Combi boiler suggestions?

FlaatusGoat

My 17 YO Potterton seems to be dying. The diverter valve has gone funny, requires seals etc etc. Clearly end of life. Plus now I'm getting E20 when trying to run a shower / cold water. Forget that. Time to replace it.

Are there any decent makes/models that are reasonably priced? Or, is having to spend £1000+ a given unless I want to cohabit with a heating engineer.

I only need to run 5 radiators (on a very bad day) and my house is very small with one family bathroom. Microbore pipe work. The Ideal Exclusive 24kw looks about the cheapest option £700 inc flue or should I push up towards a Worcester, or Baxi.

Any general advice would be great.

jefaz07

Main Eco Compact is a decent budget boiler. Basically the same as the more expensive offerings just rebadged. Around £750 with flue. 5 year warranty. 

Paulrn

Lo/ok at ATAG ours is very efficient and has a 12 year+ warrenty 

FreeBear

FlaatusGoat said: I only need to run 5 radiators (on a very bad day) and my house is very small with one family bathroom. Microbore pipe work. The Ideal Exclusive 24kw looks about the cheapest option £700 inc flue or should I push up towards a Worcester, or Baxi.

With a combi boiler, there are two numbers you need before selecting a make/model to fit.

Water flow rate - This will determine the kW rating of the boiler.

Heat loading of the radiators (the power output) - Measure the size of each radiator and use this site to get an idea of the power output -> https://www.simplifydiy.com/plumbing-and-heating/radiators/power

Now look at the minimum output of the boiler. If it is higher than your radiator power, then the boiler will be short cycling for most of its life (bad for efficiency, and bad for reliability). Ideally, you want a boiler that can modulate down to a very low level. 3kW is excellent, 5kW is not so good, 7kW or higher is poor.

The next stage is to measure the room sizes along with windows, and crunch the numbers here -> https://www.stelrad.com/basic-heat-loss-calculator/ - This will tell you if the radiators are big enough to keep the place warm. The default Delta T (ΔT) is 50°C for a flow temperature of 70°C. Ideally, you want to run the calculations with a ΔT of 40°C or even 30°C so that you can run at a lower flow temperature.

A lower flow temperature means that the boiler will be pushed further in to condensing mode which means higher efficiency and lower running costs.

It is a shame that you have microbore plumbing - If you had big bore pipes (22mm feed/return & 15mm to the radiators) I would have suggested look at a heat pump. With the £7500 government grant, you might have been able to get an ASHP from someone like Octopus for under £2K. Depending on the layout of the property, you might want to consider replumbing and get a quote for a heat pump. The next time the boiler is up for replacement, you may not have that choice, so better to bite the bullet now...?

FlaatusGoat

FreeBear said:

FlaatusGoat said: I only need to run 5 radiators (on a very bad day) and my house is very small with one family bathroom. Microbore pipe work. The Ideal Exclusive 24kw looks about the cheapest option £700 inc flue or should I push up towards a Worcester, or Baxi.

With a combi boiler, there are two numbers you need before selecting a make/model to fit.

Water flow rate - This will determine the kW rating of the boiler.

Heat loading of the radiators (the power output) - Measure the size of each radiator and use this site to get an idea of the power output -> https://www.simplifydiy.com/plumbing-and-heating/radiators/power

Now look at the minimum output of the boiler. If it is higher than your radiator power, then the boiler will be short cycling for most of its life (bad for efficiency, and bad for reliability). Ideally, you want a boiler that can modulate down to a very low level. 3kW is excellent, 5kW is not so good, 7kW or higher is poor.

The next stage is to measure the room sizes along with windows, and crunch the numbers here -> https://www.stelrad.com/basic-heat-loss-calculator/ - This will tell you if the radiators are big enough to keep the place warm. The default Delta T (ΔT) is 50°C for a flow temperature of 70°C. Ideally, you want to run the calculations with a ΔT of 40°C or even 30°C so that you can run at a lower flow temperature.

A lower flow temperature means that the boiler will be pushed further in to condensing mode which means higher efficiency and lower running costs.

It is a shame that you have microbore plumbing - If you had big bore pipes (22mm feed/return & 15mm to the radiators) I would have suggested look at a heat pump. With the £7500 government grant, you might have been able to get an ASHP from someone like Octopus for under £2K. Depending on the layout of the property, you might want to consider replumbing and get a quote for a heat pump. The next time the boiler is up for replacement, you may not have that choice, so better to bite the bullet now...?

Goldmine.

What I'd never considered is minimum flow rate, often I run only one radiator to keep costs down and if it's causing what you've described (short cycling) then that can't be a good thing.

I've measured the radiators but the calculator thingy you sent wasn't working plus I've got a towel heater in the WC, I believe that's about 8500 BTU.

Based on your suggestions, any thoughts on the Baxi Main Eco Compact 25? I was able to gleen the below from https://mediacdn.mainheating.co.uk/-/media/websites/mainheating/files/main-eco-compact-combi/main-eco-compact-combi-user-guide.pdf?v=1&d=20240213T150909Z

I'm not sure if they refer to 2KW anywhere in the document in terms of modulation.

Maximum pressure bar 3 3
Minimum pressure bar 0.5 0.5
Central heating temperature adjustment °C 25/80 ± 5° 25/80 ± 5°
Tab.4 Domestic hot water circuit specifications
25 30
Maximum pressure bar 8 8
Dynamic minimum pressure bar 0.5 0.5
Minimum working water flow rate l/min 2.0 2.0
Domestic hot water temperature range adjust
ment
°C 35/60 ± 5° 35/60 ± 5°
Specific flow rate (D) l/min 10 12.5

FlaatusGoat

I called Baxi and their boilers modulate down to 6 KW - so like you say not so good! I'll try Worcester.

FreeBear

FlaatusGoat said: I've measured the radiators but the calculator thingy you sent wasn't working plus I've got a towel heater in the WC, I believe that's about 8500 BTU.

Based on your suggestions, any thoughts on the Baxi Main Eco Compact 25? I was able to gleen the below from https://mediacdn.mainheating.co.uk/-/media/websites/mainheating/files/main-eco-compact-combi/main-eco-compact-combi-user-guide.pdf?v=1&d=20240213T150909Z

When looking for specs for a boiler, you should read the installation instructions rather than the user manual - Technical information is usually more comprehensive in the former. From https://mediacdn.mainheating.co.uk/-/media/websites/mainheating/files/main-eco-compact-combi/main-eco-compact-combi-installation-guide.pdf we can see the 25kW and 30kW models have identical central heating performance:

Nominal heat output 80/60°C — Maximum rate 20kW
Nominal heat output 80/60°C — Minimum rate 6.1kW

Nominal heat output 50/30°C — Maximum rate 21.2kW
Nominal heat output 50/30°C — Minimum rate  6.7kW

Useful efficiency at rated heat output and high temperature regime 88.0%

Useful efficiency at 30% of rated heat output and low temperature regime 97.8%

As you can see from the above figures, highest efficiency is obtained when running at a low flow/return temperature of 50/30°C. But to get the best from low flow temps, you need bigger radiators. And if your towel heater really is 8500 BTU (~2.5kW), that is a pretty impressive beast - That 8500 BTU will probably be at a ΔT50°C (or a 70°C flow temperature), and lower at a ΔT30°C (50°C flow).

There are other boilers on the market with greater modulation ranges, and hence lower minimum heat outputs. But this comes at a cost. I have a Viessmann 050 which will go down to 3.2kW, but is twice the price of a Baxi. But I also get a 10 year warranty. Some of the Worcester Bosch boilers will also go down to ~3kW, but you often have to dig through the technical specs to find that information.

Unfortunately, I replaced my boiler last year. If I were to do it now, I think I'd have gone for an ASHP - Cost me £2500 for the Viessmann, and Octopus were quoting £2700 earlier in the year (subject to survey). Scope to get that figure down as I already have suitable pipework, and radiators have been oversized.