croydoncorgi said:
Can't understand WS' simultaneous (and conflicting / conflicted?) interests in storage combis, heatstores and (see other post) high efficiency condensing boilers. Different planets!
Do you mean people should only be interested in one thing?
The 'efficiency' of a heatstore depends on how hot it is, hotter the better. If you heat it only to (say) 70 degrees, it will be difficult to get an acceptable flowrate of DHW at 55 degrees.
Wrong. The efficiency for DHW purposes is how efficient the heat exchanger is. Plate heat exchangers are highly efficient. A store of 70C can produce DHW at 60C, which coils cannot match.
Also, the store itself will need to be larger to store enough watts to be worthwhile.
Not so, if it is larger it is marginal. Thermals stores (using coils) and Heat Banks (using plate heat exchangers) are different, with the heat bank more efficient and giving much higher flowrates.
On the other hand, if you have a smaller heatstore running at (say) 80 degrees or hotter, and connected to a condensing boiler, it's most unlikely that the boiler will ever run in condensing mode - so 5 to 8% of energy efficiency is lost straight away. Return water from the store will be hotter than 56 degrees all the time.
Not so. Using a TMV on the flow and return of the boiler will ensure return water below dew point for most of the re-heat period. From cold the water only enters the store at store setpoint and heats the store top down. The temp at the top may be 75C and yet a few inches down in the cylinder 30C – a line of say 70 to 75C will just work its way down the cylinder with the water at the bottom very cool indeed. Also the energy in the store and the boiler can be combined to give greater flows, or drop the store size or have tall thin cylinder giving high condensing efficiencies – the scope is there. Few heat banks are at 80C store temp these days. Most are at 70C to 75C. I know one DHW only heat bank set to 65C and gives 55C at the taps. Efficiency is guaranteed.
Heat banks tend to be taller and thinner to take advantage of stratification ensuring that water at the bottom (the return to the boiler) is cool and forces condensing of the boiler. The TMV ensures heat only at setpoint enters the top of the cylinder and the bottom remains cool. It also ensures the delta-T of the boiler is maintained too. A boiler can only gives certain temp rise, say 35C. If the store is 25C at the bottom then it can only deliver a flow of 60C. The TMV ensures 70 to 75C (depending on how it is set) enters the top of the cylinder. All falls into place.
The ACV HeatMaster Thermal store/boiler (well an unvented cylinder/thermal store hybrid)
condenses at all times. I “think” their web site has the instruction manual to download. Have a look.
Having a hybrid of a DHW coil at the bottom of the cylinder and a plate heat exchanger, will ensure at the bottom of the cylinder, near the return take off to the boiler will always be cool water. The cold mains cools the water here and ensures condensing in the boiler. Then from the coil, which pre-heats the cold mains water, to the high efficiency plate heat exchanger. This gives: high flowrates, low store setpoint temperatures, maximum condensing efficiency.
In a test using a standard direct cylinder, I rigged up a second plate heat exchanger on the boiler return, with mains water running through cooling the return temp - this was in place of the coil at the bottom of the cylinder, as described above. Cool water from the bottom of the store entered the plate which was cooled further by incoming cold mains water. Worked very well indeed ensuring the boiler was working at max condensing efficiencies. The plate like a coil was entirely passive. A small coil at the bottom of the cylinder would have less main pressure restrictions, especially if made of stainless steel.
So decide what you want: energy efficiency or improved hot water response. Heatstores are not a way to achieve both.
You need to understand heat stores better and don’t go on old wives tales. Look at
http://www.heatweb.com for an introduction - I have no connection with this company. Read carefully what I have written and how the ACV HeatMster works too. Understand all. Things have moved on a hell of a lot since the old coiled thermal stores.