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Bit of background.... we are just completing the structure of our single story extension to our two bedroom, two bathroom (one shower, one bath/shower) house. This will add an extra bedroom and extra shower. We are on LPG and currently have a Glow-worm 30cxi combi boiler, which is in good working order and meets our needs more or less. We will be relocating the boiler into the ground floor extension as part of this project. One of the things we dislike about our current setup is that every time you turn on a hot tap, even for a short period, the boiler fires up. My impression (and I'm sure I have read this somewhere) is that firing up the boiler often and for short periods is inefficient and costly in fuel. We are on LPG so fuel is expensive. One thing we do like about our current combi is that we don't run out of water in the tank. Although the existing combi has been well maintained I have been considering our options. One option is replacing the boiler with a system boiler and tank. Tight for space, but possible. Another option I have also been looking at is to replace the combi with a storage combi eg Glow-worm Energy 35 Store combi. My hope is that the HW tank in the storage in the combi will stop the constant start/stop of our existing setup, is that true? Any thoughts and advice welcome.

Needing to focus on my next project after my UFH fiasco and being in a holiday rental place where the DHW is very hit and miss reminds me that I need to get some specs laid down before I start ordering stuff! Having read so much over the past couple of years, I feel a lot more informed but also a bit bamboozled at the same time. UVC or TS, buffer tanks, what size..... So my main questions (so far): 1) what type of gas boiler to go for? 2) Thermal store or UVC? 3) Buffer tank needed? My setup will be: 4kw solar PV Mains gas 2 UFH manifolds (upstairs and groundfloor - so I can choose the flow temps accordingly to get the maximum comfort levels) Room stats in all areas 2 Bathrooms (2 showers, 2 baths) MHVR Although not going to be a passivhaus, I’m hopeful it'll be a bloody whole let better than your standard newbuild: 2 SIP extensions 300mm EPS floor insulation 3G windows/doors 100mm EWI on existing house DHW requirement: 2 adults and two soon to be teenagers (so they will hardly wash reducing the DHW figures a bit) 2 morning showers 2 evening baths Hot water for washing up (though may get a boiler-type tap for most hot water needs in the kitchen) Occupancy: Wife and kids out and I although I work from home, my office will be one of the sheds out the back so main house can either be set to off or very low temps during the day. UFH demands: Groundfloor: Water volume: 129L; Estimated loading: 9.45Kw Upstairs: Water volume: 25L; Estimated loading: 2.03Kw BOILER QUESTION: Before I start getting plumbers out to fit a boiler system, I’d like to get people’s advice on whether to go for system or combi. Combi = don’t need a water storage cylinder = no need for cistern in loft = combi boilers don’t supply enough hot water = better to use a separate cylinder for the hot water supply. = combi boilers can be quite noisy. System = Store hot water in a cylinder, so they can feed several outlets at once at mains pressure. = no need for a cistern in the loft and the expansion vessel is built in. = Flow rates will be high as water is delivered at mains pressure, and hot water is instantaneous. I'm thinking to go for a system boiler at this stage (easier/quicker install) and then to use excess PV with the cylinder. THERMAL STORE OR UVC? My thoughts so far are to have a 300L UVC fed partly by the solar PV during the day (using an Immersun type of device) with maybe E10/E7 at night and then also the gas boiler. I would end up with two circuits – one for DHW and one for UFH. The boiler would kick in for the UFH when the UVC doesn’t require heat. BUFFER TANK? I'm also thinking that if I go for a UVC rather than thermal store (from what I have read on here) I would need a buffer store seeing I will have UFH. But I've read that if I opt for a TS, that can be my buffer tank also. So perhaps I should go for a TS rather than a UVC? Confused.... Hoping someone (AKA: @Nickfromwales etc) is able to assist..... Thanks, OB

Excerpt from a previous thread. A member asked..... "Starting our extension and renovation fairly soon so I am now looking at the type of heating system to go for. The house is roughly 100m² footprint and 450m³ volume. Insulation levels are expected to be better than building reg standard but not upto PH standard. Plan for heating and DHW in the house is as follows: UFH throughout with no rads, two towels warmers to be used though. One bath, two showers and four sinks (not decided whether both showers will be DHW or one electric yet but say both DHW for this example). Average usage of one shower and one bath per day From jsharris spreadsheet I figure that my worst month at minimum OAT heat input would be 1398kWh based on daily heat loss power of 1878W. Ideally we would like the boiler hidden from view and this could be house in either the garage or loft as both have ample space. We have both gas and electricity to the house so we are open to a wide range of possiblilities. Anyone have any thoughts on which type of boilers I should be considering? As far as manufacturers go I am willing to invest a little more for better quality (brands I have investigated so far are Valliant, Baxi and Ideal).