I have a couple of things of which I am currently uncertain in the fine tuning of my setup:
1) Currently I just have a single cylinder stat which is about 400mm or so from the bottom of the tank. I have read elsewhere regarding the use of 2 cylinder stats and a latching relay. I understand this is to allow the tank to work within a temperature range before the boiler re-fires to prevent cycling. I think I get far more cycling from my CH than from the Heatbank though - which leads on to an additional question below. But first: what height does the 2nd stat go on the tank? How are the 2 stats wired together? Can you buy an off the peg latching relay to do this? If not what components are required? Is it worth doing?
2) The CH does not go via the heatbank; this is currently just for DHW. I have a programmable Horstmann roomstat for the CH, but am very dissapointed at how much cycling this causes. It switches in and out very regularly. Can anyone recommend a better unit? I have seen the Honeywell CM67 highly spoken of, but am not sure whether it is still made; screwfix don't list it.
3) I am aware that CH can also be fed off a heatbank, but am not sure exactly what additional plumbing I'd need to do - would I need new tappings into the tank, or could I just tee of the existing output from the top of the tank and use a flow valve after the CH arm of the tee? Currently the flow from the boiler goes system pump > 3port valve -> 1 leg to CH flow // 1 leg to top essex flange/diffuser on the cyclinder .
4) One curious thing I have noticed whilst becoming more intimate with my system is that when the system pump is running a trickle of water is coming over into the FHE tank. The problem originally started when I moved the boiler and pump from the cellar - the flue location did not meet current building regs when the boiler was replaced, so no option but to relocate to 1st floor. Now the pump is obviously much closer to the FHE tank. There is no actual overflow, but hot water is flowing round and back down the fill pipe, which isn't ideal as the tank is acting as a small inefficient plastic radiator in the attic, and causing condensation to boot!! The only way I can prevent it is to close the valve on the fill pipe, once I'm sure the system is devoid of air. This stops the flow around through the tank, effectively making the enclosed, but not unvented as any expansion can still occur up the overflow pipe towards the FHE tank. I don't think running the pump on it's lowest setting is viable - it is a 4 storey house. The boiler and pump (and the heatbank) are all on the 1st floor and heat has to go down 2 storeys and up 1 storey and return obviously, so the pump needs a bit of grunt. What on earth can I do to prevent this occuring?
5) And finally what are the optimum flow and return temps of a condensing boiler - I have a Worcester Bosch Greenstar 24Ri (24kW). I have seen 70/50deg C, but the heatbank needs 75-80degC, so I'd need to go a bit higher than this. If I set the boiler too high, it tends to cut in and out too, even if the system pump is set high, so it's a delicate balance I need to find.
For background and if anyone is interested, the following specific info may be a useful addition to what is on the DIY Heatbanks wiki:
- The first significant improvement I to the blueprint on the wiki was using a 100kW PHE from GEA which was about £180 incl VAT.
- The second improvement is diffuser pipes within the cylinder. I have a c200litre direct tank [1700x500(dia) mm] double insulated. I had the tank made to spec to fit the space I have available with a single top port - a standard BSP 1" male connector. The top connector takes the flow to the PHE primary and is tee'd off to the FHE tank overflow. I added 2 essex flanges myself through which I passed my own diffuser tubes prior to assembley of the flanges into the tank. The diffusers are 22mm copper tube with a yorkshire end stop soldered on and between 30 and 40 (can't recall exactly now!) 6mm holes carefully drilled and cleaned of swarf as much as poss. The diffuser runs across the full width of the tank from the flange to just short of the the opposite internal side of the cylinder. The bottom flange is 100mm from the bottom of the tank and is tee'd to take the PHE and boiler returns (they actually flow in different directions - check the wiki if that confuses). The top flange is for the boiler flow and is just below the top of the cylinder side, below the join with the dome cap. The holes on the diffuser are oriented to aim sideways and upwards from the top diffuser and sideways and downwards from the bottom to diffuser to preserve stratification in the tank.