Pumped circuit from back boiler. Is gravity heat dump needed with open header tank?

[stegra]

Planning the pipework from my solid fuel back boiler to the calorifier wondering if it's necessary to have a permanently open gravity circuit to a radiator when there will be an open header tank. I want to get as much of the heat as possible from the back boiler to the calorifier rather than to a radiator next the stove. I'd also like to avoid the chunky 28mm pipe running to top of the radiator. As the vent and header tank act as the safety device, is it necessary to have the gravity circuit at all? Also interested to know what folks are using for their header tanks and whether they automatically refill or are topped up manually. I was wondering if a back-to-wall toilet cistern with the outlet converted to act as an overflow might work.

I found this post by Smileypete very helpful; both the diagram and the link to a 12v pump with 2m head:

 

[smileypete]

Hiya,

If you forgo the gravity circuit then you need a working pump whenever the fire is on, and power to run it, otherwise with no pump the system will boil aka 'kettling' and knock pretty loudly, until a working pump is restored.

A gravity heat dump rad doesn't need to be permanently open, it could be switched on and off with a full flow ball valve, as show in the diagram. I'd expect a 22mm ball valve with reducers will do.

I doubt a header tank on a typical narrowboat will need much topping up unless the system is allowed to boil which is unlikely due to the noise it makes. Some sort of metal or suitable thick plastic should do (like the stuff car expansion bottles are made from)

If the backboiler is rather small say <1kW and the gravity rad is very near the stove, it may be possible to get by OK with 22mm gravity pipes instead of 28mm.

Having a gravity heat dump rad in the saloon may give some benefits, eg heating the saloon up as quick as poss, saving fuel if the rest of the boat or the calorifier doesn't need heating so much, or drying space for safely drying some clothes away from the fire.

Do you know the heat output of the backboiler and the fire overall? Usually in kW (kilowatts) or BTUs. If it's a largish stove in a smaller boat then downsizing the heat dump rad a bit may be OK. You should still get a reasonable amount heat for the boat without things boiling if the pump to the rest of the rads etc fails.

HTH

Edited August 1, 2017 by smileypete

[stegra]

Thanks for the reply, Pete. I think I'm going to give up on the whole idea of heating the calorifier from the stove. The availability of plentiful amounts of free firewood got me a little too obsessed about. It seems obvious that the initial investment in time and money won't be recouped for decades; not mention the compromises to the boat. I'll look into fan driven venting or heat recovery ventilation to the other rooms as an alternative way of getting the most from the stove. I'm fitting a Webasto for hot water and central heating anyway so it shouldn't be a great loss.

[Tony Brooks]

I would comment that your choice of central heating boiler is probably of  a type not known for long term reliability, especially if it is oversized for the boat. I think that if you have the chance you would do well to do as you originally proposed. Being without hot water over the Christmas week because the boiler has broken down and everyone is away from work is probably not a good prospect.

I did not read the topic you linked to but once the stove is going it will not take long to get the water up to temperature and once that happens without a heat sink of adequate size both the stove water and the domestic water will get hotter and hotter until it boils. That is downright dangerous at the tap end.

If you do not want large bore pipes (not what Pete said about 22mm) then fit each circuit with its own pump (12V solar pumps seem cheap enough and many appear to use them) and use thermostats to control each pump so only the calorifier pump runs when the calorifier is heating up and then stops when the calorifier is up to temperature at which point the heat dump pump runs. If either failed then the other could be run (fit a switch in parallel with each stats) to dump heat. You may have to run your hot water off.

Alternatively fit a pair of pumps in parallel with a selection switch and use a T port manual valve to control water flow. It needs to be a left, right, both outlet valve so when leaving the boat with the stove running it can be set to pass heat to both the heat dump and calorifier just in case the stove runs away.

I would not want to be reliant on that type of boiler as my only source of heating.

Edited August 2, 2017 by Tony Brooks

[smileypete]

15 hours ago, stegra said:

Thanks for the reply, Pete. I think I'm going to give up on the whole idea of heating the calorifier from the stove. The availability of plentiful amounts of free firewood got me a little too obsessed about. It seems obvious that the initial investment in time and money won't be recouped for decades; not mention the compromises to the boat. I'll look into fan driven venting or heat recovery ventilation to the other rooms as an alternative way of getting the most from the stove. I'm fitting a Webasto for hot water and central heating anyway so it shouldn't be a great loss.

If planning to liveaboard then maybe a Morco instant gas water heater would be a more dependable source of hot water, if a little limited in winter.

Unless a hydronic Webasto is installed as well as a registered installer should do it (they charge premium prices and are alway busy, why is this? ), it may not be as reliable as it should be in the longer term.

Edited August 2, 2017 by smileypete