Will I regret getting a combi?

kittennose

ThisIsWeird said:

FreeBear said:

kittennose said: Not many locals want to do a standard boiler, either 40KW combi or system boiler and unvented.

With a flow rate of just under 12l/min, a 40KW combi is way over the top. I have a 10l/min flow, and a ~25KW combi was the recommended size. Opted for a (nominal) 30KW as I wanted a bit of spare capacity for when I fit a booster pump. This particular boiler is good for 12l/min according to the manufacturer's spec sheet. But I suspect a 32-35KW would be better.

I suspect a 'slip'. I doubt any plumber was recommending a 40kW combi.
Kitten's house must be pretty large, tho', as the heat-only/system options are themselves 30kW - that is BIG!.

Good point about going for a slightly larger combi than what would 'suit' their current pitiful water supply, tho', should Kit go the combi route. A 24kW jobbie is just pitiful, and doesn't leave any margin for improvement if the mains water is somehow boosted.

Kitten, are the plumbers really recommending a 30kW Heat-Only boiler? That's big.

Yes, we are in an average 4 bed, just 13 rads. All my own research suggests 30kw max for combi, way less for heat only. All online tools suggest 35kw and one in-person quote was 40kw.

theoretica

Something else you might want to think about is your shower head - there are specially designed ones to work with low water pressure - eg https://www.amazon.co.uk/Pressure-Shower-Universal-Handheld-Double/dp/B07NL7SV41 (not one I have, an example that came up on search)  and they are much cheaper than a new boiler!

ThisIsWeird

Anyhoo, Kitten, to cut to the full circle, given what you said in your first post - "our water pressure is bog standard average: 11.8 Litres per min, 1.6 Bar pressure" (I'd call that less than bog, but hey) - then I would personally never suggest a mains-driven system. It would 'work', but not be the most effective way to deliver a decent performance. 
With a vented stored system, you at least have the water in your house, and can then use methods such as pumps to provide what you want.

Could you answer, please:

1) Are you happy with the performance of your current system?

2) Make and model of current ol' boiler?

FreeBear

kittennose said: we are in an average 4 bed, just 13 rads. All my own research suggests 30kw max for combi, way less for heat only. All online tools suggest 35kw and one in-person quote was 40kw.

I'd suggest measuring each radiator and looking up the heat output by comparing the sizes with somewhere like Screwfix (or download the Stelrad brochure). Add up the total heat output, and that will determine the size of the heat only boiler you actually need. I'd be surprised if you need more than 15KW (18 at a push).

Combi boilers are selected based on cold water flow, and will (nearly) always be able to produce a lot more heat for the central heating system than you actually need.

ThisIsWeird

Or, what's the power of your existing HO boiler, and did it do the job?

ThisIsWeird

And, if as part of this job you need to replace any radiators, seriously consider up-sizing them so they can output the same as before, but with a lower water temp = more efficiency.
(Especially for your 'living' areas  which are heated for longer.)

kittennose

ThisIsWeird said:

Anyhoo, Kitten, to cut to the full circle, given what you said in your first post - "our water pressure is bog standard average: 11.8 Litres per min, 1.6 Bar pressure" (I'd call that less than bog, but hey) - then I would personally never suggest a mains-driven system. It would 'work', but not be the most effective way to deliver a decent performance. 
With a vented stored system, you at least have the water in your house, and can then use methods such as pumps to provide what you want.

Could you answer, please:

1) Are you happy with the performance of your current system?

2) Make and model of current ol' boiler?

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.

FreeBear

kittennose said:

ThisIsWeird said:

Anyhoo, Kitten, to cut to the full circle, given what you said in your first post - "our water pressure is bog standard average: 11.8 Litres per min, 1.6 Bar pressure" (I'd call that less than bog, but hey) - then I would personally never suggest a mains-driven system. It would 'work', but not be the most effective way to deliver a decent performance. 
With a vented stored system, you at least have the water in your house, and can then use methods such as pumps to provide what you want.

Could you answer, please:

1) Are you happy with the performance of your current system?

2) Make and model of current ol' boiler?

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.

Depending on exactly which version of the Space Saver you have, it could be anything from ~9KW up to ~23KW output. All versions are around 65% efficient, so a modern condensing boiler should cut your gas bill by at least a third, maybe even as much as a half.

With a 50% reduction in gas consumption, pay back (assuming gas prices remain at the current level) will be 5-7 years. That alone should dispel any initial regrets.

ThisIsWeird

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.

Albermarle

FreeBear said:

kittennose said:

ThisIsWeird said:

Anyhoo, Kitten, to cut to the full circle, given what you said in your first post - "our water pressure is bog standard average: 11.8 Litres per min, 1.6 Bar pressure" (I'd call that less than bog, but hey) - then I would personally never suggest a mains-driven system. It would 'work', but not be the most effective way to deliver a decent performance. 
With a vented stored system, you at least have the water in your house, and can then use methods such as pumps to provide what you want.

Could you answer, please:

1) Are you happy with the performance of your current system?

2) Make and model of current ol' boiler?

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.

Depending on exactly which version of the Space Saver you have, it could be anything from ~9KW up to ~23KW output. All versions are around 65% efficient, so a modern condensing boiler should cut your gas bill by at least a third, maybe even as much as a half.

With a 50% reduction in gas consumption, pay back (assuming gas prices remain at the current level) will be 5-7 years. That alone should dispel any initial regrets.

I was using a non condensing boiler ( installed 1995) in a similar4 bed  house to the OP, also with 13 radiators . Also an older house, with a family so needed keeping reasonably warm, but I also made some efforts to keep the bills down.
We used 22to 25 000 KWh of gas a year .
A switch to a modern condensing boiler 18 KW ( with flow temperature not too high ) brought a drop in usage down to 20 Kwh  so about 20% or thereabouts .
Have seen a further drop to around 17000Kwh in 2022 and this year, presumably due to relatively mild winters down here in the South. Sort of proves direct comparisons are a bit difficult as the weather has a significant effect.