webasto 90 plate heat exchanger unit for instant hot water

[Poppin]

Our thermo top C is taking too long to heat the hot water tank. I think it used to be better, but who knows, maybe not! 

I'm considering one of these 90 plate heat exchanger units that are designed to be plumbed into the system. As far as I can tell, these act as a mini radiator that instantly heats hot water that passes through it. However I cannot find clear installation notes or videos online. Can somone clarify how they work and where they are installed exactly? My webasto system is a closed, pressurised system with 5 radiators and a hot water tank (twin calorifier). Does this get plumbed into the system as another radiator, with the hot water tank output coming into this plate unit and exiting to the boat's hot supply? This would mean that water is always at least a bit hot, and then very hot once the tank has caught up?

[Eeyore]

You are basically turning it into a combi boiler. You then need a means of diverting the full flow from the boiler to the exchanger when a hot tap is opened.

 

Edited June 9 by Eeyore

[jonathanA]

I wonder even if you could divert the full flow from the webastard into the plate HE would the thermo top cycle on/off or even lockout due to high temp ?

I would also speculate  if it did work, there is a real risk of scalding if hot water is then heated up further. A bit like those folk who take calorifier output into instaneous gas heaters but need a way to bypass the gas heater if the water is already hot. How would you stop the plate thing heating already hot water. I see a complex arrangement of motorised valves/solenoids and a box of electronics to make it work... 

 

The usual boat use of those plate heat exchangers is to transfer heat from the engine cooling system to the domestic heating circuit. 

 

Maybe worth investigating why your hot water is so slow to heat, if it is ! before looking at a potentially complex solution. 

[Poppin]

What I'm asking is where the usable water is plumbed into the system. I have a hot water tank that is heated by the webasto, but it takes ages to heat fully. Would it be appropriate to have the output from the hot tank enter into the input of this unit, to be heated before supplying the boats hot taps?

 

I don't think the exchanger would make the webasto cycle, it would be the first thing heater in the system before the coolant goes on to the calorifier and 5 rads. Yes once the tank gets up to temp, this unit would heat already hot water, but I cannot see it getting ridiculously hot in the course of a few hours webasto running

[David Mack]

If this system is going to work (and I'm not convinced it will) you need some means of overriding the webasto's normal time/room temperature controller so that it runs full time (subject to the overheat thermostat) whenever a hot tap is turned on. You might be able to do that with a flow-operated switch in the main hot water line.

[jonathanA]

6 hours ago, Poppin said:

What I'm asking is where the usable water is plumbed into the system. I have a hot water tank that is heated by the webasto, but it takes ages to heat fully. Would it be appropriate to have the output from the hot tank enter into the input of this unit, to be heated before supplying the boats hot taps?

 

I don't think the exchanger would make the webasto cycle, it would be the first thing heater in the system before the coolant goes on to the calorifier and 5 rads. Yes once the tank gets up to temp, this unit would heat already hot water, but I cannot see it getting ridiculously hot in the course of a few hours webasto running

So usually these plate HEs have 2 ports for the heat source and and an in/out for the water to be heated. So you think you will put it in between the weber and the first rad. (Heat source) and between the cauliflower hot/warm out and taps . Worth a try I suppose I'm just doubtful you will get enough heat transfer to the hot water but ive never tried it..

 

If your cauliflower and rads  are all in series one after the other then I'm not surprised its taking an age to heat up. 

[Poppin]

2 minutes ago, jonathanA said:

So usually these plate HEs have 2 ports for the heat source and and an in/out for the water to be heated. So you think you will put it in between the weber and the first rad. (Heat source) and between the cauliflower hot/warm out and taps . Worth a try I suppose I'm just doubtful you will get enough heat transfer to the hot water but ive never tried it..

 

If your cauliflower and rads  are all in series one after the other then I'm not surprised its taking an age to heat up. 

The thing is, it worked fine a year ago... not sure why it's happening. Yes in series, how else can it be done?

[MtB]

22 hours ago, Poppin said:

Can somone clarify how they work and where they are installed exactly?

 

 

Pathetically, would be my prediction.

 

The hot water output from a direct-acting boiler such as a Morco 61 is barely adequate. An indirect system on a plate HE such as you describe will be worse.

 

It all hinges on the power output of the heat source. What is the power of your ThermoTop C? I bet it is less than a Morco. 

 

[ditchcrawler]

7 minutes ago, MtB said:

 

 

Pathetically, would be my prediction.

 

The hot water output from a direct-acting boiler such as a Morco 61 is barely adequate. An indirect system on a plate HE such as you describe will be worse.

 

It all hinges on the power output of the heat source. What is the power of your ThermoTop C? I bet it is less than a Morco. 

 

Well an electric shower is 8Kw+

[MtB]

2 minutes ago, ditchcrawler said:

Well an electric shower is 8Kw+

 

The D61 is 9.4kW, having just checked. The thread title says "Webasto 90" so maybe 9kW too.

[Tacet]

1 hour ago, MtB said:

 

The D61 is 9.4kW, having just checked. The thread title says "Webasto 90" so maybe 9kW too.

If it is 9kw - and you can make the exchange 100% efficient (unlikely) and the rise in temperature is 40c (not much), you'll only get about 2 litres per minute.  A typical pump is more like 10 litres per minute, open flow.

 

All assuming I have worked it out correctly.

[jonathanA]

4 hours ago, ditchcrawler said:

Well an electric shower is 8Kw+

And to quote mtb barely adequate...

9 5 and 10.5 kW are quite common now 

 

I think the webastard is about 5kw although not sure if there are different power output versions 

 

 

[Tony Brooks]

12 hours ago, Poppin said:

The thing is, it worked fine a year ago... not sure why it's happening. Yes in series, how else can it be done?

 

The radiators and calorifier in parallel with each other using a two pipe system, The calorifier closest to the boiler, although with a two pipe system it should not make much difference.

[Bod]

13 hours ago, Poppin said:

The thing is, it worked fine a year ago... not sure why it's happening. Yes in series, how else can it be done?

What has changed in the last year?

Has there been any changes to the boats plumbing system?

Do the batteries have enough strenght to run the water heater fully?

Is the fuel supply to the heater good enough?  No blocked filters?

Has the system been checked for air locks?

When was the system last serviced?

A whole heap of questions that need investigating, before an expensive solution is put into a possibly simple problem.

 

Bod

[MtB]

On 09/06/2025 at 20:03, Poppin said:

Our thermo top C is taking too long to heat the hot water tank. I think it used to be better, but who knows, maybe not! 

 

How long is "too long"? 

 

Could it be that 'user expectations' have changed? 

 

[ditchcrawler]

One slight but unlikely possibility is the water furring up the coil in the calorifier if the water is very hard or maybe the heating system needs flushing to remove sludge 

[Poppin]

27 minutes ago, ditchcrawler said:

One slight but unlikely possibility is the water furring up the coil in the calorifier if the water is very hard or maybe the heating system needs flushing to remove sludge 

The rads get hot so I'm wondering if this is it. Is there an easy solution?

[MtB]

40 minutes ago, Poppin said:

The rads get hot so I'm wondering if this is it. Is there an easy solution?

 

The tank will definitely heat up quicker if the rads are not heating at the same time. 

 

Turn them OFF!

[BEngo]

It worked OK last year.  So it must be fixable.

 

It has a twin coil calorifier.  Does the other coil ( engine fed?) warm the water  quickly?

 If so I suspect a good calorifier descale with central heating descaler will help.

 

Also check that the temperature at the webasto outlet is what its meant to be. If the water at inlet to the  calorifier is not hot enough, it willl take longer to heat up.

 Finally, where are you checking the hot  water temperature?  Many set ups have a Thermostatic Mixing Valve  in the hot water line.  If this is mis-set, or has failed/scaled up the hot water will not be at the right temperature.

 

N

[jonathanA]

Well if the cauliflower is in series with the rads as the op suggests, then it's possible that the rads have to heat up before the cauliflower even starts warming. 

 

Rather than look for unlikely furred up coils nd such I think the op should start by tracing his pipework and drawing it out. Then any fault finding would be on a firm basis.

[IanD]

18 hours ago, MtB said:

 

The tank will definitely heat up quicker if the rads are not heating at the same time. 

 

Turn them OFF!

 

That might be true if the heater can either throttle down or cycle on and off, usually a calorifier coil on its own doesn't absorb heat fast enough to keep the heater running. If the heater doesn't like continuous cycling on and off, turning all the rads off might not be a good idea... 😞 

[nicknorman]

If the decline was gradual, I’d say limescale on the calorifier heater pipes. The trouble is that it will be on the outside of the pipes, on the potable hot water side, not inside from the coolant circulation. So descaling is a bit tricky as you have to pour nasty chemicals into the hot water supply. Lots and lots of rinsing out required!

 

If the calorifier has an electric immersion heater element, you could undo this which allows you to look inside at the state of the heating coil pipes and see if there is a lot of limescale.

Edited Friday at 09:21 by nicknorman

[blackrose]

On 10/06/2025 at 18:31, Poppin said:

The thing is, it worked fine a year ago... not sure why it's happening. 

 

So finding out why it's taking so long to heat the calorifier is what I would focus my attention on if it were me, rather than fitting a new heater. 

 

It might just be something simple like an airlock in the system, unbled radiators, lack of coolant, etc. 

 

I haven't read the whole thread so these things may have already been suggested or ruled out.

Edited Friday at 09:34 by blackrose