it's a good idea to operate them in both heating and cooling modes (alternating) 5 or 6 times a year.

Ted

 

[slowgoesit]

At the dock, reverse cycle hands down for cost here in the PNW. The water temp will never get below the point where it's more efficient to use hydronic.
Away from the dock, reverse cycle only when the generator will be run for other reasons. If no generator need for other reasons, then hydronic, even at $5.50 a gallon diesel.

 

[Hippocampus]

Got my answer. Thanks gentlemen.

 

[slowgoesit]

Got my answer. Thanks gentlemen.

Ahhh, come on Lee, we've got to beat this up for another 5 or 6 pages, allowing everyone to basically say the same thing over and over again! You're going against the natural order of things, and therefore no phun!

 

[twistedtree]

At the dock, reverse cycle hands down for cost here in the PNW. The water temp will never get below the point where it's more efficient to use hydronic.
Away from the dock, reverse cycle only when the generator will be run for other reasons. If no generator need for other reasons, then hydronic, even at $5.50 a gallon diesel.

Just curious, how are you determining that reverse cycle is more efficient? Is it based on cost of electric vs diesel? Is it based on unmetered shore power? I haven’t really thought about it, but imagine there are many ways to compare the two.

 

[tiltrider1]

I ran hydronic only and measured my diesel consumption and then computed my cost. I then ran Reverse cycle and computed the increase in electrical cost. It wasn’t even close. Reverse cycle from shore power was by far cheaper, even if you add back the cost of wear and tear on the AC equipment.

I then did a comparison between Reverse cycle and resistance heaters. Again, Reverse cycle was cheaper but not by much. However, the resistance heat put the boat close to maximum amp usage. When you add back the cost of replacing heat pumps it made the two costs very similar.

Hydronic heat is by far the most comfortable heat. It’s quieter and more consistent temperature throughout the boat. If your are not a liveaboard or you have expensive shore power hydronic is probably worth the extra cost.

In my case I have cheap electricity and I liveaboard so hydronic is just too expensive for everyday use.

 

[rslifkin]

As far as wear on the A/C units goes, I can't say I've ever really seen one wear out, save for something like a pump or blower fan. The overall units always seem to die of either a refrigerant leak (typically due to age and corrosion, not usage) or have issues from lack of use like a stuck reversing valve.

 

[Pete Meisinger]

If you own a 40 to 50 foot boat who cares about efficiency? Marina electricity is at a fixed rate which doesn't fluctuate based on efficiency. If you are running your genny efficiency also doesn't make much difference.

About the only thing I would consider is which variety makes the least noise and vibration and which is less costly to replace.

pete

 

[rslifkin]

If you own a 40 to 50 foot boat who cares about efficiency? Marina electricity is at a fixed rate which doesn't fluctuate based on efficiency. If you are running your genny efficiency also doesn't make much difference.

But if you're talking about burning diesel directly for heat vs using an electric source (like reverse cycle), you could easily be talking about a 2 - 3 fold difference in cost. That's significant for anyone who uses their boat a lot in cold weather.

 

[Hippocampus]

Now underway. Looking at available instant information given genset is running as well reverse is better for fuel burn. If genset off hydronic is better. Seems clear alternator is sufficient for hydronic load. It’s warmed up so both will be off for recast of trip except maybe at night. Will post if have more observations.

 

[O C Diver]

Now underway. Looking at available instant information given genset is running as well reverse is better for fuel burn. If genset off hydronic is better. Seems clear alternator is sufficient for hydronic load. It’s warmed up so both will be off for recast of trip except maybe at night. Will post if have more observations.

If you're underway, without question, free heat off the engine with a "bus heater" is the absolute cheapest.



As in a car, your freshwater cooling loop transfers heat via a coil and blower. It costs a few amps 12 VDC. Currently running three 20,000 BTU units while the engine is burning 1.2 GPH. The ultimate efficiency is repurposing waste heat otherwise dumped overboard.

Ted

 

[Hippocampus]

On prior boat had loops off engine cooling for hot water and fed into the hydronic heat as well. Don’t know why they didn’t do that with this one. Simple and works well.

 

[twistedtree]

I ran hydronic only and measured my diesel consumption and then computed my cost. I then ran Reverse cycle and computed the increase in electrical cost. It wasn’t even close. Reverse cycle from shore power was by far cheaper, even if you add back the cost of wear and tear on the AC equipment.

I then did a comparison between Reverse cycle and resistance heaters. Again, Reverse cycle was cheaper but not by much. However, the resistance heat put the boat close to maximum amp usage. When you add back the cost of replacing heat pumps it made the two costs very similar.

Hydronic heat is by far the most comfortable heat. It’s quieter and more consistent temperature throughout the boat. If your are not a liveaboard or you have expensive shore power hydronic is probably worth the extra cost.

In my case I have cheap electricity and I liveaboard so hydronic is just too expensive for everyday use.

Thanks. Sounds pretty compelling.

 

[Portage_Bay]

Should be easy to add into your system. One considration is the risk of overheating the engine if the loop develops a significant leak. This can be mitigated by using a heat exchanger and short runs of well protected hoses between the engine and heat exchanger. International Thermal Research the makers of Hurricane Hydronic systems sells heat exchangers that will work very well for this. https://itrheat.com/wp-content/uploads/2020/01/ITR-Accessories-Rev-2019.pdf

On prior boat had loops off engine cooling for hot water and fed into the hydronic heat as well. Don’t know why they didn’t do that with this one. Simple and works well.