A bit of advice regarding my central heating, please.
Having almost completed work on my bungalow extension it's time to specify the heating requirements and choose radiators. Back in the day when I had a proper job I learned how to do heat transfer calcs so I`ve a good understanding, I`ve been looking at radiator specs and there are some useful calculators on the internet that take all factors of the room/house into account. However, the specs for radiators quote output ratings for different delta T values, typically T30, 50 and 65. Here's where I`m struggling, I don`t know what delta T value to use when sizing the rads. The current set-up has worked very well despite the cold weather recently, so I could base calcs on existing rooms and the radiators fitted, but I`m worried about ending up with insufficient heating capacity. I could leave it up to the builder/plumber, I'll certainly listen to their advice, but I prefer to run the numbers myself too.
I currently have a combi boiler rated at 28 watts, it's getting old and we'll replace it shortly when the rest of the house gets sorted out. Using the best data I can find I reckon the current heating system (prior to changes) constitutes a load of 15.9kw based on delta T 65°, so on that basis the boiler is significantly oversized. The extension adds 28% extra floor and roof area so the increase in load will scale up to 20.5kw which should be within the capacity of the boiler. When quoting specs for boilers, are the figures based on heat output from the boiler or are they gross figures that don't account for boiler efficiency losses? I`m guessing a modern boiler is close to 90% efficient so it doesn`t make a huge difference.
Choosing a new boiler is some way in the future, but I`m tempted to go for bigger capacity to allow for filling the bath reasonably quickly. This is my first experience with a combi boiler and so far I'm reasonably impressed, but we do like nice hot baths and this is the aspect that the current boiler struggles with compared to the hot water tank we had in the old house. It's tempting to take the 'bigger is better' route and over-specify the boiler to provide the optimum instant hot water and keep Mrs Walker happy, other than extra cost are there any drawbacks to consider?
I`m also considering siting the boiler in the loft to free up space, are there any pitfalls to consider? Loft access will need improving but I plan to do that regardless of the boiler .
All advice welcome, I'm keen to get this right!
I am no expert but I always promote my mate's company when I see heating threads. He runs this place:
https://www.discreteheat.com/
I had it in my old house and when I renovate I'll be putting it in here. I'm no evangelist but I love the no visible radiator look and flexibility. They do all the calcs for you too.
Anyway, an option to consider before you go classic rads just because that's what you've always done.
Can't help on the math, but putting the boiling in the loft doesn't sound like a good idea. A leak later on (if it happens or) means water running down through the floors.
Mine is in the basement next to the furnace which makes it easier to hook up the gas lines
If your getting a new system. Wouldn't the company doing the install know how to calculate the specs for your needs?
Get est.'s from 3 companies selling the same system and see what they recommend
DON
The first page I Googled says: "Delta 50 is the UK standard for all domestic gas boilers. If you’re looking into newer, more renewable heating systems, you can also purchase radiators with a lower level output. Delta 30 and Delta 40 work well for lower water temperature systems."
If you're going for a conventional boiler / radiator system, and your bungalow isn't insulated to the latest standards, I'd go with Delta 50.
Combi boilers are sized for the HW load required & this almost always mean they end up oversized for the CH system - it takes a lot of power to instantaneously heat a bathful of water. Increasing the boiler size means you can heat a greater HW flow rate, which is good, but there are considerations when it comes to the CH side.
If the boiler can't modulate down it's output very far then there is a risk of short cycling if the radiators can't get rid of the heat. The boiler heats the water, it runs through the rads but they can't dump the heat fast enough so the return water temperature rises & the boiler shuts off before the rooms are warm. You either need to pick your boiler carefully so that it can modulate down or alternatively it's common to oversize the radiators so that they can output as much heat as possible. Unless the return water temperature is down below around 55 degress the boiler won't be in condensing mode & with a outgoing water temperature around 70 degrees you need about a 15 degree drop across the radiators. In the old days the drop was supposed to be 11 degrees but my reading indicates that 20 degrees is more common these days.
I've struggled to find a good online resource for combi system design but if I find one I'll post it.
The loft location is ok but make sure there is flooring around it so the plumber doesn't go though the ceiling when he services the boiler. You'll need to get a 22mm gas pipe up there & run the condensate drain away somewhere - if you have a soil stack in the loft it's a good place to run it to.
Thanks for all the comments, especially the one from Pointy which has reminded me how my knowledge of combi boilers is limited, confirms the old adage that you don’t know what you don’t know! By coincidence the plumber called today and we had a chat, he’s a young guy but he seems to know his stuff.
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