Somebody mentioned that maybe I should invest in a thermal store rather than a megaflow boiler. What are you experiences with them and any particular manufacturers that are favourite?
I hadnt come across thermal stores before so Ill do some info searching, I did think that megaflow and thermal stores were the same system...
Our house has lousy pressure from the gravity system upstairs and the two bathroom showers and baths are crap. Originally I was going to fit a pump however they break and are noisy. Id prefer to fix it the professional way!
I fitted a Range thermal store about 4 years ago (all plumbed in plastic :-), and we're very pleased with it. It's particularly well suited to our place, because we have an oil boiler. The large thermal capacity totally eliminates boiler cycling - the boiler does long, clean burns. This makes for very efficient operation
I have a DPS Pandora heat bank. Here's a non-exhaustive comparison of the various systems:
Way they work:
Unvented cylinder: Basically the mains pressure water (regulated by a pressure reducing valve) goes into the bottom of the cylinder. It is heated by immersion or indirect coil. Hot pressurised water is taken from the top to the taps. A pressure vessel or integral air bubble handles expansion of the water as it is heated.
Thermal store: A vented cylinder is filled with chemically corrosion inhibited water. This water is heated by immersion, indirect coil, or is heated by directly pumping it through a boiler. The mains pressure cold water enters through a high surface area coil and is hot when it leaves on the way to the tap. Temperature is regulated either by primary water temperature, or using a thermostatic mixing valve on the output.
Heat bank: Identical to a thermal store, except in the way that hot water is generated. Instead of an indirect coil, the mains pressure cold water enters a plate heat exchanger and a flow switch on the way to the tap. The flow switch turns on a pump, which pumps hot inhibited water from the top of the cylinder, through the plate exchange and back into the bottom. Temperature is regulated using a thermostatic mixing valve on output, or by regulating the pump speed to ensure output temperature. The latter is more efficient, whilst the former usually provides better regulation.
Advantages/disadvantages of systems:
Unvented cylinder:
Very high potential flow rate.
Requires yearly servicing and special training for installers.
Possibly safety risk, if not maintained or installed properly.
Can't use very high recovery rates.
More complicated plumbing, with pressure/temperature relief required.
Must have pressure reducing valve.
Excellent stratification (water goes cold quickly when run out, showing efficient energy storage).
Thermal store:
Stores water at atmospheric pressure. (Safety and installation/maintenance benefit).
Medium flow rates at full temperature. Better than instantaneous combi.
Can run rads off it.
Can add multiple heat sources (solid fuel/solar)
Cylinder water is corrosion inhibited.
Stratification less good.
Heat bank:
Stores water at atmospheric pressure. (Safety and installation/maintenance benefit).
Much better flow rates than a thermal store (but potentially less than an unvented cylinder, although this depends on circumstances).
Can run rads off it.
Can add multiple heat sources (solid fuel/solar)
Cylinder water is corrosion inhibited.
More complicated than thermal store.
Stratification less good (particularly when using low flow rates), unless pump modulation (rather than TMV) used.
Hum, I like that idea or beast of a boiler (38kW) cycles liike crazy and overheats unless we wind it's stat well down when it only has the HW to heat or just a little bit of CH. Solar would be nice to add along with a woodburner so a thermal store or a heatbank sound like a very good idea if it can soak up 38kW from the boiler as well...
The boiler size is probably overkill but not by much when you consider it may have to heat 2 3 bed semis with external temps down to -10C and a gale blowing...
I would definitely recommend it - it's a pretty simple arrangement, and works very well with oil boilers. As long as the boiler to cylinder pipe run is adequate, you should be able to dump your 38kW into the cylinder without any problems. Probably go for 28mm, with as few bends as possible.
As long as the coil can take 38kW, or if heating the store direct that is not an issue. The beauty of thermal store/heat bank is that when connected direct the boiler size can be as big as you like. I know of one man who picked up a 75kW oil boiler for a few hundred pounds (the contractor messed up and had it spare) . He only needed 16kW for the CH. I advised a thermal store with 32mm connections and CH run off the store too. The store was heated in no time and his 3 bathrooms could be run simultaneously and never run out of hot water when filling them all. 75kW in combi terms gives approx 30 litres/min of instant hot water, so when re-heating the store it poured in the heat.
With gas you are limited to around 55kW (spare for a hob) because of the U6 meter.
Take the CH off the store using a Grundfoss Alpha pump. Heat the store "directly" with 28mm piping. Put a TMV on the boiler flow and return to maximise efficiency.
The store can take solar and stove input by adding coils to the bottom of the cylinder.
Or can be run direct without coils. Stove mustn't normally be pumped, unless the instructions tell you otherwise, which they probably won't. They need a reliable gravity circulation with no valves. The heater must be well below the heat bank.
On my Gledhill even when running the bath, the plate only ever gets just over half of it hot, with the other half very cold. It will deliver about
100kW, so it is delivering about 60kW which is about 25 litre/min in combi terms at 60C delivery. That fills the bath very fast. Although on mine a have a bath mixer with TMV reducing the bath temp to 46C. So a tot can't jump in a bath full of 60C water giving 2nd to 3rd degree burns. This slows down the flow a bit but still fill fast.
100kW in combi terms is about 45 litres/min.
On a heat bank/T store, I would always install a Magnaclean on the CH return to the store (I would put one on all types of system really). Sludge can build up in an "integrated" store because the iron oxide particles collect there as the store is the part the water moves slowest through. The filter will grab the a particles before they enter the store. The boiler should present no problems as the rads are the problem.
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