What is the point of having a heater if you can't use it when its cold out?

[Rohan]

OK, this is as much a rant as a question, but I have a typical reverse cycle heating/AC system, and it won't work when the water temperature is below 40 degrees F.

So basically, what that means is, I can't use the heater when it's cold outside!! So what is the point?

Currently, the boat is in fresh water (the Potomac) Previously it was in partially salt water at a similar latitude (the Cheseapeake), and I didn't have this problem there. Does the the salinity/freezing point of the water make a difference for the operating threshold?

[1affiah]

Many heaters will have an electric coil back up for this reason. It is the same with home heat pumps, there is not enough therms left at 40F.

[rslifkin]

The minimum water temperature for reverse cycle heat will be a little lower in salt water than fresh. Probably about 3 degrees difference. A bigger seawater pump will often buy another 1 - 2 degrees, as more water flow will mean the water temperature drops less as it passes through the unit, so you can handle slightly colder input water before it starts to freeze in the coil.

In general, reverse cycle heat is good for chilly days early and especially late in the season, but it's not an ideal all-winter solution unless you're somewhere where the water just doesn't get all that cold.

[CharlieJ]

The engineering behind this is as the heat is removed from the raw water, its temperature naturally drops. Water increases in volume when it freezes so if the raw water temperature drops low enough so that it can freeze, it will expand. If this condition occurs, the expansion may burst the condenser tubes. Hence the restriction on injection temperature of about 40F.

[S/V Illusion]

Originally Posted by Rohan

So basically, what that means is, I can't use the heater when it's cold outside!! So what is the point?

It's all about the water temperature. Here it gets into the 40s sometimes in winter (BRRRRRRR) and we need heat. Not often but that's the "point" to those of us who dont live in a snow belt. Location, location, ....

[mvmojo]

This is the reason I'm a fan of resistance heat vs reverse cycle. A reverse cycle system is more complex, more things to go wrong. The only advantage is that it's more efficient, i.e., it doesn't use as much electricity as a resistance type unit. But it also doesn't produce as much heat and doesn't produce any heat at all with water temps below 40F. I've got 3 a/c units on MOJO, each with resistance heat via a cal-rod. When it gets cold, we still have nice warm air coming from the ducts and a nice, warm, dry boat! Well worth the extra few bucks in electricity if you marina charges by the kw - mine doesn't.

[Niall leslie]

- That issue isn't uncommon for heat pump systems including water cooled house ones.
- Likely the most effective cure is to use either an electric resistance coil in the system below 40 F. or a completely separate diesel powered heater or similar such as a Dickinson.
- Even our air - to - air heat pump system at home is ineffective below about 43 F. ambient which is why there is a propane furnace system in the mix as well.
- Heat pumps that are effective to a lot less than 40 F. ambient are available but expensive which is a reason that most people don't install them but use another heat source when it is at "BRRRRRR" temperatures.
- A similar issue happens to the cooling side of things with heat pumps when that ambient temperature is very high.
- Almost always is really "boils down" to upfront price and cost effectiveness where either people don't want to pay for really high efficiency or the projected use does not warrant the expenditure.
Cheers,
Niall.

[Gripfast]

Another solution is to sail south, until the winter water low temp is 50 degrees or above. No more problems with your reverse cycle Heat/AC not producing heat.

This is my chosen solution. Intend to do that this fall, and never winterize the #@%& thing again.

Cheers!

[thinwater]

IMHO, reverse cycle is disfunctional in climates that get actual freezing, like the northern Chesapeake area. I had AC and propane heat on my PDQ.

The way I see it, I want to be able to winterize the AC system and still have heat in any weather. Having heat you can't use in frosty weather is frankly, useless.


The other thing is power. I don't need AC in the summer when anchored. It's nice, but I really only need it in marinas, where there is no wind. But in the spring and particularly the fall, there is nothing like a warm cabin at anchor. Glorious sailing, and then a nice little cabin on the water to relax in at night. Fuel based heat takes little power and can be run quietly on the hook. Some of my favorite nights were in the fall, with my wife, the heat purring along.


Install fuel-based heat.

[Bill O]

Originally Posted by 1affiah

Many heaters will have an electric coil back up for this reason.

+1 Assumed the water would be too cold to get any heat out of it, purchased one w/an oil filled coil heating element for heating when I was living aboard and working in DC A couple of those years, the Potomac froze solid and we had some major snow storms. Was glad I had the heating element and the electric bill for the winter was ~$30/mo. for a 40' sailboat, so not too bad.

Our current boat has no heat pump and we use electric oil filled radiator type heaters to heat the boat in the winter.

[Chotu]

Originally Posted by Bill O

Our current boat has no heat pump and we use electric oil filled radiator type heaters to heat the boat in the winter.

These deserve a lot more credit than they get.

What warm, lovely heat they produce that really radiates out giving you a much nicer feeling than forced hot air.

It’s the next best thing to wood stove heat.

[Cadence]

Originally Posted by Rohan

OK, this is as much a rant as a question, but I have a typical reverse cycle heating/AC system, and it won't work when the water temperature is below 40 degrees F.

So basically, what that means is, I can't use the heater when it's cold outside!! So what is the point?

Currently, the boat is in fresh water (the Potomac) Previously it was in partially salt water at a similar latitude (the Cheseapeake), and I didn't have this problem there. Does the the salinity/freezing point of the water make a difference for the operating threshold?

A reverse cycle heat pump isn't going to work with an ambient temp. 40 or less unless it has a heat strip as backup. It is what it is.

[rslifkin]

Originally Posted by Cadence

A reverse cycle heat pump isn't going to work with an ambient temp. 40 or less unless it has a heat strip as backup. It is what it is.

With water cooled units, it's water temp that matters. High 30s is about the limit there, but the air temp can be as cold as you want provided the water is warm enough.

[Cadence]

Originally Posted by rslifkin

With water cooled units, it's water temp that matters. High 30s is about the limit there, but the air temp can be as cold as you want provided the water is warm enough.

I believe he said the water temp was 40 or below.

[thinwater]

Originally Posted by Rohan

OK, this is as much a rant as a question, but I have a typical reverse cycle heating/AC system, and it won't work when the water temperature is below 40 degrees F.

So basically, what that means is, I can't use the heater when it's cold outside!! So what is the point?

Currently, the boat is in fresh water (the Potomac) Previously it was in partially salt water at a similar latitude (the Cheseapeake), and I didn't have this problem there. Does the the salinity/freezing point of the water make a difference for the operating threshold?

There isn't enough difference in freeze point between DC and the ~ 20% salinity of the Annapolis area to measure.


However, the Bay does stay a little warmer than the Potomac due to shear size. But only a few degrees.



Get fuel-based heat. You can use it away from the dock too. Reverse cycle is for areas that merely get cool, not freezing.