Webasto Water Heater - Pressurised or not?

[Tim Good]

I have an old Webasto DW80 water heater which hears my calorifer and air blower type matrix blowers. In an effort to figure why it is not blowing warm air and not hot air I checked the system installation guide. It says its should be pressurised. In the diagram (Screenshot 2020-10-23 at 14.55.39.png) it shows an expansion tank, pressure guage and pressure release valve.

However my system has a rigid plastic header tank with a pressure cap on it and also a pressure release valve. Not an expansion tank. See Webasto DW80.jpg and pressure cap.jpg

Now... when i power up the system the prssure guages doesn't go up and I can easily remove the blue pressure cap from the tank without any hissing or pressure being released. For completeness I have attached the Webasto DW80 installation manual.

Questions:

1: Is my system actually meant to be pressurised and if it were pressuised would the circulation be improved / make the air blowers more efficient?

2: Is it possible the blow cap is faulty and not allowing the system to pressurise?

3. Was there meant to be an expansion vessel or is this header tank sufficient? Like a radiator on a car for example?

 

[CLB]

I know nothing if this system, but the presence of a pressure gauge would lead me to believe it should be pressurised.

 

[pyrojames]

If it was pressurised it will make a difference to the peak temperature you can run, but I doubt that is your problem. The thermostat in the hot water side will be set to cut down in the heating if the water gets close to the set point, and I doubt this will be pressure compensated. I suspect that the water circulation is poor, or that the heat exchanger is not heating the water as it should. I think that the header tank you have should be ample of the expansion you might get. I have a much smaller header tank on my thermotop, and run the water though conventional radiators. They get very hot and i have to take my feet off them.

 

[TernVI]

These things are designed to be used in vehicles, the coolant circuit is normally pressurised.
It only pressurises ,because the coolant heats up.
so a lack of pressure should not stop things getting hot!.

Things that can go wrong:
Air in the eber/basto: The heat won't get into the water, the eber will turn the flame down
Lack of flow: The hot water stays in/near the eber, rest of system stay cold
'Short circuit': hot water gets back to the eber by th shortest route, eber turns the wick down to regulate temp, rest of system stays cool.

The 'header tank' allows for expansion. The coolant expands and the air space is compressed. Too little air space, coolant will be ejected.
Too little coolant, air drawn into eber.

So, you have to look around the whole system and see what it's doing.
I found I had to adjust flow through various bits so that enough water moved through the eber to keep it at full throttle, but flow was shared among calorifier and heater matrices.
Heater matrices have to get v hot and shift a lot of air to shift a kW of heat. It's not easy to get the heat into the air without lots of close fins, which the needs a hefty fan to move the air.
My first experiment used a heater from a VW, the fan drew 10A....

 

[Ian_Edwards]

I have an Eberspacher Hydronic 10, a later version of the one you have.
The air from the matric fans will never be as hot as that from a hot air system, the water probably gets to 80C at max, not hot enough to give very hot air.
Things to check:

• Make sure that all to pipes are well insulated.
• Check that there is no air in the system, air can get trapped at high points and reduce the flow, try opening air vents, if there are any, and clean any automatic air vents if you have them, they get corroded in time, or just crack any joints near obvious high points, just loosen then a bit and see if water drips out or you get a hiss of air. Or you could close all the valves on the heating devices, except one, hopping that the increase water flow will push the air out. Do that for each device in tern, the heating element which is in circuit should get very hot quickly, if it doesn't there's a restriction somewhere.
• Try shutting off all the heaters except the matrix, ie close all the other valves. This will force all the water through the matrix and see if it makes any difference.
• If it doesn't make a difference then you probably have to live with it as is.
• If it does then open the valves to the other heaters in turn, leave the calorifier until last, I suspect that the calorifier is a thermal short circuit, it's the shortest loop and probably offer minimum resistance to the flow, and preventing the hot water getting to the rest of the system.
• You could try balancing the system, the general idea is to increase the thermodynamic efficiency of the system. Think of this way, if the water entering the boiler is at the same temperature as the water going out, then no heat has been transferred, if the water entering the boiler is cold and it leaves hot, then a lot of heat has been transferred. Practically, to get near optimum efficiency , feel the inlet and outlet temperatures of each heating element, partially close the valve controlling the device until the outlet temperature feel much cooler than the input. Do that for each device in turn.
• Other things that could be wrong are the pump is old or blocked and is no longer efficient, the heat exchanger in the boiler is partially blocked (although this shouldn't happen if the system has always had antifreeze or inhibitor in it). Both these are much harder to check, so do the easy things first.

I hope this helps.

 

[Tim Good]

I have an Eberspacher Hydronic 10, a later version of the one you have.
The air from the matric fans will never be as hot as that from a hot air system, the water probably gets to 80C at max, not hot enough to give very hot air.
Things to check:

• Make sure that all to pipes are well insulated.
• Check that there is no air in the system, air can get trapped at high points and reduce the flow, try opening air vents, if there are any, and clean any automatic air vents if you have them, they get corroded in time, or just crack any joints near obvious high points, just loosen then a bit and see if water drips out or you get a hiss of air. Or you could close all the valves on the heating devices, except one, hopping that the increase water flow will push the air out. Do that for each device in tern, the heating element which is in circuit should get very hot quickly, if it doesn't there's a restriction somewhere.
• Try shutting off all the heaters except the matrix, ie close all the other valves. This will force all the water through the matrix and see if it makes any difference.
• If it doesn't make a difference then you probably have to live with it as is.
• If it does then open the valves to the other heaters in turn, leave the calorifier until last, I suspect that the calorifier is a thermal short circuit, it's the shortest loop and probably offer minimum resistance to the flow, and preventing the hot water getting to the rest of the system.
• You could try balancing the system, the general idea is to increase the thermodynamic efficiency of the system. Think of this way, if the water entering the boiler is at the same temperature as the water going out, then no heat has been transferred, if the water entering the boiler is cold and it leaves hot, then a lot of heat has been transferred. Practically, to get near optimum efficiency , feel the inlet and outlet temperatures of each heating element, partially close the valve controlling the device until the outlet temperature feel much cooler than the input. Do that for each device in turn.
• Other things that could be wrong are the pump is old or blocked and is no longer efficient, the heat exchanger in the boiler is partially blocked (although this shouldn't happen if the system has always had antifreeze or inhibitor in it). Both these are much harder to check, so do the easy things first.

I hope this helps.

Thanks. I suspect you might be right with the thermal short-circuit. My system has a handle which controls the ball valves. In one setting it does the hot water tank only and in the other it does the heating and hot water tank. There is no option for just heating. I think when it is on heating and hot water there may well be a short circuit which firstly sends the system into standby as the water remains hot coming back to the unit and secondly prevents a good flow of water just to the matrix heaters.

 

[Ian_Edwards]

If your system has rubber hose in the calorifier cicuit, you could try clamping a hose to reduce the flow, just to test it.
It my be worth fitting isolating valves in the calorifier circuit to reduce the flow, they are also useful if you ever have to drain part of the system.
My system has isolating valves on each of the 3 matrix heaters and the calorifier, which I have found useful.

 

[Tim Good]

If your system has rubber hose in the calorifier cicuit, you could try clamping a hose to reduce the flow, just to test it.
It my be worth fitting isolating valves in the calorifier circuit to reduce the flow, they are also useful if you ever have to drain part of the system.
My system has isolating valves on each of the 3 matrix heaters and the calorifier, which I have found useful.

Yeah i was going to try and isilate the hot tank but I need to double check the heating and hot tank are not wired in series. Or else i might blow something up with the pressure.

Good idea regarding the valves. Certainly because I have 4 matrix blowers and rarely need to heat the bunk room and forward cabin and yet water still goes all the way up there and back.

 

[TernVI]

Is the heater heating the engine?
Are your matrices plumbed in series?

 

[Tim Good]

Is the heater heating the engine?
Are your matrices plumbed in series?

No and kinda of.

I have an engine heating circuit which heats the calorifier and a matrix heater buts that’s simple and works fine.

In terms of the Wabasto heating then I have a loop that goes up to the bow and a loop that goes to the stone. The bow loop hates the bunkroom and the forward cabin and the stern loop.

In each case The matrix blowers run off in parallel but probably not ideal. For example the bunkroom comes off the forward loop at right angles so I imagine it is short-circuiting slightly and the flow of water is much better going to the forward cabin but it does the bunkroom.

 

[Tim Good]

I have an Eberspacher Hydronic 10, a later version of the one you have.
The air from the matric fans will never be as hot as that from a hot air system, the water probably gets to 80C at max, not hot enough to give very hot air.

Do you know what temp you are getting out of the matrix fans? I seem to be getting about 43c.