Calorifier heating coils and connections

[IanD]

It's getting close to the stage where I have to make some decisions, and I have a couple of questions that I'd like some feedback on...

 

My (hybrid) boat will have a calorifier with two heating coils, one for the generator and one for the pressure-jet diesel boiler. The normal way to connect this up would be for the main cooling connection for the (9kW) generator to be the skin tank with a (smaller) branch off to the calorifier, and the main connection for the boiler to be to the radiators with a (smaller) branch off to the calorifier -- in both cases most of the water bypasses the calorifier which only has a small coil and fittings (15mm?).

 

Both generator and boiler are capable of dumping relatively large amounts of heat into the system, certainly much more than the coils in a standard calorifier can absorb, which is only a bit over 1kW -- see here for actual measurements:

 

https://www.perseverancenb.com/post/how-to-measure-the-calorifier

 

The issue I see is that this means it will take a long time to heat up the water, heating rate from either coil is only about 15C/hour. Since I only expect to run the generator for battery charging for an hour or so per day in summer, this isn't very helpful for providing hot water. The same applies when running the diesel boiler if only hot water is needed (radiators off), it will end up cycling on and off rapidly since the calorifier won't absorb enough heat.

 

Some calorifier suppliers offer customised cylinders with much bigger/finned heating coils to heat them up more quickly (can be less than an hour), the biggest are often called solar coils because these are intended to accept lower-temperature "hot" water (45C?) than would normally come from an engine/generator/boiler (85C?) -- for example:

 

https://www.coppercylinder.co.uk/customise-your-hot-water-cylinder.html

 

This should mean the water can be heated from cold to hot in much less time -- I don't have exact numbers yet, but maybe up to at least 5x faster than a "standard" coil, so an hour or less -- and absorbing much more heat from the incoming water, so less boiler cycling. The coils and fittings are also bigger diameter (up to 28mm?) to allow much more water flow than standard 15mm.

 

It also than seems that there's no reason to have the coils on a side branch any more, it should be feasible to put the entire water flow through them -- so the boiler loop would go to the calorifier first and then radiators (with a radiator bypass valve for hot water only in summer), the generator loop would go to the calorifier first and then to the skin tank. This would heat up the calorifier as fast as possible without restricting cooling water flow to either loop. The feed to the radiators would be cooler than normal if the tank is full of cold water, but probably still 50C or so which is OK -- and would rapidly get hotter as the tank warms up.

 

Does anyone have any experience with doing something like this? Can anyone see any problems with building a system like this?

[Tony Brooks]

Don't know if this helps but when I fitted a calorifier to the Bukh (35bhp) there were no "normal" calorifier take-offs on the engine, being a modified direct raw water system directed to the skin tank. I made sure that the calorifier would not cause excessive flow resistance and as it looked about 1" ID I felt it was OK, so I put the calorifier coil in front of the skin tank that was in series. It worked well and although the main heating effort had to wait until the thermostat opened (I image that would be fast on a generator) the bypass stated warming the water at once although it would take between 15 minutes and half an hour to get some hottish water. I can't see any reason the same would not apply to the radiator circuit. Just make sure the coil ID is large enough not to restrict flow.

[PaulJ]

4 minutes ago, Tony Brooks said:

Don't know if this helps but when I fitted a calorifier to the Bukh (35bhp) there were no "normal" calorifier take-offs on the engine, being a modified direct raw water system directed to the skin tank. I made sure that the calorifier would not cause excessive flow resistance and as it looked about 1" ID I felt it was OK, so I put the calorifier coil in front of the skin tank that was in series. It worked well and although the main heating effort had to wait until the thermostat opened (I image that would be fast on a generator) the bypass stated warming the water at once although it would take between 15 minutes and half an hour to get some hottish water. I can't see any reason the same would not apply to the radiator circuit. Just make sure the coil ID is large enough not to restrict flow.

Done exactly the same on my Sabb and it works fine- main reason for doing so was the flow of cooling water is so minimal I wanted as much heat going through the calorifier as I could get.

 

[IanD]

12 minutes ago, Tony Brooks said:

Don't know if this helps but when I fitted a calorifier to the Bukh (35bhp) there were no "normal" calorifier take-offs on the engine, being a modified direct raw water system directed to the skin tank. I made sure that the calorifier would not cause excessive flow resistance and as it looked about 1" ID I felt it was OK, so I put the calorifier coil in front of the skin tank that was in series. It worked well and although the main heating effort had to wait until the thermostat opened (I image that would be fast on a generator) the bypass stated warming the water at once although it would take between 15 minutes and half an hour to get some hottish water. I can't see any reason the same would not apply to the radiator circuit. Just make sure the coil ID is large enough not to restrict flow.

 

Thanks -- I've asked the supplier what the biggest coil they can fit is.

 

5 minutes ago, PaulJ said:

Done exactly the same on my Sabb and it works fine- main reason for doing so was the flow of cooling water is so minimal I wanted as much heat going through the calorifier as I could get.

 

 

My reasoning too, though for slightly different reasons (fast water heating, reducing boiler cycling)

[Tony Brooks]

1 minute ago, IanD said:

My reasoning too, though for slightly different reasons (fast water heating, reducing boiler cycling

 

But once the calorifier is up to temperature it can't load the boiler. In fact the load reduces as the calorifier starts to heat up. It will only reduce cycling during the warming up period.

[IanD]

23 minutes ago, Tony Brooks said:

 

But once the calorifier is up to temperature it can't load the boiler. In fact the load reduces as the calorifier starts to heat up. It will only reduce cycling during the warming up period.

 

I realise that, but either way it's always better to have all the flow through the calorifier than only part of it. With a smaller coil the output water will always be hotter (until the tank gets up to 85C or so) and the boiler will shut off sooner. There is some water storage/thermal mass in the boiler to allow it to run for longer and then shut off for longer (same duty cycle) but this is limited.

Edited November 23, 2022 by IanD

[Tracy D'arth]

How are you going to heat just the calorifier in summer when you do not want the radiators on if the flow has to go through both?

[Tony Brooks]

Three port valve to isolate the rads.

[David Mack]

3 hours ago, Tracy D'arth said:

How are you going to heat just the calorifier in summer when you do not want the radiators on if the flow has to go through both?

And similarly, how are you going to heat the boat quickly (e.g. on arrival in the depths of winter) if the calorifier is absorbing all the boiler heat?

Better I think to keep the conventional parallel arrangement (with valves) for the boiler circuit, but series for the generator circuit.

[Tracy D'arth]

1 hour ago, David Mack said:

And similarly, how are you going to heat the boat quickly (e.g. on arrival in the depths of winter) if the calorifier is absorbing all the boiler heat?

Better I think to keep the conventional parallel arrangement (with valves) for the boiler circuit, but series for the generator circuit.

My thoughts exactly.

[Jen-in-Wellies]

1 hour ago, David Mack said:

And similarly, how are you going to heat the boat quickly (e.g. on arrival in the depths of winter) if the calorifier is absorbing all the boiler heat?

Better I think to keep the conventional parallel arrangement (with valves) for the boiler circuit, but series for the generator circuit.

I have this with the way the stove back boiler is done on my boat. All the flow goes through the cauliflower before reaching the radiators. Lighting the stove in a cold boat means that for the first couple of hours most of the heat is being sucked in to the stone cold calorifier, rather than heating the boat. I don't know how a 1kW back boiler can do this to a 5kW stove, but some how it manages! Not enough of a problem for me to alter it, as I live on board, so it is only the combination of coming back from days away in winter that does this. Not applicable to @IanD, but it is also gravity circulation, so a risk of messing that up with too many valves and extra plumbing.

Edited November 24, 2022 by Jen-in-Wellies

[GUMPY]

If you are running the generator to charge the batteries, to me the obvious thing to speed up the water heating would be to switch on the immersion heater🤔

 

Edited November 24, 2022 by Loddon

[IanD]

3 hours ago, David Mack said:

And similarly, how are you going to heat the boat quickly (e.g. on arrival in the depths of winter) if the calorifier is absorbing all the boiler heat?

Better I think to keep the conventional parallel arrangement (with valves) for the boiler circuit, but series for the generator circuit.

The calorifier won't absorb all the boiler heat even with big "solar" coils, as I thought I'd explained -- the pressure-jet boiler can put out something like 10kW maximum, even if the calorifier absorbs 5kW (unlikely!) this leaves 5kW for the radiators.

 

In summer there will be a valve after the calorifier to isolate the radiators. The boiler is designed to cycle on and off like a domestic boiler, there's a water thermal store to slow down the rate at which this happens.

 

Running the immersion heater (2kW so it can absorb all the spare solar panel power in summer) when the generator is on will work but isn't an efficient use of fuel; the generator is about 25% efficient (fuel calorific value to electricity out), the diesel boiler is at least 80% efficient (fuel calorific value to electricity out). If the generator doesn't provide enough hot water by the time the batteries are charged, it's better to run the diesel boiler then to run the generator for longer and use the immersion heater. And yes there will be extra heat from the coil if the generator is run longer, but this still ends up with about double the fuel used for water heating alone compared to the boiler.

 

I have actually thought of all this, the boat design process (with Ricky) has been long and quite thorough, with a lot of iterations... 😉

Edited November 24, 2022 by IanD