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blackrose

Radiators for a Webasto thermotop C

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I tried to find a pipe hear loss/ output calculator online but none seem very simple.

1 hour ago, Chewbacka said:

I was told the Webasto evo throttles back about 75c, so I would use 72c as output, by which point the boat is probably becoming unpleasantly hot, say 25c giving a deltaT of 47.  But it is all a bit academic as you will have probably turned it off as the boat is hot and diesel is expensive. 

I'm not sure about that. I don't think it's academic. Everyone's been telling me how important it is. And I know people who have their webastos running for many hours. 

 

If the delta-T is 47 do you know how to calculate the kW output of 26m of 22mm dia plastic pipe? I'm simply trying to match my system to the output of the webasto.

Edited by blackrose

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6 hours ago, blackrose said:

I tried to find a pipe hear loss/ output calculator online but none seem very simple.

I'm not sure about that. I don't think it's academic. Everyone's been telling me how important it is. And I know people who have their webastos running for many hours. 

 

If the delta-T is 47 do you know how to calculate the kW output of 26m of 22mm dia plastic pipe? I'm simply trying to match my system to the output of the webasto.

On my boat the radiator load is close to the boiler output.  However when I run the boiler (this is from memory and touch, I would measure it if allowed to travel 😢) the boiler has an output temp maybe 20c hotter than it’s input.  I have about 40l of coolant in the system that also gives some thermal load.  So in my case the boat is plenty hot enough that I turn off the boiler long before the output climbs above about 60c.  So the boiler never throttles back.  This then leads to the problem that the calorifier is not as hot as I would like, so I had to put a valve in the radiators circuit to restrict the flow a little so that more flows through the calorifier.  The restrictor valve is also bypassed by a butterfly valve so I can have full flow through the rads in very cold weather or I can close the bypass valve and get a bit more heat into the calorifier.

 

Added - the rads are on one circuit and the calorifier & towel rad (bathroom) on the other, with the coolant passing through the calorifier before the towel rad.  

Advantages -

The main radiator circuit also has a shut off valve for water heating in summer
The towel rad  will give enough thermal load when the calorifier is hot (with the main rad circuit off) to prevent the boiler getting too hot too quickly.

You get a rough idea of how hot the calorifier is by the temp of the towel rad.

 

Disadvantages 

The bathroom get very hot in summer, but with the door shut & window open it’s manageable if not a little wasteful- Don’t tell Greta.

Edited by Chewbacka

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13 hours ago, TheBiscuits said:

Yes.  By quite a long way.

 

Have you done the calculation for your pipework?  My pipework dumps more heat than the largest radiator does, admittedly by design (28mm copper pipe thermosyphon).

 

You need length of pipe, diameter of pipe, material of pipe and expected delta-T.

 

A few years back I had a boat heated entirely with a continuous double loop, around the whole boat, of 2" copper pipe all exposed and polished it really worked very well

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7 hours ago, blackrose said:

I tried to find a pipe hear loss/ output calculator online but none seem very simple.

[snip]

do you know how to calculate the kW output of 26m of 22mm dia plastic pipe?

 

Yeah, sorry about that - most of them are used for calculating heat losses in steam plant or oil refineries, so they get a lot more involved than you need for central heating.  Worse, most of them are American so don't use SI units!

 

https://www.engineeringtoolbox.com/copper-pipes-heat-loss-d_51.html

 

This table gives 60 W/m at dT 55C for bare copper pipe, and the only figure I can find for PEX pipe suggests it dissipates heat at 1.07 x the rate of copper (3/4" pipe, dT 140F)

 

JG Guest helpfully state for speedfit pipe: "The insulation requirements for Speedfit pipe are the same as those for copper" so this is probably close enough.

 

I'd just use the copper figures, add a bit for PEX, lose a bit for lower temperature delta.   It's close enough for your purposes, so 60W/m x 26m = 1560W.

 

That doesn't allow anything for your calorifier, so you will easily be able to load the system as desired.

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1 hour ago, TheBiscuits said:

 

Yeah, sorry about that - most of them are used for calculating heat losses in steam plant or oil refineries, so they get a lot more involved than you need for central heating.  Worse, most of them are American so don't use SI units!

 

https://www.engineeringtoolbox.com/copper-pipes-heat-loss-d_51.html

 

This table gives 60 W/m at dT 55C for bare copper pipe, and the only figure I can find for PEX pipe suggests it dissipates heat at 1.07 x the rate of copper (3/4" pipe, dT 140F)

 

JG Guest helpfully state for speedfit pipe: "The insulation requirements for Speedfit pipe are the same as those for copper" so this is probably close enough.

 

I'd just use the copper figures, add a bit for PEX, lose a bit for lower temperature delta.   It's close enough for your purposes, so 60W/m x 26m = 1560W.

 

That doesn't allow anything for your calorifier, so you will easily be able to load the system as desired.

 

Cheers. Thanks for that. So it looks like I don't really need to upgrade more than one of my rads after all!

 

Currently 4 single panel rads @ approx 820W = 3280W + 1560W for the pipes = 4840W

 

I'll ignore the calorifier as once that's up to temperature it doesn't count.

 

If I change just one of the rads to a double panel @ 1111W it will add 291W which will give me 5131W.

 

And it saves a bit of work too!

Edited by blackrose

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I'm trying to identify which connection in my engine room is the central heating flow and which is the return. Without dismantling all the boxing in and tracing the pipes under the kitchen units and stern steps it's quite difficult. I came across this old picture before I'd fitted the boat out. It's a bit grainy when you zoom in and difficult to see but would you say I'd marked the flow and return pipes correctly? 

fitout1 - Copy.jpg

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11 minutes ago, system 4-50 said:

Can't look. Agoraphobia.

🤣

 

This is what it looks like now.

 

DSC_5965.JPG

Edited by blackrose

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1 hour ago, blackrose said:

I'm trying to identify which connection in my engine room is the central heating flow and which is the return. Without dismantling all the boxing in and tracing the pipes under the kitchen units and stern steps it's quite difficult. I came across this old picture before I'd fitted the boat out. It's a bit grainy when you zoom in and difficult to see but would you say I'd marked the flow and return pipes correctly? 

fitout1 - Copy.jpg

It is normal to connect the header tank to the return pipe, so probably you are correct.  To confirm you could fire up the boiler and see which pipe gets warm first.

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24 minutes ago, Chewbacka said:

It is normal to connect the header tank to the return pipe, so probably you are correct.  To confirm you could fire up the boiler and see which pipe gets warm first.

Ok thanks. I guess it's going to be simple enough to change it around if it's wrong. I just need to make sure I'm connecting it up the right way  so that the coolant flows through the rads the right way around. I will just check that the TMV legs of the rads heat up first. 

Edited by blackrose

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I'm going to install one of these into the CH circuit/calorifier loop so I can run it from the Webasto.

 

https://www.kuranda.co.uk/product/kalori-silencio-fai/

 

Question: If I installed a separate pump into that loop could I run the fan heater while the engine is running with the webasto switched off? Does it matter if a bit of hot water coolant gets pushed into the webasto if it's not on? Since the webasto pump wouldn't be running I assume the coolant won't circulate through the webasto but does it matter if it does?  

 

I don't know if that idea would work as the additional pump might stop the webasto from circulating water to it if it wasn't switched on but I'm just truing to work out how I could run the fan heater from the webasto and from the engine. But I don't want to mix the CH and engine coolants.

Edited by blackrose

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Circulation pumps are (normally) centrifugal pumps, coolant will flow fairly freely through a non running pump.  The engine coolant system is pressurised the Webasto system in your photo looks like it is not pressurised so if you connect then together you will have problems.  To transfer heat from one circuit to the other, I used a plate heat exchanger.

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5 hours ago, blackrose said:

I'm going to install one of these into the CH circuit/calorifier loop so I can run it from the Webasto.

 

https://www.kuranda.co.uk/product/kalori-silencio-fai/

 

Question: If I installed a separate pump into that loop could I run the fan heater while the engine is running with the webasto switched off? Does it matter if a bit of hot water coolant gets pushed into the webasto if it's not on? Since the webasto pump wouldn't be running I assume the coolant won't circulate through the webasto but does it matter if it does?  

 

I don't know if that idea would work as the additional pump might stop the webasto from circulating water to it if it wasn't switched on but I'm just truing to work out how I could run the fan heater from the webasto and from the engine. But I don't want to mix the CH and engine coolants.

Add a plate heat exchanger between the two circuits. I have added after calorifier, over cooling not a problem on my setup (wait for engine to be up to temp). The CH circuit also has a Webasto, works a treat, pressurised system.

20190512_084450.jpg

Edited by Cas446

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If your CH system has a calorifier on it for Webasto heat, then to stop that leg when in engine heat mode connect motorised valve to the relay on the Webasto loom. The relay should operate at about 40 Deg water temp, hence will only open leg to calorifier when webasto on and up to temp.

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Thanks I have an old tubular heat exchanger laying about somewhere so I'll use that. But will it matter if the Webasto gets warm when it's not on?

Edited by blackrose

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14 minutes ago, blackrose said:

Thanks I have an old tubular heat exchanger laying about somewhere so I'll use that.

Ideally you want about 5kW rating as per the Webasto so that your radiator balancing is not messed up. Also ideally install it next to the Webasto w.r.t. the pipe layout for the same reason.

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16 minutes ago, blackrose said:

Thanks I have an old tubular heat exchanger laying about somewhere so I'll use that. But will it matter if the Webasto gets warm when it's not on?

Regarding the webasto getting warm - nope not in the slightest. They are effectivly operated in this configuration in automotive setups. The only thing that might happen is if you have an ex auto one - is that if you run on engine heat, stop, then start webasto it may just circulate until water temp drops below ~60deg before firing up. All depends how auto manufacturer asked webasto to set the software, I have this, but it is never an issue as water temp drops so quickly.

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