Fitting 2 air heaters - advice

[Elessar]

I have a 5 kW air heater that struggles in the coldest weather.
I have bought another.

Current situation:

Air heater in middle of boat under forward end of saloon. Galley is open to and forward of the saloon.
Duct goes aft to aft end of saloon and aft cabin.
Duct goes forward to forward cabin.

I plan to fit the second heater under the galley sole.
The existing heater will then do the aft end of the saloon and aft cabin only. I was planning to remove the ducting that goes forward and attach it to the new heater.
The new heater will then do forward cabin, and gain new outlets in the galley, heads and (tiny) 3rd cabin.

Then I thought why not keep the ducting all connected and feed both heaters into it.
Advantages:
In mild weather 1 heater will heat whole boat
When 1 gets clagged up as they do, I still have some heating for the whole boat whilst the heater is serviced.
Disadvantages:
That’s what I’m asking!

 

[Kelpie]

What diameter is the ducting?

 

[Elessar]

100 from the heater to the aft end of the saloon reducing to I think 80 when it goes onto the aft cabin.
100 going forward. I intend to use 80 for the heads and tiny cabin extra outlets.

 

[Skipper_Christoph]

Might be a problem with back pressure in the ducting if both heaters are operated at the same time?

 

[Daverw]

What is to stop the warm air blowing through the none working unit when the other is operating?

 

[peteK]

May work with a couple of flap valves in the pipework but then you would have to make sure you do nt operate the system with both valves closed by mistake.

 

[fredrussell]

What size is your boat? My 5kw Chinaspacher heats my 31ft boat well via 4 metre and 1 metre duct lengths. Is your ducting insulated?

 

[Kelpie]

May work with a couple of flap valves in the pipework but then you would have to make sure you do nt operate the system with both valves closed by mistake.

Would be best to have a splitter valve so that the first heater in line always has an opening to vent out of.

 

[Murv]

What is to stop the warm air blowing through the none working unit when the other is operating?

That would be my concern. As above, you'd need some sort of valve system to make that work.

I had exactly the same problem, the original heater fitted in the boat was 8 or 9kW, I replaced it with a D4 that struggled a bit in the cold weather. Following a string of expensive failures I replaced it with a Chinese one, and then fitted a second one (as they're so cheap.)
However, I did split the outputs so that the first heater only has to heat the saloon and aft cabin, the second heater heats the forward cabin and heads. (I utilised the original ducting then added a second "T" for the forward heads)
The only disadvantage was having to fit a second skin fitting but otherwise I'm delighted with the set-up.

If we're only on for the day, I tend to just run the primary heater but most of the time both are in service.
When both are running at full power I'm obviously pulling a bit of power from the batteries, but in practice, they heat so quickly that they're both running at a low power setting most of the time. I would guess that I'm probably not drawing much more power and fuel than the original 8/9kW heater would have done.

The other big advantage is that I can run them both at different settings, almost an extremely budget dual zone climate control!
I also like that I could swap the primary heater with the second if it did ever fail, although I've actually just bought a third to carry as a spare.

I suspect that you may get overheat failures due to restricted airflow if running two heaters into the same ducting at full power, but I'm only guessing.

 

[Elessar]

Thanks for all the replies! Sorry I've been slow getting back. Had to go and earn some money.

What is to stop the warm air blowing through the none working unit when the other is operating?

Nothing!

May work with a couple of flap valves in the pipework but then you would have to make sure you do nt operate the system with both valves closed by mistake.

Would be best to have a splitter valve so that the first heater in line always has an opening to vent out of.

Yes these are good ideas but I think I would be complicating things for little gain.

What size is your boat? My 5kw Chinaspacher heats my 31ft boat well via 4 metre and 1 metre duct lengths. Is your ducting insulated?

The longer runs are insulated - good point to raise though.
Its a 40ft boat but has a very large interior volume compared with most 40fters as it has an aft cabin, aft shower and aft loo plus all the usual stuff forward. And being a flybridge lots of glass too.

Might be a problem with back pressure in the ducting if both heaters are operated at the same time?

That would be my concern. As above, you'd need some sort of valve system to make that work.

I had exactly the same problem, the original heater fitted in the boat was 8 or 9kW, I replaced it with a D4 that struggled a bit in the cold weather. Following a string of expensive failures I replaced it with a Chinese one, and then fitted a second one (as they're so cheap.)
However, I did split the outputs so that the first heater only has to heat the saloon and aft cabin, the second heater heats the forward cabin and heads. (I utilised the original ducting then added a second "T" for the forward heads)
The only disadvantage was having to fit a second skin fitting but otherwise I'm delighted with the set-up.

If we're only on for the day, I tend to just run the primary heater but most of the time both are in service.
When both are running at full power I'm obviously pulling a bit of power from the batteries, but in practice, they heat so quickly that they're both running at a low power setting most of the time. I would guess that I'm probably not drawing much more power and fuel than the original 8/9kW heater would have done.

The other big advantage is that I can run them both at different settings, almost an extremely budget dual zone climate control!
I also like that I could swap the primary heater with the second if it did ever fail, although I've actually just bought a third to carry as a spare.

I suspect that you may get overheat failures due to restricted airflow if running two heaters into the same ducting at full power, but I'm only guessing.

Sounds like you have the same boat as me the issue is so similar.
Taking this and all the above into account I will do the same except I will have an outlet in the galley (which is open to the saloon) and abandon all plans of complicated linkages.
As it happens I used to have a diesel hob, so I already have a blanked off skin fitting in the right place.