Boiler help!

Posts: 34
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I need a boiler that can accommodate about 7 to 8 radiators and underfloor heating. It's a terraced 3 bed house with 2 bathrooms.
I am only planning to get underfloor heating in the hallway and kitchen. Rest will be carpeted so will have radiators?
What Boiler is the best and how many kilowatts would I need? Different ppl are telling me different things and now I am confused.

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• Posts: 50
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Viessmann 200 is good with underfloor heating, it can modulate very low and tick over efficiently at low flow temperatures
• Posts: 14,257
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Are you wanting a combi boiler or system/heat only ?

For a combi, you need to measure the water flow from a tap fully open (the bath is probably best). With that number to hand, look at the specs for a selection of boilers and find one that matches the flow rate. If you have say 10l/min at the tap, a combie around 28KW will probably do. The final selection should be based on the minimum central heating output (i.e. what it will modulate down to).
I went through the same exercise last year (10l/min flow, 9 radiators), and settled on a Viessmann 050 30KW. The great thing with that particular model is the modulation range - It can go as low as 3.2KW, and once there, is pretty darned efficient (probably around 98%). Last few days, heating has been kicking in at minimum level for around 30 minutes at a time.

With a system boiler, you need to measure up the size of the radiators & UFH and calculate how much heat is required. Chances are, you will only need 8-10KW. You also need to consider how fast you want to heat a tank of water, so you may find 14-18KW gives a reasonable time - What you won't need is a 30KW boiler which some installers recommend (just waaaay to big unless you are running a small hotel). Again, look at the minimum heat output.

Whilst on the subject of radiators, run the room sizes through this calculator - https://www.stelrad.com/basic-heat-loss-calculator/
Consider replacing some of your old ones with new, but oversized by 30-50% - This will enable you to run at a lower flow temperature (which you will need for UFH) and maximise efficiency from the boiler. It also puts you in a better position for the day when heat pumps are all that are available.

Her courage will change the world.

Treasure the moments that you have. Savour them for as long as you can for they will never come back again.
• Posts: 21,146
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I have a system boiler ( so not a combi) 4 bed, 3 bathroom semi. 13 radiators of various sizes, quite large hot water tank.  The boiler is 18 KW and copes fine. So probably max 15Kw for you unless it is a combi.
• Posts: 24,331
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2 bathrooms, so two people trying to shower at the same time sometimes?

No reliance should be placed on the above! Absolutely none, do you hear?
• Posts: 4,458
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ALacoste said:
I am only planning to get underfloor heating in the hallway and kitchen. Rest will be carpeted so will have radiators?
No UFH in the sitting room as well? Is it not on the same floor level as the kitchen?
From what I understand, you can have carpet over UFH, tho' you may need to take into account its 'tog' value. So don't rule it out - UFH is lovely 🤩
For all other rooms, rads will be much cheaper to install, but do take good technical advice on over-sizing them, so that they, too, can run on cooler water from the boiler.
Cooler flow = cooler return = more e££iciency.
• Posts: 34
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Thank you all for your responses. I'm getting various different answers and was told I need a 35kw now.

Assume its good idea to have UFH as well as radiators?

Just thought because of the budget would leave the UFH out from the downstairs living and fron room as will opt for an carpet. Might actually think about having it on all ground floor then?

Yes kids might shower at the same time and would like to do the loft up in a couple of years too. And since we are replacing the old boiler we thought we would get a powerful decent one.
• Posts: 14,257
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ALacoste said: I'm getting various different answers and was told I need a 35kw now.
Combi or system (do you have a hot water tank) ?

Very, very unlikely that you really need a 35KW boiler - Bigger boilers cost more, and often won't modulate down low enough for the central heating system. So you end up burning more gas and suffer from short cycling.

Her courage will change the world.

Treasure the moments that you have. Savour them for as long as you can for they will never come back again.
• Posts: 21,146
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You need to supply  the critical info of whether it will be a combi ( instant hot water) or one with a hot water tank ?
• Posts: 14,257
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You need to supply  the critical info of whether it will be a combi ( instant hot water) or one with a hot water tank ?
Also maximum water flow if combi.
Her courage will change the world.

Treasure the moments that you have. Savour them for as long as you can for they will never come back again.
• Posts: 34
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Would like combi so hot water on demand especially with kids. We don't have a tank.

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