Boiler Sizing

[DieselPig]

Afternoon All

My first post!

Have a two bedroomed bungalow with a 15 year old BAXI Bermuda suppling heat and hot water from a cylinder. 6 radiators three of which are small, hallway and towel dryer in bathroom and a Myson kickspace heater in kitchen.

Looking to replace with a combi for loft installation, I have an electric shower I wish to remove and use the combi to supply with a mixer or some temperature controlled feed.

I have read many posts on CH requirements and 24Kw looks sufficient as the building is lagged with everthing possible!

My lack of understanding is what flowrate/power input is required for a good shower.

Have also pinpointed either ATAG or Viessman as the boiler to use, the BAXI has run for 15 years without failure just a yearly clean!

Many thanks for your advice please.

 

[bob the plumber]

I would imagine, well i know that 24kw boiler will easily supply your heating set up (6rads etc..) with a combi its all about the hot water performance, the higher the kw the higher your ltrs per minute your boiler will produce. At 24kw your probably looking at 10-11 ltrs a minute if that is good enough for your requirements then go for it. When i put them in I usually use combi's with around 30kw as they produce a flow rate of around 14-15 ltrs a minute which is alot better

 

[Drew Greenfield]

I agree, 24kw unless you want massive amounts of hot water.

 

[Billy Bob Bob]

i agree with the above posts stick with baxi look at the duotec

 

[stevetheplumber]

a 24kw will give you a great shower but will be slow filling a bathpersonally i try not to fit less than 28kw

 

[doitmyself]

I would imagine, well i know that 24kw boiler will easily supply your heating set up (6rads etc..) with a combi its all about the hot water performance, the higher the kw the higher your ltrs per minute your boiler will produce. At 24kw your probably looking at 10-11 ltrs a minute if that is good enough for your requirements then go for it. When i put them in I usually use combi's with around 30kw as they produce a flow rate of around 14-15 ltrs a minute which is alot better

The flow rate of the hot water leaving a combi boiler can never exceed the flow rate of the cold water entering the boiler. So installing a larger combi just means that the water will get hotter, not faster.

Combi performance is based on the temperature rise, which is assumed to be 35°C. So, if the water entering the boiler is 5°C, it will leave at 40°C, which is about the recommended temperature for a bath or shower. There is no need to heat the water up to 60°C as it is not being stored, so the problems with legionella bacteria do not occur.

The OP needs to measure his incoming flow rate at the kitchen tap and size the boiler based on that. 2.5kW per litre per minute will give a 35°C temperature rise.

You also need to consider the heating requirement, even though it will be much less. You can estimate this by using the Boiler Size Calculator and deducting 2kW from the result. It should be within the CH modulation range of the boiler and the lower the boiler modulates, compared with the heating requirement, the better.

 

[GQuigley67]

if your flow rate is good enough for a decent boiler, look at the vaillant range and you can have a 30kw one with a decent boiler and range rate the heating down to what ever kw you need, this can be done easily on these boilers by following the instructions on the MI's

 

[bob the plumber]

The flow rate of the hot water leaving a combi boiler can never exceed the flow rate of the cold water entering the boiler. So installing a larger combi just means that the water will get hotter, not faster.

Combi performance is based on the temperature rise, which is assumed to be 35°C. So, if the water entering the boiler is 5°C, it will leave at 40°C, which is about the recommended temperature for a bath or shower. There is no need to heat the water up to 60°C as it is not being stored, so the problems with legionella bacteria do not occur.

The OP needs to measure his incoming flow rate at the kitchen tap and size the boiler based on that. 2.5kW per litre per minute will give a 35°C temperature rise.

You also need to consider the heating requirement, even though it will be much less. You can estimate this by using the Boiler Size Calculator and deducting 2kW from the result. It should be within the CH modulation range of the boiler and the lower the boiler modulates, compared with the heating requirement, the better.

Your right in what you are saying but fact is with a 24kw combi, if you open a bath tap, or your hoping for a shower when your incoming mains are coming in at a very low tempreture a 24kw is going to struggle like mad!

 

[fuzzy]

Afternoon All

My first post!

Have a two bedroomed bungalow with a 15 year old BAXI Bermuda suppling heat and hot water from a cylinder. 6 radiators three of which are small, hallway and towel dryer in bathroom and a Myson kickspace heater in kitchen.

Looking to replace with a combi for loft installation, I have an electric shower I wish to remove and use the combi to supply with a mixer or some temperature controlled feed.

I have read many posts on CH requirements and 24Kw looks sufficient as the building is lagged with everthing possible!

My lack of understanding is what flowrate/power input is required for a good shower.

Have also pinpointed either ATAG or Viessman as the boiler to use, the BAXI has run for 15 years without failure just a yearly clean!

Many thanks for your advice please.

24kw is probably 4 or 5 times bigger than needed for CH.

The only reason to fit that or bigger boiler is for the HW flow rate. most 24kw boilers have decent flow rates these days

 

[justlead1]

It sadden me to read posts regarding the installations of combi;s. Purely for selfish reasons. As during my training in the trade the design codes of practice were 4 gallons (18Ltr) a minuet at a bath tap (10 gallons per min for public baths) Which required calculations regarding head, length of pipe, discharge and pipe size. Surly we have not got more patients now than then, when filling a bath. Or is it that cost is the prime concern?

 

[doitmyself]

Your right in what you are saying but fact is with a 24kw combi, if you open a bath tap, or your hoping for a shower when your incoming mains are coming in at a very low temperature a 24kw is going to struggle like mad!

But it all depends on your incoming flow rate. If it's only 10 litres/min a 24kW boiler will supply a 40°C shower when the incoming water is only 5°C. There is no need to mix cold with hot as the hot water is not at 60°C, as it would be if it came from a cylinder.

Of course if you are the type who likes a high pressure shower throwing out 20 litres/min you will need a good flow rate and a large boiler.

Baths are a different matter as everyone likes the bath to fill up quickly and 10 litre/min is a tad on the slow side. The problem is that you can't get 20 litres/min by turning on hot and cold taps at the same time, as you can with a HW cylinder and cold water tank.

 

[DieselPig]

Many thanks for the good advice here.

Is my mains flowrate important with regard to boiler selection? I have noticed the electric shower has to be turned up and down sometimes as the flowrate varies throughout the day, its either too hot or too cold.

Another option I considered is to buy a system boiler and a modern horizontal cylinder and place all this in the loft, will this give the same energy saving against a combi boiler?

 

[Billy Bob Bob]

Many thanks for the good advice here.

Is my mains flowrate important with regard to boiler selection? I have noticed the electric shower has to be turned up and down sometimes as the flowrate varies throughout the day, its either too hot or too cold.



yes your mains flow rate is important if its really poor you may not be able to have a combi,you may be forced to have a system boiler and cylinder set up,when the job gets quoted for your plumber should check your flow rate