Diesel Heating Questions...

[rbcoomer]

Hi All,

I'm seeking to tap into the communities knowledge of diesel heating please... :cold:

As many here will be aware, I'm refitting an early Fletcher Arrowbolt (21' open sportsboat with cuddy) and part of my objective is to make more of an ultra-compact coastal day cruiser. Basically, I want to be able to use all year around and either 'pootle along' [(C) Hurricane ] with canvas up when very cold or at least be able to anchor, pull covers up and defrost the crew!

I'd planned to fit a diesel heater in the stern beside the batteries and duct this forward under the port side raised floor into the cuddy, with 3 closable outlets into the lower central floor section. I'm thinking a D3 or D4 Eberspacher or Webasto equivalent c/o our favourite bargain basement auction site... However I need to think about ducting now as I'm installing the stringers/floor!

The first issue is relating to ducting and I'm interested in first hand experience of how much heat transfers through the wall of the ducting? I'm going to have to seal this under the floor and ideally fill the surrounding void with buoyancy foam. Obviously heat could be an issue if the walls of the ducting get hot! Should I install a 75mm duct inside a 90mm one (or other tube)? If so, what could I seal the ends with that can contact the 'hot' ducting without transferring heat? (The 75mm 'inner duct would connect to the vent outlets, so just the out tube would need sealing to prevent the foam entering as it's poured in in liquid form.)

Secondly, is a D3/D4 too higher rating? What would be a typical wattage for a 21' vessel? I was working on the basis that a higher wattage unit and thermostat would be more efficient than a smaller unit running flat out!

The central floor section is approx 230mm lower than the side floor areas, but the void is triangular due to the deadrise of the hull - the vent outlets would open through the 230mm vertical face. The floor needs to be sealed to prevent moisture ingress and thus will be glassed over and flow-coated - hence the need to insulate the run and prevent escaping heat in the wrong places!

I suppose I'm particularly interested in how this ducting is usually routed and what is the radiated heat loss like, proximity to GRP etc etc? Am I worrying about nothing?

Many thanks in advance,

Robin

 

[vas]

Robin,

no experience with diesel heaters at all am afraid, but let me ask a few qs that will probably help you out.

What is the temp of the air exiting the heater?
Assuming it's in the 40-60C range, I doubt heat transfer from flex pipe to hull/trusses/whatever makes any difference.
Further, there are insulated flex pipes around. Thing is you have to find one that the insulation (rockwool usually) will be externally sealed/covered/waterproof. That's to make sure an water ingress wont make insulation a solid piece on the bottom of the duct...

IMHO unless the system brings air hotter than 55-60C you should be alright. OTOH I'd be very impressed if it did

cheers

V.

 

[superheat6k]

I would have thought a D2 would be fine for a 21' sports boat. I have a D4 (4 kW) on my 36', and it gets as warm as toast. Losses from the duct hose would be minimal, far higher losses will be losing the warm air from your cover, so make sure this is snug.

I bought a 'bargain' D2 for my last boat, and then shortly after bought a New ECU, just be wary, and they do go for silly money on EBay.

Easy to install. PM me if you want the manuals.

 

[rbcoomer]

Thanks both - very helpful and kinda what I wanted to hear!

No idea what the air temp is leaving the heater, but I'd guess that will depend on airflow and as my duct will be perhaps 3M, it might be hotter than it would with a half meter duct? I guess I could add extra fans to 'pull' the air, but that'd probably get complicated too! I think you're both perhaps right with the insulating properties and minimal losses so perhaps I am indeed over-analysing? I just don't want to goof it up and have to pull out the floor later... :disgust:

I'll look for D2 or perhaps D3 instead.

 

[volvopaul]

I was looking on eBay today, there are some seriously cheap 2kw bers and webastos on there at less than £800.

Keep the ducting run as short as possible, I'd have two outlets maybe 3 at a push. Should be toasty.

A 3kw heater will do a 30ft boat easily so don't overdo it. I once fitted a 4kw heater to an s23 on advise from the owner, gues what it just kept shutting itself down.

 

[superheat6k]

I'll look for D2 or perhaps D3 instead.

The current models are the Airtronic D2 and D4. I personally would avoid the older ones. The ducts on my last boat were a lot longer than 3m. It's also worthwhile having a return duct, so you keep the warmed air recirculating. No matter how hard you try you will have plenty if make up from the smallest gaps around your covers. Keep the exhaust well away from any vent inlets - Eberspacher instructions are very clear on this aspect.

 

[maby]

The ducting is fairly thin walled plastic stuff and will get pretty much up to the temperature of the air outlet - hot, but not damaging for most materials. It is recommended that you should insulate it in order to minimise losses and make sure that the heat gets to where you want it - Eberspascher (and the other manufacturers) offer sleeving that you can install over the ducting. I guess you could use alternatives such as rockwool if you want to save money.

 

[rbcoomer]

I was looking on eBay today, there are some seriously cheap 2kw bers and webastos on there at less than £800.

Keep the ducting run as short as possible, I'd have two outlets maybe 3 at a push. Should be toasty.

A 3kw heater will do a 30ft boat easily so don't overdo it. I once fitted a 4kw heater to an s23 on advise from the owner, gues what it just kept shutting itself down.

Thanks, that's useful and D2 looks like the way to go. I need one outlet in the cuddy and thought 3 along the main run of the floor, but perhaps 2... I can't really shorten the 3M unless I relocate the heater...

 

[rbcoomer]

The current models are the Airtronic D2 and D4. I personally would avoid the older ones. The ducts on my last boat were a lot longer than 3m. It's also worthwhile having a return duct, so you keep the warmed air recirculating. No matter how hard you try you will have plenty if make up from the smallest gaps around your covers. Keep the exhaust well away from any vent inlets - Eberspacher instructions are very clear on this aspect.

Thanks - I hadn't realised that the D3 wasn't current. I wonder if the 3K2 Webasto is? I can't really justify new as the project is going to overrun on costs anyway I'm sure! Yes, aware of the CO issues and planned to vent at the stern - hence the location I need to revisit the bilge blowers too as current design blows inside the canvas and I'd like that outside too!

 

[David2452]

If you intend to encapsulate a length of ducting in buoyancy foam I suggest you use 60mm (standard for 2kw heaters which is all you need on such a small boat) APK or other ali duct with thermo duct insulation and then pop the whole lot in 100mm APK or other ali duct before encapsulation. The surface of normal un insulated PAK duct can reach 140c within 50cm of the heater and build up on the rest would be significant, I have no real idea of the temperature tolerance of buoyancy foam but suspect it is not that high. Two outlets is really all you should need otherwise air volume will be low with a small heater. A heater running hard for its living in this application would fare much better than a 3.5kw constantly cycling or running on low heat as it would prematurely coke up.

 

[longjohnsilver]

Robin, better to have a smaller unit working hard than a larger unit cutting in and out, which can cause issues with sooting up. I installed a Mikuni in my last boat and it was excellent, and cheaper than the other 2 makes.

 

[rbcoomer]

The ducting is fairly thin walled plastic stuff and will get pretty much up to the temperature of the air outlet - hot, but not damaging for most materials. It is recommended that you should insulate it in order to minimise losses and make sure that the heat gets to where you want it - Eberspascher (and the other manufacturers) offer sleeving that you can install over the ducting. I guess you could use alternatives such as rockwool if you want to save money.

I was looking at the 75mm version of this: http://www.ebay.co.uk/itm/251214995038?ssPageName=STRK:MEWAX:IT&_trksid=p3984.m1438.l2649 and using their Y pieces and vents: http://www.ebay.co.uk/itm/250739729334?ssPageName=STRK:MEWAX:IT&_trksid=p3984.m1438.l2649 & http://www.ebay.co.uk/itm/250904399421?ssPageName=STRK:MEWAX:IT&_trksid=p3984.m1438.l2649 If I go with D2, then 60mm may be enough - depending on length... I'll look into surrounding sleeving/insulation - Rockwool would probably absorb the buoyancy foam before it set and thus I'd need to wrap a waterproof barrier around that too I think if I went down that road?

 

[David2452]

The ducting is fairly thin walled plastic stuff and will get pretty much up to the temperature of the air outlet -

Errr... no it isn't plastic it's ali and it will get very hot, especially if the heat can not dissipate because it is insulated.

 

[rbcoomer]

If you intend to encapsulate a length of ducting in buoyancy foam I suggest you use 60mm (standard for 2kw heaters which is all you need on such a small boat) PAK or ali duct with thermo duct insulation and then pop the whole lot in 100mm PAK or ali duct before encapsulation. The surface of normal un insulated PAK duct can reach 140c within 50cm of the heater and build up on the rest would be significant, I have no real idea of the temperature tolerance of buoyancy foam but suspect it is not that high. Two outlets is really all you should need otherwise air volume will be low with a small heater. A heater running hard for its living in this application would fare much better than a 3.5kw constantly cycling or running on low heat as it would prematurely coke up.

Yes, that's what I feared on the temps - hence original question! I figured a 3KW electric heater got pretty hot, so this should be similar... I'll perhaps stick with three outlets then on the basis of heat to cuddy or heat to 2 x cabin outlets... I think they do a 90mm, so perhaps a 60mm inside a 90mm? I doubt it would be run for very long durations as we're unlikely to want to overnight in the middle of winter and we all currently suffer an open boat! It will probably be used more to stop and warm up from time to time rather than continual running over a long cruise, but then again once we get used to the comfort... That and I need to factor in future owners and how others might use, so needs to be safe above all else.

Robin, better to have a smaller unit working hard than a larger unit cutting in and out, which can cause issues with sooting up. I installed a Mikuni in my last boat and it was excellent, and cheaper than the other 2 makes.

Hadn't considered the coking up - thanks both. Will look at the Mikuni too - have seen a couple come up on eBay...

 

[maby]

Errr... no it isn't plastic it's ali and it will get very hot, especially if the heat can not dissipate because it is insulated.

Our recently installed (professionally) Webasto on our previous boat was definitely ducted with plastic - I accidentally disconnected the ducting at one union while I was installing an extra cable. I'm pretty sure that the (also recently professionally installed) Eber on our current boat is also ducted with plastic. There's no reason for it to get any hotter than the air passing through it and I can hold my hand in the flow out of any of the vents - it's hot, but only hot in the way that air from a hair dryer is hot.

 

[longjohnsilver]

Robin, I could be wrong, but pretty sure that its recommended to have at least one fixed (ie non closable) outlet, as its all too easy to accidentally close them with the resultant problem of overheating.

 

[rbcoomer]

Robin, I could be wrong, but pretty sure that its recommended to have at least one fixed (ie non closable) outlet, as its all too easy to accidentally close them with the resultant problem of overheating.

Yes, that makes sense! I can have an open duct in the main cabin area or the cuddy. The cuddy is small enough that a couple of candles would heat anyway so 2kw all in there would be a sauna TBH! I am however think of the kids and somewhere out of the elements if it's a bit nippy!

 

[David2452]

Our recently installed (professionally) Webasto on our previous boat was definitely ducted with plastic - I accidentally disconnected the ducting at one union while I was installing an extra cable. I'm pretty sure that the (also recently professionally installed) Eber on our current boat is also ducted with plastic. There's no reason for it to get any hotter than the air passing through it and I can hold my hand in the flow out of any of the vents - it's hot, but only hot in the way that air from a hair dryer is hot.

If plastic were used I question the professional part, ducting for these heaters is specified to have an operating temperature of 140c and I measure the temperatures quite often so I know precisely how hot they get. AKP ali duct is the norm and supplied with kits, however there is also some silicone duct used by some installers that you may have confused with plastic.

 

[rbcoomer]

If plastic were used I question the professional part, ducting for these heaters is specified to have an operating temperature of 140c and I measure the temperatures quite often so I know precisely how hot they get. AKP ali dust is the norm, however there is also some silicone duct used by some that you may have confused with plastic.

It was ali that I was looking at anyway, so 60mm ali+insulation+90/100mm ali sounds like the way forwards? Do you supply out of interest? I note that you install and would rather give the business to a fellow forumite than a faceless corporate giant...

 

[maby]

If plastic were used I question the professional part, ducting for these heaters is specified to have an operating temperature of 140c and I measure the temperatures quite often so I know precisely how hot they get. AKP ali duct is the norm and supplied with kits, however there is also some silicone duct used by some installers that you may have confused with plastic.

I certainly would not be able to distinguish between silicone and any other plastic type material - perhaps that is the explanation. The temperature of the air coming out of the duct is far less than 140c - that would be scorching the floor that the vent is playing on.