Sorry, I meant on the primary side. If a heatbank dedicated to DHW has it's own pump feeding the plate exchanger this could also be used indirectly to keep the DHW hot in the pipes.
Also a thermostat inside the heat exchanger in the right place might be an alternative to a flow switch.
cheers, Pete.
In most cases a DHW pump serving a plate is on the return to keep it cooler. You could have the same pump heating the store from the boiler. It would need to be on the return of the boiler and plate, and when heating the store the boiler, hot primary water will circulate through the plate too. Also the hot water from the store would flow into the boiler being cooled. The control system would need to be arranged, so that when the flow switch cut in the boiler is cut out and the DHW pump only operates and a zone valve isolate the boiler. This would create a lag in getting hot water fro the store to the plate. The cost of an extra pump, rather than extra controls and a zone valve is well worth it, to separate the two functions.
"inside the heat exchanger"? There is no provision for one.
Gledhill do not have a flow switch. They use a temp senor on the DHW draw-off that times the rate of temp change and modulates the pump to suit to maintain a DHW draw-off temp of 55C. If low flows a low pump speed.
The Gledhill heat bank is very good. I have one. The electronics on the Gledhill are very sophisicated learning the boiler warm up characteristics. It also will not cycle the boiler and will only bring it in when the store temp drops below 60C. Just using sensors, not stats, temperaures around the store can be monitored too.
Other lesser heat banks use flow switches to detect when their DHW demand. When using a flow switch, even when you draw off a little amount of water the pump is taking hot water out of the store at a rapid rate. This can upset stratification in the cylinder. That is why it is best to have a hybrid heat bank: a coil and a plate heat exchanger. The coil for low flow rates, where no pump operates and the plate with pump for high flowrate. Only when high DHW flows will the pump and plate cut in. The coil also acts as a pre-heat to give higher flows in winter when the mains water may be colder, although a correctly sized plate would make no difference.
The DHW is oxygenated mains pressure water.[1] It will rust all the iron/steel fittings it runs through. Bronze pumps are around £150.
[1] It is usual to reduce the pressure to around 2.5-3 bar from the mains. 1) Becasue the is a good enough pressure to provided a very good flow through typically sized pipe work. 2) You can split the pressure reduced cold water into cold and hot feeds which can greatly improve the usability of the manual mixers. 3) It is not too high to create a lot of wear and strain in average taps.
Ahh thanks, getting prices or figures or out of IMM is like getting blood from a stone :)
True, more benefits besides I would have thought.
cheers, Pete.
Sounds like a DIY electronics project in there.
Could an immersion cylinder be used as a heat bank?
cheers, Pete.
A cheap plain direct cylinder can be used. One with a shower tapping on the side for the flow from the boiler, and have two tapping at the bottom (Range have these mostly as standard) One bottom tapping for the return to the boiler and one for the return from the plate. the draw-off at the top is the flow to the plate. Have two strap on cylinder stats to prevent boiler cycling. The top stat set to approx 65C and the bottom to approx 70-75C. A relay is required using these and latching is required. When starting from cold the store heats up and the boiler is cut out when the bottom stat is
75C. The store cools as water is being drawn-off. Only when the water at the top is below approx 65C does the boiler cut in. Then it heats the whole store all in one long efficient burn. No boiler cycling on a few degrees as with one stat.
Hi,
Would that only be OK for a vented system? What pressures can sealed systems and direct cylinders operate at?
Also I wonder if it's possible to put a blending valve in so that primary water from the boiler is used by preference, with water from the heat bank used when necessary.
cheers, Pete.
Yes. Just have a stadard CH F&E tank top the heat bank up.
Sealed systems operate at approx 1.5 to 2 bar and a max of 3.5 bar, the blow-off setting.
There is grade 1, 2 and 3, with grade 1 being the highest.
Grade 1 cylinders, the highest pressure are rated at 2.5 bar working pressure and a test pressure 1 bar above that.
I don't quite get you. Best let the boiler heat the cylinder using a blending valve on the flow/return, that only alows 75-80C water enter the top of then cylinder. Heats up top down.
Oops, meant using a cylinder with a sealed system, but that answers my question.
True, sounds good.
cheers, Pete.
If you have a sealed boiler/CH system and want a DHW only heat bank, you can use a cylinder with a quick recovery coil. You will still need the F&E tank to top up the heat bank, or use a combination cylinder if you want an all in one solution. Extra tappings, exactly where you want them, can be inserted by the supplier/maker, usually for free, or a slight extra charge.
If having a condensing boiler and quick recovery cylinder then you can dispense with the blending valve.
Heat Banks provide Secondary Circulation by the usual means, except the return tees into the mains in (above flow switch). The heat exchanger pump can be brought on by a pipe thermostat fitted to secondary return that will result in circuit being reheated once it has dropped to stat setting. A clock should always be used.
Note that Pumps are more reliable, less vulnerable to grit, easier to service, cheaper, and provide pressure gain, rather than loss, over motorised zone valves. Where an expensive modulating pump is to be used, zone valves may be more economical however.
Don't you mean the secondary circulation pump?
Pumps require a check valve in the circuit. Check valves restrict flow/pressure, unless the next size up is used, i.e., 28mm on a 22mm pipe.
Sorry, I misread. The pipe stats can switch in the secondary circulation pump and the DHW plate heat exchanger pump simultaneously. No sense in having both running when the draw-off pipe is hot.
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