Mild Steel boiler - can it cope with the heat from a batch box?

Thanks to all the information and help on this forum I'm finally at the stage where I need to decide on which boiler to fit at the top of my 5" batch box heat riser.

I recently bought a used Clearview 750 wrap around mild steel stove boiler (45,000 btu/hr) which I plan to connect via a 'Neutraliser' to our Heating and DHW circuit. The system is open vented (unpressurised) and is fitted with a heat leak radiator. I'm making provision to fit a circulating pump on the rocket boiler side of things in case the boiler 'kettles' when exposed to the fierce heat of the batch box unit. The whole batch box firebox and boiler is to be encased in a brick 'bell' with the final exhaust routed via an existing masonry chimney.

I'm concerned that after talking to different plumbers (who are all giving me different advice...!) that the boiler may spall when in contact with the 700 degrees C plus output from the heat riser. Some say the cooling action of the water in the boiler will save the steel, others say the boiler will fail after a few weeks use. In addition some have mentioned that the steel boiler may rob too much heat from the exhaust and, combined with the cooling effect of the masonry bell and chimney, stall the draught of the system. I've reduced the planned size of the brick bell somewhat to hopefully compensate for the cooling effect of the boiler.

What does the forum think?

Well, i'm not absolutely sure the outside of the boiler would spall. But it's so easy to fit a stainless shield to the boiler.  I would weld the stainless shield on the part facing the heat riser's output. With 312 rod.

As for the stalling, take the outside of the boiler as ISA

For a 150mm. You need 53sqft or 5m2. Including boiler's outside surface.

http://donkey32.proboards.com/thread/1822/sizes-single-bell-systems

As long as the boiler is filled with water that 700º C doesn't mean anything. The boiler is open vented so the water won't be able to get above boiling point. Whether or not the water will take too much heat I don't know, sorry for that.

My opinion on the relative effect of the water-filled boiler is that it will be more efficient at absorbing heat, since its surface may be much cooler (212F at most) than the interior masonry surfaces of the bell (no real limit on maximum temperature). Therefore, I would weight it higher, by an unknown factor, than the masonry surfaces in ISA calculations. If an ISA of 5m2 is indicated, I would make your total combined ISA slightly smaller.

To be safe, I would build in the ability to expand or shrink the ISA of the bell as your experience shows to be warranted. There could be many ways to do this.

Glen, Walls of the bell, when cold, are at the same temp as the boiler if cold. I seem to recall the heat absorption was dependent on the delta of temperature. Due to convection in the tank, and circulation, it can't reach more than 90C°. But But up to the point the tank is at 90, the walls should heat up at the same rate, pretty much, or faster may be. But this shouldn't effect the bell's ISA too much, if at all.

Anyway, your idea of adjustable bell's size is good. I would implement this with an adjustable plunger tube.

John, a plunger tube is the chimney descending in form of a tube, inside the bell from the top. If you do this with two tube one sliding inside the other, you can adjust the height of the chimney entrance, and hence adjust the size of the active bell. Anything bellow the bottom of the plunger tube being some kind of lost heat exchange surface. So your effective ISA is less.

Yes, during initial heating, there will be little or no difference between the masonry and boiler surfaces of the bell. If heated for long periods, the difference could grow to be significant, with the boiler staying effective at absorbing heat while the masonry absorbs less due to the smaller delta T. I suppose this would not be any kind of detriment, as the system needs to work well while coming up to temperature anyway, and would only become relatively more efficient compared to an all-masonry system at higher heat states.

Glenn, as well, heat radiated from the walls of the bell will heat up the boiler until everything is at equal temps.

Satamax, Glenn and Peter - thanks very much for your quick and helpful reply's (as always). Sorry for the delay in thanking you all, our internet signal in the wilds of Wales is very poor at the moment...

The idea of the adjustable bell is pure genius!

I'm planning on installing an inspection door in the bell where the boiler will be located to allow me to monitor how the exhaust from the heat riser is affecting the boiler surface. I think I'll try a thin stainless steel 'protector baffle' between the heat riser exit and the boiler and see how the system performs.

Thanks again for your help Gentlemen - I'll keep you posted as the build progresses.

Well my 5" BB Mass/Water heater has been running since mid December 2017. Due to the recent very hot weather over here in the UK it has not been used for a few weeks, but prior to that it was in use almost daily.

The unit performs very well as a room heater. From 'cold' it soon emits heat to the room via the glass door and after only a few minutes the cast iron inspection door gets very warm and acts as a radiator. The cast iron oven that I incorporated in the brick bell then becomes warm. After 20 minutes or so (from a cold start) the top section of the brick bell warms and this warmth transfers downward towards the floor as the burn progresses. After 45 minutes or so from cold the Central Heating pump kicks in and sends the water from the boiler around the radiators of the CH system.

We tend to use the BB all day. This is to maintain a reasonable level of heat from the boiler to the CH system. The outside of the single skin brick bell gets very hot - too hot to touch over most of its surface.

As a stand alone room heater you could easily get away with 2 or maybe 3 BB firings in a day which keep the room at a very pleasant temperature - even on the coldest of days. To maintain enough water heat to power all the radiators in the house you really need to have a constant fire in the firebox to supply this demand.

In summary I am very pleased with the performance. Ideally the water system would be connected to a large thermal store but I currently don't have the funds or space to incorporate one... maybe one day

Many thanks again to everyone for their help, advice and comments.