right size boiler spec

[Desktop987456]

Hi, I am not in UK and the rising fuel and gas prices are forcing me to change my boiler to an efficient one. Right now I have an old non condensing boiler which is way oversized and consumes 1.75m3/hr so I am looking at an efficient gas boiler to support attached load. I plan to buy it from UK though and it will be shipped to me in my country of residence. I have attached a template with the required BTUs or Kw for knowledable members. I would be grateful if you could suggest an efficient boiler. The boiler will be heating rooms highlighted in green most of the time but I have guests/extended family in holidays in December, January so should be able to support on full load. I have given pipe lengths in feet from boiler as well if that makes any sense.
Since I have a non pressurised boiler I have been told that I need to change all single panel radiators to double panel prior to changing boiler as they may not been able to sustain the 2 bar pressure of a condensing boiler? Is this true. The spec sheet of radiators say that it can hold pressure of upto 7 bar. I don't want to test it the hard way by busting the rads but I am also wary of changing all radiators for no reason. I have changed two radiators in first floor rooms to double panel rads since old ones were undersized for the room and temperature differential between first and ground floor. Thanks in advance.

 

[Last Plumber]

What system have you got?
I know you said it is open vented but is it gravity hot water or fully pumped?

You could use a Vaillant ecoTec 424 or an ecoFit 425 if the system is compatible.

 

[Desktop987456]

What system have you got?
I know you said it is open vented but is it gravity hot water or fully pumped?

You could use a Vaillant ecoTec 424 or an ecoFit 425 if the system is compatible.

Thanks for your response. Gravity hot water not pumped via a separate hot water tank (separate heating arrangement for DHW).
This boiler will be only for CH, so 24kw would be enough for my Central heating needs?

 

[Timmy D]

24kw will be fine. There would be enough left over to also do the hot water as you’ll never have the full 18kw estimated load going full 24/7.

 

[Desktop987456]

Thanks, I thought so 24kw would be enough. Two questions:

• The last radiator on the pipework extends to ~80 feet (upstairs) from boiler which is downstairs. Would the pump be enough for circulation?
• I have been told that I have to change all my radiators not because they are undersized but because in his view they will not be able to sustain the 1-2 psig from the new boiler?

I don't want to change the radiators without substantiating first. The spec sheet of the radiators say that it can sustain pressure upto 7psig. I understand that it depends on age of radiator and rust in it but they are only 7 years old (installed in 2014) and are big radiators 4.5ft in length and do the job of heating big rooms fairly easily.

 

[Timmy D]

Thanks, I thought so 24kw would be enough. Two questions:

• The last radiator on the pipework extends to ~80 feet (upstairs) from boiler which is downstairs. Would the pump be enough for circulation?
• I have been told that I have to change all my radiators not because they are undersized but because in his view they will not be able to sustain the 1-2 psig from the new boiler?

I don't want to change the radiators without substantiating first. The spec sheet of the radiators say that it can sustain pressure upto 7psig. I understand that it depends on age of radiator and rust in it but they are only 7 years old (installed in 2014) and are big radiators 4.5ft in length and do the job of heating big rooms fairly easily.

It’s hard to answer the first question without knowing the property layout etc.

80ft is high.

Most internal boiler pumps have a head of between 4m-6m. That doesn’t mean that is the max height, as they are not lift pumps but circulators. The head and flow rate help compare.

Smaller pipes introduce more resistance. If there is too much resistance, the pump can’t cope. If the pipe size is increase, a pump that couldn’t cope, may now cope.

This is why it’s hard to answer with out knowing the layout/pipe sizes etc.

For large houses, I often put in a heat only boiler (pump/expansion vessel are installed separately/external to the boiler), rather than a system boiler. It allows me to spec the pump size and expansion.

With regards to the rads, I think you may have your units of pressure wrong. A sealed boiler will not run on 2 psig. I think you mean 1-2 bar, with rads rated at 7 bar.

Now, whenever converting a system from vented to unvented (pressurised). There is a risk of leaks. If there is going to be a leak it’s highly unlikely from a rad (I ant even remember a time), but from a valve or pipe joint that previously didn’t leak, due to the lower pressure.

I wouldn’t chance the rads. I typically don’t recommend to customers to change rads until they get to 30+ years old, unless it’s rusted or leaking. I see no point.

If your plumber didn’t explain this, which it seems he didn’t, I’d be looking for another quote.

 

[Desktop987456]

It allows me to spec the pump size and expansion.

Thanks, you have been very helpful and have answered my questions.
Apologies I did get the units wrong it is 1-2 bar and rads 7bar.
1. pipe sizes are 22mm from boiler and branches off 15mm just before or to rads;
2. The pump is coping up with these lengths so far in a non pressurised system but I have only tested it over 2-3 days how it performs during whole season, I don't know.
3. Boiler pipe layout involves pipes going upstairs twice; once from front (3m) and second from backside of the house after running 40ft horizontally on ground floor then 3m upstairs (all 22mm lengths). The farthest rad ~80 ft from boiler (upstairs on the back side of the house) gets hot too so this means pump is working fine, how will it perform in pressurised system I am not familiar with hence the question. How can I calculate pump requirement in a pressurised system?
4. Installers in my area aren't very helpful and knowledable and when it comes to central heating I am on my own mostly. A while ago I wanted to extend the pipework and he was insisting on PEX pipe which after consulting on forums like people such as yourself I was advised (rightly so) to go for copper which I did and I am happy that I made the right call. I don't want to spend money where I don't need to. I am making improvements in my system by bringing efficiency and reducing my gas usage that is why I wanted to buy an energy efficient boiler. My current boiler is not energy efficient and consumes 12m3/day for 15 hours of heating only (6am-9pm). Hot water arrangement consumes another 3m3/day. I am under the impression rightly or wrongly that a condensing or newer boiler will consume much less than this but I have no way of substantiating this theory or info.

 

[Desktop987456]

see attached floor plan and pipe layout for reference and guidance.

 

[Timmy D]

I wouldn’t chance the rads. I typically don’t recommend to customers to change rads until they get to 30+ years old, unless it’s rusted or leaking. I see no point.

I didn’t mean chance, meant to write change. I wouldn’t change the rads.

I also for some reason, thought you meant 80ft height from boiler not total distance.

You’ll be fine with pumping power.

Upgrading the boiler to high efficiency/condensing, might save you 10%.

Biggest saves are made reducing heat losses. Have you had someone have a look with a thermal camera inspect the exterior of the property for heat loss?

 

[Desktop987456]

Biggest saves are made reducing heat losses. Have you had someone have a look with a thermal camera inspect the exterior of the property for heat loss?

Thanks for your thoughts.
I have plugged all draughts and holes to minimize heat losses from heated rooms including insulation on pipework and it has made a difference.
You are spot on using the thermal camera but there are hardly any heating engineers with thermal camera in my area, and honestly I cannot justify the cost of using it once.

 

[Timmy D]

Thanks for your thoughts.
I have plugged all draughts and holes to minimize heat losses from heated rooms including insulation on pipework and it has made a difference.
You are spot on using the thermal camera but there are hardly any heating engineers with thermal camera in my area, and honestly I cannot justify the cost of using it once.

Out of interest, where in London are you?

 

[Desktop987456]

Out of interest, where in London are you?

Sorry, I am not in UK, I live in Pakistan and have learned a lot about heating systems from forums and people such as yourselves. We installed a locally made heating system in 2013 during major house renovation. The system wasn't designed properly and they put two boilers in the property; one in front and one at the back, creating two zones. The front one was way oversized for my needs (35kw) and now after calculating my property needs I have connected all radiators to this boiler which is still way oversized for 13 radiators installed at our house. I have extended pipework creating heating zones and upgraded some undersized radiators to improve the system. At some point in time I need to change the boiler to a more efficient one.
There aren't many certified or registered heating engineers or plumbers in my country of residence. Winters here are not as cold as and not as long as in UK. It gets 5degC in January and winters lasts a couple of months. It was very cold in January but the chill in air is gone in February, I think because of short duration of winters we we don;t have many qualified people here. People either use gas or electric heaters for this short duration and very few people have central heating system.

 

[Timmy D]

Sorry, I am not in UK, I live in Pakistan and have learned a lot about heating systems from forums and people such as yourselves. We installed a locally made heating system in 2013 during major house renovation. The system wasn't designed properly and they put two boilers in the property; one in front and one at the back, creating two zones. The front one was way oversized for my needs (35kw) and now after calculating my property needs I have connected all radiators to this boiler which is still way oversized for 13 radiators installed at our house. I have extended pipework creating heating zones and upgraded some undersized radiators to improve the system. At some point in time I need to change the boiler to a more efficient one.
There aren't many certified or registered heating engineers or plumbers in my country of residence. Winters here are not as cold as and not as long as in UK. It gets 5degC in January and winters lasts a couple of months. It was very cold in January but the chill in air is gone in February, I think because of short duration of winters we we don;t have many qualified people here. People either use gas or electric heaters for this short duration and very few people have central heating system.

Thanks for the clarification. I asked as I notice your profile said London. Was going to pop round with my thermal camera if a bit closer than Pakistan!

 

[Desktop987456]

Thanks for the clarification. I asked as I notice your profile said London. Was going to pop round with my thermal camera if a bit closer than Pakistan!

Thanks.

 

[Desktop987456]

Thanks for the clarification. I asked as I notice your profile said London. Was going to pop round with my thermal camera if a bit closer than Pakistan!

Thanks Timmy D, I would have given you a red carpet welcome if you were!

 

[John.g]

Thanks, you have been very helpful and have answered my questions.
Apologies I did get the units wrong it is 1-2 bar and rads 7bar.
1. pipe sizes are 22mm from boiler and branches off 15mm just before or to rads;
2. The pump is coping up with these lengths so far in a non pressurised system but I have only tested it over 2-3 days how it performs during whole season, I don't know.
3. Boiler pipe layout involves pipes going upstairs twice; once from front (3m) and second from backside of the house after running 40ft horizontally on ground floor then 3m upstairs (all 22mm lengths). The farthest rad ~80 ft from boiler (upstairs on the back side of the house) gets hot too so this means pump is working fine, how will it perform in pressurised system I am not familiar with hence the question. How can I calculate pump requirement in a pressurised system?
4. Installers in my area aren't very helpful and knowledable and when it comes to central heating I am on my own mostly. A while ago I wanted to extend the pipework and he was insisting on PEX pipe which after consulting on forums like people such as yourself I was advised (rightly so) to go for copper which I did and I am happy that I made the right call. I don't want to spend money where I don't need to. I am making improvements in my system by bringing efficiency and reducing my gas usage that is why I wanted to buy an energy efficient boiler. My current boiler is not energy efficient and consumes 12m3/day for 15 hours of heating only (6am-9pm). Hot water arrangement consumes another 3m3/day. I am under the impression rightly or wrongly that a condensing or newer boiler will consume much less than this but I have no way of substantiating this theory or info.

12m3/day for a 15 hour heating day is 0.8m3/hr or ~ 8.8kw/hr boiler output, don't know what the efficiency of your boiler is but assuming 70% then your heating requirements are ~ 6.2kwh/hr, hardly outrageously excessive?. although you state elsewhere that the boiler consumption is 1.75m3/hr.

Your HW requirement of (gas) 3m3/day equates to a HW demand of 440LPD at 60C assuming the same 70% boiler efficiency, seems a very high HW demand?.

A condensing boiler might give a max of 20% greater efficiency IMO or less depending on the present boilers efficiency.

What is the make/model of your existing boiler(s)?.

 

[Desktop987456]

12m3/day for a 15 hour heating day is 0.8m3/hr or ~ 8.8kw/hr boiler output, don't know what the efficiency of your boiler is but assuming 70% then your heating requirements are ~ 6.2kwh/hr, hardly outrageously excessive?. although you state elsewhere that the boiler consumption is 1.75m3/hr.

Your HW requirement of (gas) 3m3/day equates to a HW demand of 440LPD at 60C assuming the same 70% boiler efficiency, seems a very high HW demand?.

A condensing boiler might give a max of 20% greater efficiency IMO or less depending on the present boilers efficiency.

What is the make/model of your existing boiler(s)?.

12m3/day is heating the essential rooms only (not the large fan coil unit which consumes 5-7kw). It is an old boiler and you I have seen its efficiency in the range of 65-70% range. 1.75 m3/hr that I stated in post is running the boiler on full load (sorry it this confuses you).

I have a separate DHW arrangement with cylinder's own burner (on gas) independent of boiler, its a 200 litres cylinder and it consumes in 2-3m3/day range.

The boiler and cylinder works independently, what would be the most efficient way for Central heating and DHW? Cylinder providing hot water to boiler or vice versa?

I have a non pressurized system and want to make it efficient if it doesn't cost an arm or a leg!

I also have 11kw PV system installed on the roof and cylinder is fitted with 2x1500 watts immersion heaters.

Thanks

 

[John.g]

You have a gas fired Hot water heater/cylinder supplemented with a PV diverter?, if so, then if you wanted to supply CH & HW from one gas fired source (the boiler) you will need a new cylinder + immersion heater(s). not sure what you mean by cylinder providing HW to the boiler.

 

[Desktop987456]

You have a gas fired Hot water heater/cylinder supplemented with a PV diverter?, if so, then if you wanted to supply CH & HW from one gas fired source (the boiler) you will need a new cylinder + immersion heater(s). not sure what you mean by cylinder providing HW to the boiler.

I don't have PV diverter installed in my system, I get credit from grid for selling excess power.
Since I have separate cylinder for heating (stored hot water), can it supply hot water to boiler and it can ciruclate hot water to radiators? Can it be done.

 

[John.g]

If I am interpreting you correctly in that you have a separate gas fired stored "DHW cylinder" then no or certainly not easily, you have plenty of spare capacity in your CH boiler to suppy a conventional DHW cylinder and can't see any efficiency gains the other way.

 

[Desktop987456]

If I am interpreting you correctly in that you have a separate gas fired stored "DHW cylinder" then no or certainly not easily, you have plenty of spare capacity in your CH boiler to suppy a conventional DHW cylinder and can't see any efficiency gains the other way.

Correct.
Thanks for your help.
Do you think that installing a PV diverter would be a good investment replacing gas for hot water?

 

[John.g]

You will get a good feel for it if you compare the price (and amount) of your grid export vs gas cost of producing the same amount of DHW.

 

[Desktop987456]

You will get a good feel for it if you compare the price (and amount) of your grid export vs gas cost of producing the same amount of DHW.

Have to do it someday and it make sense for conversion, gas is still cheaper than electricity. When I had installed PV in Jan 2022, my unit cost for PV generation has come down 40% in 1 year owing to increase in electricity tarriffs whereas the cost of installing a PV system has doubled literally ($ parity). Gas is still cheaper but it is a matter of couple of years when we'll run out of our own and will rely on imported gas which will be at par with electricity.