Condensing boiler with rads possible? (no UFH)

[tellme]

Hi,

I have an 20yrs heat only potterton 50e boiler with 196x old house - suspended floor - designer radiators, open vent system and hot water cylinder on first floor with two motorised valves and stat.

I want to replace the boiler with minimal cost and max efficiency so thinking of replacing the old boiler with a new condensing boiler.

Currently, the old boiler is set to 70C supply temp to ensure the cylinder gets to 60C to avoid legionella. Also the designer radiators - Milano Aruba - have low BTU output - so the 70C helps getting enough heat out of it.

In order to save money I only heat up the hot water cyl for 30 min before shower.

Question: Can I get a new boiler and new rads that does both 1) provide 70C to the hot water cyl during for 30mins and 2) Operate central heating at low condensing temp?

My solution would be 1) buy a heat only boiler that can normally operates at 50C to feed the rads and changes output temp 70C upon an dry contact signal trigger by cylinder hot water demand and 2) replace designer rads with triple panel radiator and operate them at low temperature.

Finally I don't know whether triple panel radiators can really enable low temp systems, but only underfloor heating can? UFH will cost a fortune I suppose.

 

[fowlerboi]

Below is a sizing chart based on a heat loss calc (its good enough as a starting point but different brands will vary.) Probably for you, you could look what sizes you have on a delta T of 50, check the kw for it and then find a comparable size in the delta T 30. Ufh runs at a delta T of 30 as well

Weather compensation might be your best bet and keep the rads the same but I’m no expert in how that works with heating the cylinder to 60. I know heat pumps have special controllers that use the immersion once a week to heat the cylinder to 60 as a legionella cycle. I expect your immersion won’t be able to do the whole tank though

https://www.stelrad.com/wp-content/uploads/2014/08/Stelrad-Compact-Horizontal-Product-and-Technical-Information.pdf