New boiler + tank recommendations please

TBH I thought there would be millions of recent/up to date threads on this…but either there aren’t, or I can’t find them.

Need a new CH boiler+HW tank. Will be system/unvented. (Partly as I do want to transition to some PV and then maybe ground source….but not just yet, also because good showers).

Anyway, local plumber likes Intergas. They do seem OK but I’m not really aware of them. I’ve had a WB and Vaillant in the past which would be fine by me, but this is 20+ and 10+ years ago and things can change of course.

Only just starting the research…but for reasons need to make a decision quite quickly. Initial thoughts are:

I know that I want a modulating boiler – this does seem to count against WB somewhat as their control interface is not well supported it seems?

Current boiler is 28kW, and although the house is being extended by ca. +50% it is also being insulated to the gills, so deffo don’t need more than 30kW (say, and online calculators confirm this, in fact they say around 22kW demand), could we go lower? Probably doesn’t really matter if the boiler is modulating, but I could easily be wrong there of course.

What else do I need to know about? e.g. I’ve seen OSO cylinders mentioned here…given the cylinder will be in the house, do I need to worry that much about heat loss? I guess it would probably make a saving for the 6-7 months a year when the heating’s off.

TIA.

We have recently had ours changed. Had a 30kw Baxi system boiler & a Main cylinder, with a Hive control.

I did here the plumber speaking about Intergas, isnt that the ones with very few moving parts?

It replaced a vented system with a 20 year old Gloworm which was probably slightly underspec’d for the task.

4 bed house with 13 rads. The cylinder apparently keeps the water up to temp for approx 24 hours, but i doubt that very much. has made a big difference to the rads though, and its quiet!

I’d bet you will be fine with a 16kW boiler. Most gas boilers are oversized, as, basically, Plumbers dont work out the correct size needed. You need to calculate your heat loss (not as difficult as it seems), then size from that. Would you really need 28kW of heat? Very unlikely unless you have a terribly uninsulated house. And, that power is only any good at the worst outside temperature, which is typically designed around -3 degree C. Days when it is above 5 degrees will have your boiler working at 3/4 power, and at 9 degrees it needs half the power to keep the house warm, thats why modulation, and a good system design is imperative to make the boiler efficient.Over sizing a boiler gives poor performance as start up losses can add up, modulation will help, but sizing it correctly will be the first thing to get an efficient system.
Start with a heat loss survey.

If you are considering ground source in future then get a heat pump ready cylinder

We have an Intergas HRE 18 SB installed a few years ago, and it’s been great. We have a 1950s 4-bed semi in SE England with 11 rads, and even though it’s the smallest in the range I would say it’s slightly overpowered. It can modulate between 6kW-18kW, and cycles on/off every 8 minutes below that. Other than when it’s freezing outside it spends most of its time cycling.
With OpenTherm it works well with our Honeywell EvoHome setup, which we’ve upgraded over the years from single room to multiroom, and recently converted to Priority DHW for further efficiency. It also plays well with the Solar iBoost that we use to divert excess solar power to the immersion element in the tank, not that that’s at all complicated.
They are very robust boilers – if system pressure falls to zero they will still work (tested several times!), and having fairly large bore pipes in the heat exchanger means they are not liable to getting blocked.