25kW or 30kW combi boiler?

FataVerde

Bendy_House said:

No, it can't be 4 lpm! Or, it can, but it won't be

Water boards (most, at least) are obliged (or were) to provide a min 9lpm at the ground level. 9lpm is utter pants, and completely unsuitable for a combi.

Not sure what your friend did - perhaps used the hot? Or didn't turn the tap on full? It has to be the cold kitchen tap, as that's supplied direct from the incoming mains.

Yes, they redid with cold water and filled 1L in about 5 second so I guess I'm looking at about 12L/min so I guess a 12.3 or 13 DHW flow rate would be fine. Or would I need a higher cold water flow rate? I can tell just by usage that my water flow is low. The pressure definitely is. From my research, you have to go much higher than 30 in a combi to 35 or 38 to get a 20 DHW flow rate

FataVerde

Bendy_House said:

FataVerde said:

From what I see, all boilers are now required to have weather and prog stats. Can't remember if it was EU or UK legislation that got this in motion.

I didn't know that, but I'm not up to date on a lot of stuff, tbh.

Cool - that would be good news. Weather comp strikes me as being a 'must', especially if the rads haven't been up-sized. (If the rads are 'over-sized', they'll be designed to provide enough heating at a fully-condensing temp, so the boiler can - theoretically - be 'set and forget'.)

My 14-year old condensing combi doesn't have this feature, so I manually adjust the rad output temp now and then as the outside temp changes enough, but I clearly cannot do this often enough for the boiler to be running at its optimum efficiency - life's too short for that malarkey. Altho' still relatively mild, our heating has just started to come on now and then for brief periods, and quickly gets the room up to temp even tho' the flow temp is lowish. As it cools further, tho', it'll struggle to do this at the current CH flow temp, so I'll need to tweak it upwards = less efficient. I suspect many folk with boilers that do not have weather comp will just have their boiler CH output set at 'hot' all the year round.

Couldn't you get a smart thermostat? My ideal boiler was/is 13-years old but had a smart thermostat connected. A nest.

But looking at the article below, I realised basically companies just install a thermostat, cheapest way to comply with the legislation. The boilers I looked at don't have weather compensation actually.

https://heatable.co.uk/boiler-advice/boiler-plus-what-it-is-how-it-works-and-when-it-s-happening

"Boiler Plus dictates that the installation must be one of a combi boiler with an efficiency rating of at least 92% and include at least one of the below features & functions. What's installed to meet Boiler Plus minimum standards is generally at the installers discretion, but you can always request an alternative feature or more than one, but this will likely incur additional costs.

Feature	Put Simply (in Heatable terms)
Flue Gas Heat Recovery System	Just as interesting as it sounds. A device capturing heat that would otherwise be wasted and using it to heat your system again, clever.
Weather Compensation	An outdoor thermometer communicating the outside temperature to your boiler, so it can adjust accordingly and be more efficient.
Load Compensation	Similar to weather compensation but measuring the ambient temperature inside your house and altering the amount of hot water that's delivered to your radiators.
Smart Thermostats/Heating Controls	One of those fancy devices that allows

Bendy_House

FataVerde said:

Bendy_House said:

FataVerde said:

From what I see, all boilers are now required to have weather and prog stats. Can't remember if it was EU or UK legislation that got this in motion.

I didn't know that, but I'm not up to date on a lot of stuff, tbh.

Cool - that would be good news. Weather comp strikes me as being a 'must', especially if the rads haven't been up-sized. (If the rads are 'over-sized', they'll be designed to provide enough heating at a fully-condensing temp, so the boiler can - theoretically - be 'set and forget'.)

My 14-year old condensing combi doesn't have this feature, so I manually adjust the rad output temp now and then as the outside temp changes enough, but I clearly cannot do this often enough for the boiler to be running at its optimum efficiency - life's too short for that malarkey. Altho' still relatively mild, our heating has just started to come on now and then for brief periods, and quickly gets the room up to temp even tho' the flow temp is lowish. As it cools further, tho', it'll struggle to do this at the current CH flow temp, so I'll need to tweak it upwards = less efficient. I suspect many folk with boilers that do not have weather comp will just have their boiler CH output set at 'hot' all the year round.

Couldn't you get a smart thermostat? My ideal boiler was/is 13-years old but had a smart thermostat connected. A nest.

But looking at the article below, I realised basically companies just install a thermostat, cheapest way to comply with the legislation. The boilers I looked at don't have weather compensation actually.

https://heatable.co.uk/boiler-advice/boiler-plus-what-it-is-how-it-works-and-when-it-s-happening

"Boiler Plus dictates that the installation must be one of a combi boiler with an efficiency rating of at least 92% and include at least one of the below features & functions. What's installed to meet Boiler Plus minimum standards is generally at the installers discretion, but you can always request an alternative feature or more than one, but this will likely incur additional costs.

Feature	Put Simply (in Heatable terms)
Flue Gas Heat Recovery System	Just as interesting as it sounds. A device capturing heat that would otherwise be wasted and using it to heat your system again, clever.
Weather Compensation	An outdoor thermometer communicating the outside temperature to your boiler, so it can adjust accordingly and be more efficient.
Load Compensation	Similar to weather compensation but measuring the ambient temperature inside your house and altering the amount of hot water that's delivered to your radiators.
Smart Thermostats/Heating Controls	One of those fancy devices that allows

I do - it's a Hive. But it cannot control the boiler's output temp - that's purely manual.

Bendy_House

FataVerde said:

Bendy_House said:

No, it can't be 4 lpm! Or, it can, but it won't be

Water boards (most, at least) are obliged (or were) to provide a min 9lpm at the ground level. 9lpm is utter pants, and completely unsuitable for a combi.

Not sure what your friend did - perhaps used the hot? Or didn't turn the tap on full? It has to be the cold kitchen tap, as that's supplied direct from the incoming mains.

Yes, they redid with cold water and filled 1L in about 5 second so I guess I'm looking at about 12L/min so I guess a 12.3 or 13 DHW flow rate would be fine. Or would I need a higher cold water flow rate? I can tell just by usage that my water flow is low. The pressure definitely is. From my research, you have to go much higher than 30 in a combi to 35 or 38 to get a 20 DHW flow rate

Around 12lpm, and with a weak pressure? Sorry, I don't think a combi is for you :-(. 

You could fit a Salamander to help with the pressure, and it would also guarantee a 12lpm min, but still pants.

There are other solutions, but they need storage space.

If that's really your rate, and with no obvious way to improve it, then no point going more than 25kW as far as I can see.

But hopefully plumbers on here will advise more.

FataVerde

I talked to a local plumber, who lives across the street from me and he said the pressure and flow rate in the area is bad. He also said because I am in a conversion, I actually share my mains with downstairs, which obviously halves my flow rate because ground floor has priority. He also explained the diff in the vaillant ranges and said the Plus is a more expensive but better combi than Pro. He advised on getting the 32kW with the idea of later getting my own mains when extending into the loft. Reading the online specs, the 32kW has modulation at 1:6 and can get down to 5.5 (min CH output), but I will see when the quote comes.

FataVerde

Btw, is there a difference between minimum CH output condensing and noncondensing? They give different figures for the boilers I'm interested in. The 30kW has 8.5 condensing but 6.2 non-condensing.

Bendy_House

FataVerde said:

I talked to a local plumber, who lives across the street from me and he said the pressure and flow rate in the area is bad. He also said because I am in a conversion, I actually share my mains with downstairs, which obviously halves my flow rate because ground floor has priority. He also explained the diff in the vaillant ranges and said the Plus is a more expensive but better combi than Pro. He advised on getting the 32kW with the idea of later getting my own mains when extending into the loft. Reading the online specs, the 32kW has modulation at 1:6 and can get down to 5.5 (min CH output), but I will see when the quote comes.

I would have to be confident that my flow and pressure will be seriously improved before entertaining a larger combi. And, when you extend into the loft, anticipate a further ~0.2bar reduction in pressure from what you currently have due to the extra height above ground level. This will equate to a lower flow.

But, if you are planning a loft conversion, then you will most likely want a better flow too, so that's what you should be planning for.

My previous suggestion of a Salamander would be problematical if you literally share the same rising mains pipe with the flat below, 'cos you'd be sucking away their water, and possibly even introducing air into your pipes if they open a tap at the same time!

So, Qs I'd be asking include - if you fit your own rising mains pipe, what P and F will you have? And, do you have room for a booster tank - the size of a hot cylinder - if necessary?

Leon_W

How about a pumped accumulator type tank ?  

https://www.salamanderpumps.co.uk/range/accuboost/28

Bendy_House

Leon_W said:

How about a pumped accumulator type tank ?  

https://www.salamanderpumps.co.uk/range/accuboost/28

Yes, one of these - or by Challis or Grundfos - is the ultimate solution. But best if you can successfully upgrade your incoming.

jj_43

24kw is fine, 30kw is overspending. Your in a flat, the heating requirement is low, and a 30kw would cause your boiler to cycle on and off all the time increasing costs. Hot water requirement can be meet. Pressure issues not sure. Don’t get Hive it does not do load compensation.