Radiator sizes and quantity

[Adam1991]

Hi all, We’re working out the size and quantity of our radiators on our narrowboat build. 
 

It’s a 55ft nb with 5 large windows (100 x60cm each) and 4 small windows (60x60cm) 
 

We’ll be using a 5kw diesel heater.

 

We’re thinking the total radiator heating output should be around 19000 btu, would this be the around the correct total output for the rads? Assuming the diesel heating puts out 17000btu 
 

We think this will work out at 2x 5000btu and 2x 3000btu rads with a 3000btu towel rad. 
 

Could anyone clarify if we are on the right track with this? Totally new to heating. 
 

thanks! 

[BEngo]

Never mind calculating  the heat loss, because 5KW is plenty for a well insulated (Sprayfoam or Kingspan)  n.b. even with big windows like yours.

You then want enough radiators to disperse all  the heat from the stove, in case it goes to full on in error.  That way you should not boil the system (provided the circulation is good)  and have scalding water coming out of the vents.  Your estimates  for radiators do this so you are on the right track.  The towel rail will not emit 3000 Btu with towels on it.  So you might want to add a bit to one of the other of the remaining rads.  Ignore any heat requirement for a calorifier as once the tap water has reached the same temperature as the heating water it will not take any heat.

 

Good pipe work is the key to good circulation, if you are not fitting a pump.  Depending on a pump is a bad idea, unless you are moored on a shore line all the time when a 240v pump is OK.  The flow pipework wants to rise vertically as far as possible from the stove and to keep rising to the furthest radiator.  The return pipework should fall all the way to the stove.   Use 28mm copper pipe if you can bear the cost (many use 22 mm and get away with it) with 15 mm tees to the radiators.  Connect  flow to a top corner;  return to the bottom; ideally, to the diagonal bottom corner.  Use either full flow lock shield valves on both tees or no valves at all.  Never any thermostatic valves.

 

  Plastic pipe goes all saggy when it gets hot, looks awful and spoils the flow.  No valves improves the flow but means you cannot take a rad out without draining down.

 

Connect a 15 or 22 mm vent to the highest point and to the top of the header tank.  That way the system should self bleed., Use  15mm to  connect the bottom of the header to the lowest point. Finally, don't forget a drain at the low point with room to get a container under it!

 

N

[Jen-in-Wellies]

21 minutes ago, BEngo said:

Good pipe work is the key to good circulation, if you are not fitting a pump.  Depending on a pump is a bad idea, unless you are moored on a shore line all the time when a 240v pump is OK.  The flow pipework wants to rise vertically as far as possible from the stove and to keep rising to the furthest radiator.  The return pipework should fall all the way to the stove.   Use 28mm copper pipe if you can bear the cost (many use 22 mm and get away with it) with 15 mm tees to the radiators.  Connect  flow to a top corner;  return to the bottom; ideally, to the diagonal bottom corner.  Use either full flow lock shield valves on both tees or no valves at all.  Never any thermostatic valves.

You saw the bit about the OP fitting a diesel heater right?

 

Sorry, to expand a bit. @BEngo's advice is good for a solid fuel stove. For a diesel heater of 5kW, assuming it is the Webasto/Eberspacher sort, rather than a diesel version of a solid fuel stove, if the pump fails, the heater will simply shut itself down as it overheats. Smaller diameter pipe work is fine, as the pump will overcome the extra resistance to flow. No need to arrange heights of pipes in the system to aid gravity flow. Similarly, flow through the radiators can be as normal, not diagonal. 

Edited August 30, 2022 by Jen-in-Wellies

[BEngo]

I decided that Diesel Heater could also mean a bubble or refleks type  device with a Toby valve.   They benefit from proper circulation as much as a solid fuel stove and in the event of the valve sticking are somewhat prone to roaring away until they trip the emergency shut off, if fitted.

 

IMO the Neverspacher/webarsteward type are so called for a reason and I certainly would not want  one as a primary hear source in any boat I owned.  Others may disagree.  Whatever, they too will come to no harm from heating a well designed circuit, and with sufficient radiators will not be as inclined to short cycle or go to low setting before the start up carbon has burned off.

N

 

Ps   Diagonal flow/return  is not essential, in any system, so I said "ideally"  but it is the way four entry radiators are rated, TBOE.

N

Edited August 30, 2022 by BEngo

[Jen-in-Wellies]

Interesting that we made different assumptions as to what a 5kW diesel heater was from the same incomplete information. I probably went towards assuming it was  webasto/eber type because I've been helping some one fit one recently.

[Adam1991]

Hi Jen and BEngo, I was referring to the install of a webasto 5kw water heater. I’m trying to work out the total btu to spread the load across a number of radiators throughout the boat and therefore work out what size and quantity of radiator to fit. 
 

thanks

[IanD]

25 minutes ago, Adam Mc Gowan said:

Hi Jen and BEngo, I was referring to the install of a webasto 5kw water heater. I’m trying to work out the total btu to spread the load across a number of radiators throughout the boat and therefore work out what size and quantity of radiator to fit. 
 

thanks

 

The new Eberspacher is reported to be quieter than the Webasto, and also throttles down to lower power levels (1.3kW?) to avoid cycling on and off when you don't need full power or hot water only.

 

 

Edited August 30, 2022 by IanD

[BEngo]

2 hours ago, Adam Mc Gowan said:

Hi Jen and BEngo, I was referring to the install of a webasto 5kw water heater. I’m trying to work out the total btu to spread the load across a number of radiators throughout the boat and therefore work out what size and quantity of radiator to fit. 
 

thanks

 5 kW is,   as you say, a tad over17000 BThU  so your radiator sizes look good, with the caveat above about towels on ⁸ the towel rail.  Some heat will be lost from the pipework though so if a slightly  bigger rad somewhere is not practicable I would not worry.  The really important things with the Eberspacher type heaters are to get them to run af full power  long enough to burn off the carbon that forms at initial light up, and to avoid short cycling.  

 

N

[IanD]

5 minutes ago, BEngo said:

 5 kW is,   as you say, a tad over17000 BThU  so your radiator sizes look good, with the caveat above about towels on ⁸ the towel rail.  Some heat will be lost from the pipework though so if a slightly  bigger rad somewhere is not practicable I would not worry.  The really important things with the Eberspacher type heaters are to get them to run af full power  long enough to burn off the carbon that forms at initial light up, and to avoid short cycling.  

 

N

 

AFAIK this is *not* the case with the newer heaters which are designed to throttle down to 1.3kW to avoid short cycling when lightly loaded. It *does* apply to the older models.