Will I regret getting a combi?

kittennose

ThanksThisIsWeird said:

kittennose said:

1) No. It's expensive (20,000kwh of gas annually despite being very frugal with heating) and showers / taps are poor

2) Glowworm Space Saver MKII non-condensing. Installed 1988.

For (1), by performance I meant how well it works - heats up your rads, and your hot tank. Does it do both ok?

But, yes, also how well that hot water comes out your taps - and that's not good.

For (2), we'd need to know the specifics, as FreeBear says.

In essence, we need to know if your current size of boiler is actually heating up your rads and hot tank ok. If it is, then it would suggest you don't need a boiler with a larger output (I'm still fascinated by the recommendations for 30kW Heat-Only boilers...)

So, my gut is thinking that a normal-sized H-O boiler should cost a lot less to install than the ~£3.5k figures being bandied about, and there are simple ways of improving the hot flow performance to your taps and showers using your existing system.

IF fitting a combi would give you full combi performance, I'd be saying 'go for it!'. IF an unvented cylinder would do ditto, I'd be saying ditto.

But I just cannot suggest either, knowing the mains flow is so poor. It would be expensive to install, and working well under its capacity to deliver.

Your options are to either boost the incoming - so you can use a combi or unvented to its full - or to store the incoming water in the loft, and then deliver it fully to the house. With the latter option, you are half way there - you have the loft store.

I've mentioned the Grundfos pumps on another thread. These strike me as being pretty much ideal. It won't make your supply 'crazy', and you won't hear a jet engine fire up, but the flow from your taps should be easily tripled or even fourpled.

Current heating is fine. All radiators warm well and evenly. Cylinder also but it's 120L and we run out a LOT. Takes a while to heat too. This is 30 years old so probably not as efficient as it could be...

What's the issue with the mains at 12l/min for a combi? Is it because we would not be able to split this between 2 showers/taps at once? If so I understand that.

Believe it or not we had someone quote yesterday - another 40kw combi 🥹🥹

I am thinking 30kw would be more than enough for a combi. That gives us 12L/min hot water flow rate which matches our mains and peers 18 radiators (we have 13). I just don't understand why they are all quoting 35-40kw! Surely they don't make more money.

 we went that route...

FreeBear

kittennose said: I am thinking 30kw would be more than enough for a combi. That gives us 12L/min hot water flow rate which matches our mains and peers 18 radiators (we have 13). I just don't understand why they are all quoting 35-40kw! Surely they don't make more money.

The installation costs remain about the same regardless of the size of the boiler, but a bigger boiler comes with a higher price tag. When I was shopping for quotes, most were specifying 30KW combis, and a couple were saying 24KW would be fine. On paper, a 24KW would indeed cope with the flow rate I have (10l/min) giving a 35°C temperature rise. I decided on a 30KW combi in the end, and feel it was the right decision. Heating cold water to 50°C at 9.4l/min. the boiler is running at 110% capacity.

Whilst I don't think a 40KW combi is required for your 12l/min flow, a 30KW would be the minimum size in my opinion. Maybe 32KW or even 35KW to give a bit of headroom. Modern boilers will modulate heat output down, so the thing to look at is the minimum it will generate - Mine goes down to 3.2KW and spends most of its time running at 4-6KW when doing the central heating. Most installers will over specify the boiler as they know it will modulate down on the heating side - This covers their back knowing the customer isn't going to turn round & say the boiler isn't providing enough heat, and the modulation range means the customer is less likely to complain about high gas bills. But if a boiler can't modulate down low enough, it will spend a lot of time short cycling which leads to a shorter working life.

kittennose

FreeBear said:

kittennose said: I am thinking 30kw would be more than enough for a combi. That gives us 12L/min hot water flow rate which matches our mains and peers 18 radiators (we have 13). I just don't understand why they are all quoting 35-40kw! Surely they don't make more money.

The installation costs remain about the same regardless of the size of the boiler, but a bigger boiler comes with a higher price tag. When I was shopping for quotes, most were specifying 30KW combis, and a couple were saying 24KW would be fine. On paper, a 24KW would indeed cope with the flow rate I have (10l/min) giving a 35°C temperature rise. I decided on a 30KW combi in the end, and feel it was the right decision. Heating cold water to 50°C at 9.4l/min. the boiler is running at 110% capacity.

Whilst I don't think a 40KW combi is required for your 12l/min flow, a 30KW would be the minimum size in my opinion. Maybe 32KW or even 35KW to give a bit of headroom. Modern boilers will modulate heat output down, so the thing to look at is the minimum it will generate - Mine goes down to 3.2KW and spends most of its time running at 4-6KW when doing the central heating. Most installers will over specify the boiler as they know it will modulate down on the heating side - This covers their back knowing the customer isn't going to turn round & say the boiler isn't providing enough heat, and the modulation range means the customer is less likely to complain about high gas bills. But if a boiler can't modulate down low enough, it will spend a lot of time short cycling which leads to a shorter working life.

Thanks Bear

How do you find a combi with 10L/min mains flow? Some people are saying it's way too for a combi.

Your showers must be 8l/min or thereabouts factoring in shower heads and travel?

kittennose

I just looked our (possibly new one) up, looks like it will modulate to 7.5 or 6.2 if we went for the 30kw.

ThisIsWeird

kittennose said:

Current heating is fine. All radiators warm well and evenly. Cylinder also but it's 120L and we run out a LOT. Takes a while to heat too. This is 30 years old so probably not as efficient as it could be...
What's the issue with the mains at 12l/min for a combi? Is it because we would not be able to split this between 2 showers/taps at once? If so I understand that.
Believe it or not we had someone quote yesterday - another 40kw combi 🥹🥹
I am thinking 30kw would be more than enough for a combi. That gives us 12L/min hot water flow rate which matches our mains and peers 18 radiators (we have 13). I just don't understand why they are all quoting 35-40kw! Surely they don't make more money.
 we went that route...

If you keep running out of hot water because of the tank size, then I can understand you seeing a combi as a saviour!

I have a 30kW combi, 13 rads in a 3+ bedroom house. But I also have 3+ bar pressure, and 20+lpm flow. The showers are great! But only one at a time...

If a combi delivers hot water at around 60oC, then it needs a good dose of cold added at the shower mixer. So the boiler will effectively only be needing to supply around 9-odd lpm of hot.

Anyway, I would also not suggest going less than 30kW. But I do not understand the reasoning behind 35 to 40kW combis for you.

From FreeBear's experience, a combi should work fine for you. As long as you know the compromises being made.

FreeBear

kittennose said:

I just looked our (possibly new one) up, looks like it will modulate to 7.5 or 6.2 if we went for the 30kw.

That range of modulation is pretty poor in my opinion. Viessmann do several that will go down to just 3.2KW - I have the Vitodens 050 which is proving to be well suited to my needs. Worcester Bosch boilers also have a similar modulation range - However, they are a litle more expensive than say RavenHeat or Baxi, but you do get a 10+ year warranty (subject to terms & conditions) unlike most of the budget boilers.

And yes, my boiler does modulate down low when in use. This morning, it fired up at 40-50% for about 20 minutes before dropping down to ~12% for another hour (so start at 10-12KW and then tick over at 3.2KW). This comfortably raised the temperature of the house by 2°C and used 12KWh of gas (according to the Bright app). If the boiler wasn't capable of modulating down so low, it would probably start short cycling.

kittennose

FreeBear said:

kittennose said:

I just looked our (possibly new one) up, looks like it will modulate to 7.5 or 6.2 if we went for the 30kw.

That range of modulation is pretty poor in my opinion. Viessmann do several that will go down to just 3.2KW - I have the Vitodens 050 which is proving to be well suited to my needs. Worcester Bosch boilers also have a similar modulation range - However, they are a litle more expensive than say RavenHeat or Baxi, but you do get a 10+ year warranty (subject to terms & conditions) unlike most of the budget boilers.

And yes, my boiler does modulate down low when in use. This morning, it fired up at 40-50% for about 20 minutes before dropping down to ~12% for another hour (so start at 10-12KW and then tick over at 3.2KW). This comfortably raised the temperature of the house by 2°C and used 12KWh of gas (according to the Bright app). If the boiler wasn't capable of modulating down so low, it would probably start short cycling.

Thanks, I can get the 050 30kw installed for £3400. 

Is your shower acceptable? We have no way of knowing what 8-9L/Min feels like but ours is 3 so I'm sure it's fine

ThisIsWeird

kittennose said:

FreeBear said:

kittennose said:

I just looked our (possibly new one) up, looks like it will modulate to 7.5 or 6.2 if we went for the 30kw.

That range of modulation is pretty poor in my opinion. Viessmann do several that will go down to just 3.2KW - I have the Vitodens 050 which is proving to be well suited to my needs. Worcester Bosch boilers also have a similar modulation range - However, they are a litle more expensive than say RavenHeat or Baxi, but you do get a 10+ year warranty (subject to terms & conditions) unlike most of the budget boilers.

And yes, my boiler does modulate down low when in use. This morning, it fired up at 40-50% for about 20 minutes before dropping down to ~12% for another hour (so start at 10-12KW and then tick over at 3.2KW). This comfortably raised the temperature of the house by 2°C and used 12KWh of gas (according to the Bright app). If the boiler wasn't capable of modulating down so low, it would probably start short cycling.

Thanks, I can get the 050 30kw installed for £3400. 

Is your shower acceptable? We have no way of knowing what 8-9L/Min feels like but ours is 3 so I'm sure it's fine

Your 30kW combi-delivered shower should be pretty much at your 'full' rate of just under 12lpm (ie - your mains), made up from 9-ish of hot, blended with a few litres of cold. 
If you do go combi, a 30kW jobbie would seem to be pretty optimum for you. 
If, after it's fitted, you find your lowish mains pressure is allowing just a touch too much wobble during showers, you do have a very simple option of fitting a salamander mains booster pump - around £300? - to the incoming mains. This should ensure you always get a solid 12lpm (the most you are allowed to pump the mains by) at a firm pressure, so it'll optimise your existing supply, short of adding a tank.

FreeBear

kittennose said: Is your shower acceptable? We have no way of knowing what 8-9L/Min feels like but ours is 3 so I'm sure it's fine

Electric shower, so can't comment on any running off the combi. Whilst it isn't blast your socks off power, it is sufficient to get wet & rinse off with. And being thermostatic, it holds temperature regardless of water flow or pressure.

If you were local, I'd say come over and give it a try, but some here might get the wrong idea

kittennose

FreeBear said:

kittennose said: Is your shower acceptable? We have no way of knowing what 8-9L/Min feels like but ours is 3 so I'm sure it's fine

Electric shower, so can't comment on any running off the combi. Whilst it isn't blast your socks off power, it is sufficient to get wet & rinse off with. And being thermostatic, it holds temperature regardless of water flow or pressure.

If you were local, I'd say come over and give it a try, but some here might get the wrong idea

🤣 thanks.