BTW - Existing vintage boiler is 37kW
37kW in, 30 kW out to radiators (and 7kW up the chimney) if I’ve read that correctly.
BTW - Existing vintage boiler is 37kW
You might want to consider a heat only boiler, rather than a system boiler. Means the pump and valves are external to the boiler, allowing you / your installer to choose them, and making you less dependent on manufacturer's prices. Can still be a sealed system with modern boilers.
Heat output.I do not know if kW is usually quoted as heat input or heat output.
Heat output.
You don't say whether your 3-port valve is either/or (Honeywell W-plan) or mid-position (Y-plan). If it's W-plan the cylinder stat must be satisfied before CH is available. But that's not usually a problem. Cylinder heat loss is small if it's well insulated (preferably pre-insulated with foam) the heat lost goes into the house anyway. If it's Y-plan you can have HW or CH or both.
Speaking as a non-professional, non-qualified, slightly-above-average* DIYer (i.e. take the following with a pinch of salt!) I'd give you one piece of advice I wish I'd understood more before having our system replaced...
Do a bit of proper research on how to calculate the correct sizing of radiators - check one of my previous threads https://www.diynot.com/diy/threads/oversizing-radiators-how-much.530890/ for info. My experience is that not all boiler installers are the same as proper heating engineers, and are great at swapping boilers and components, but not always so great at the overall system design. If you like and trust the installer you've got but aren't convinced on this side of things, do your own research and work with them to design the system together?
* based on my own ratings, of course.
It's an even higher output boiler than we thought! Do you know what setting it's currently on? I assume it doesn't have 3 switchable settings, but varied continuously by adjusting the burner pressure at the gas valve, but you or somebody else might tell me I'm wrong. If you don't know the setting it can be checked by measuring the gas pressure with a manometer. But if it's a very old boiler it could be scaled up, with efficiency below nameplate data.Those are minimum values. See table attached.
If the timer works OK I doubt it is causing poor economy.3 port valve is Y Plan. Programmable timer is very vintage so maybe this is the problem? Going to be replaced with a Nest.
The table you show is for a Glow-worm Ultimate 120FF.Those are minimum values. See table attached.
If you need to find a tradesperson to get your job done, please try our local search below, or if you are doing it yourself you can find suppliers local to you.
Select the supplier or trade you require, enter your location to begin your search.
Are you a trade or supplier? You can create your listing free at DIYnot Local