Will I regret getting a combi?

kittennose

We need a new boiler and have been considering a combi. On paper it gives us lots of benefits over our old gravity fed boiler:

✅ Better shower pressure
✅ Instant hot water

✅ More storage

However I have found plumbers appear to either love combos or hate them. Depending on who we've had round, it's either 'combis are the way to go' or 'you're mad to get a combi mate'.

So I am looking for real life experiences from families and kids in 4 bed homes with a combi.

Note: our water pressure is bog standard average: 11.8 Litres per min, 1.6 Bar pressure.

Brie

Heating up hot water you may not be using seems a bit mad to me so I'm happy to have a combi.  That said - it is not instant hot water as the pressure triggers the boiler to turn on an you do get a fair bit of cold water before the hot comes through*.  Reduce the pressure in the household (toilet flush, other tap, washing machine) and you can lose the hot.  That may be due to us having had combis on the cheaper end - I would expect someone with a larger household (ours was 3 adults, now 2) would need something more top end.  

*getting hot water was an issue with my MiL having showers.  She needed assistance and so would be sitting on her shower chair waiting for the hot water and be shivering due to the cold vapor kicked out initially.  Not so much an issue with someone who doesn't feel the cold so much.  

daveyjp

Shower pressure is directly related to mains water flow rate, with less than 12 l per minute you may be limited in boiler output.  In a 4 bed property I'd stick with a system boiler.

elsien

Sibling had a combi in a five bedroom house with three children. Worked fine for them. 

However they did retain the electric shower rather than having it fed off the hot water system and I suspect that may have made a difference as they were a family who took a lot of showers. Combi heated the water for the sinks and running the bath, which worked fine for them. 

FreeBear

Ripped out my gravity fed heating & hot water system earlier in the year. Spent quite a bit of time replumbing the system and fitted larger radiators in the process. I was looking to the future when heat pumps are the only option, hence the new plumbing (larger pipes, and more sensible routing).

The DHW cylinder was in a bedroom, so getting hot water out of any of the taps always involved a long wait. With the combi, I could have put it in the kitchen or next to the bathroom - One or the other would get (almost) instant water, whilst the other would still incur a wait. Gave the bathroom priority as it simplified the plumbing considerably. Have an electric shower, so pressure & flow rate is not much of a concern for me..

Was it worth doing - From a financial aspect, with the reduced gas usage, it is going to take a very long time to see a return on the investment (was already a low usage @ <3500KWh per year). Having (almost) instant hot water on demand without having to heat a tank full is worthwhile. Heating seems to be a bit more responsive, and initial indication is that it is more efficient. Still playing with the settings to find that sweet spot. Overall, I am happy with the ~£3K investment and should have done it years ago. But modern boilers are capable of modulating output down much lower than they used to (mine will drop to 3.2KW when ticking over).

Postik

Sorry, I can't comment on what it's like having a combi in a 4 bedroom house.  My old 3-bed had a combi and my new 4 bed has a system boiler.

From experience neither of them provide "instant" hot water.  They are both about the same in terms of having to run the tap for about 10 seconds before hot water comes through.

At my old house we had an electric shower and it used to run cold mid-shower if someone turned a tap on downstairs, which was a minor irritation (I suspect related to water pressure).

I do find the idea of heating a huge tank of water very wasteful.  We tend to turn the hot water on for a minimal amount of time (15 mins if one person if having a shower, 30 mins if two people are having a shower, etc).  But one way or another you're always going to heat either too much water or too little.

I know the vast majority of 4 bed new builds have system boilers, but Bloor Homes started fitting their 4 beds with combi boilers, so they must work.

ThisIsWeird

kittennose said:

We need a new boiler and have been considering a combi. On paper it gives us lots of benefits over our old gravity fed boiler:

✅ Better shower pressure
✅ Instant hot water

✅ More storage
However I have found plumbers appear to either love combos or hate them. Depending on who we've had round, it's either 'combis are the way to go' or 'you're mad to get a combi mate'.
So I am looking for real life experiences from families and kids in 4 bed homes with a combi.
Note: our water pressure is bog standard average: 11.8 Litres per min, 1.6 Bar pressure.

Hi Kitten.

Will you regret getting a combi? At 11.8lpm (very specific! - but barely adequate) and 1.6bar (limp), I'd say 'yes'.

You currently have a vented hot tank and heat-only boiler. The quality of shower you'll be getting from that will largely depend on how high up the CWS (cold tank) is in the loft. If it's high enough, you should be getting a great shower. Baths should fill very quickly. If they don't, you can always add pumps.

A wee word about 'pressure' and 'flow' volume. I don't know how good the flow from your hot bath tap or shower currently is, but it should be gushing. Is it? If it is, then that means it's giving a good 'flow' (ooh, I'll have a stab at 20lpm or so?) but this will be a low pressure - 0.2 bar or so. The pressure doesn't matter, 'cos the pipes are big, so allow a lot of water out. The hot bath tap might gush with water, but you can stop that flow in an instant by putting your hand over it, or sticking yer big toe up t'spout.

So, you currently have a stupidly low 'pressure' - a small fraction of one bar - but presumably a good 'flow'? If you swap to a combi, you'll get an increase in 'pressure' - you won't be able to stop the flow with your thumb any more - but a 11.8lpm flow is really pretty pitiful. For that, there's no point getting a boiler any larger than 28kW - and that's 'weak'.

If, for some reason, your current hot water flow isn't good, then it can usually be sorted fairly easily - by raising the CWS, improving the pipe layout, or even by adding gentle pumps.

So, as it stands, I personally wouldn't go for a combi, because your pressure and flow will not get the best out of it.

I replaced my vented system (like yours) with a combi shortly after moving in, as we were converting the loft and needed to lose the tanks up there - that was the main driver. But, no regrets. We brought up two kids in this house, from babies to recently leaving Uni. The only issues have been the classic combi one of 'only one hot tap at a time'! (And the combi sludging up 'cos the system hadn't been cleaned properly - not the boiler's fault...) Other than that, it's been great. Terrific showers, but that's because we have 3+bar pressure, and 20+lpm flow. Of course, if anyone flushes a loo during a shower, they get beaten up afterwards (in practice, 'cos the shower is thermostatic, 'all' that happens is that the flow drops off dramatically for 20 seconds or so).

Our 'airing' cupboard has a zig-zag of 15mm copper pipe up one side to keep it warm when the CH is on, so we ain't missing the old hot cylinder.

Your options:

1) Keep your vented hot cylinder, and replace your 'heat-only' boiler with a ditto. That is your cheapest option, and should be around £1.5-2k. Your new boiler will almost certainly be more efficient, so expect some energy cost savings. Nothing else will change; your tap and shower flows will be as they currently are.

2) Keep your vented hot cylinder, and replace your 'heat-only' boiler with a 'system' type. That will remove the small tank in the loft, and give you a simpler and more reliable setup. Your new boiler will almost certainly be more efficient, so expect some energy cost savings. Nothing else will change; your tap and shower flows will be as they currently are.

3) Either of these^ two options, but also addressing any hot water flow issues you may have. Can the CWS be raised? Can the pipe runs be simplified? Could pumps be added?

4) Change to a combi. This will free up cupboard space, lose the tanks in the loft, be a more efficient boiler (like the other new ones above), possibly save you hot water heating costs - but likely minimal - and provide you with adequate showers. But only one at a time.

5) Swap your existing 'vented' hot cylinder for an 'unvented' type. Unvented are supplied by the mains, and deliver hot water at mains pressure and flow. But, since yours are carp in both regards, that is the last thing I'd do. Cost - along with the required new boiler - £4k upwards?

6) Either of 4 or 5, but with the addition of a pumped accumulator. These are plumbed into the cold mains as it comes into your house. It's a storage cylinder with a pump, so takes your pitiful 1.6bar pressure and boosts it to ~3bar (some to 4.5) within this tank. That will then blow your mind. It'll provide whatever flow you could possibly want, at whatever pressure. If you use it with a combi, you could fit even a 38kW jobbie, and have stupendous 'rain' showers, even two at the same time. If you use it with an unvented hot cylinder, you will definitely have stupendous showers - as many as you like at the same time (until the hot cylinder runs cold, which could be quite soon...)

Cost of accum? Around the £1k+ mark, but you need to have room for it. An attached garage could be used, for example.

Other issues to consider. As mentioned by others, what's the future for energy in this country? If it involves ASHPs or similar, then likely you'll need a hot water storage system. Tbh, these will then likely be so advanced that you'll need to replace whatever you may have anyway.

The first Qs I'd be asking myself in your position are;

1) What are my existing showers and bath-fills like? If not 'good', then why not? What can I easily and cheaply do to improve this?

2) Do folk like to run showers at the same time?

3) Just how much do I want to lose the ol' hot tank?

In short, what is it I actually want to achieve? What's important; Better performance? Gaining of a cupboard? Saving energy costs?

On that latter point - energy savings - I'd suggest that if you were a singleton or just a couple, then a combi will likely save you on running costs. That's because it'll only run for DHW when required, for just two of you, and that won't be so much - barely worth heating a whole hot tank for. And when you do heat up the whole tank, it'll sit there losing any heat that you haven't used - it's a steady loss. However, for an active family, needing regular showers, heating up a hot tank will be the norm, and be required, so there will be no hot water sitting around seeping away. So making this a combi instead ain't going to save much, if anything; what you need is what you'll heat and what you'll use!

There - I hope the above hasn't confused matters :-)

spo2

We have a combi in a 4 bed house with 2 kids, and no issues. If someone turns a tap or another shower on when you're in the shower you can feel the pressure drop slightly but not so much to annoy the person in the shower

Section62

Postik said:

I do find the idea of heating a huge tank of water very wasteful.

The problem with the above is people imagining the heat from a hot water cylinder vanishing into thin air at a rapid rate.  The reality is a well-insulated hot water cylinder kept in a space like an airing cupboard is not losing heat at anything like the rate people might imagine.

If the hot water isn't being used then the cylinder is not going to be stone cold by the time the next heating cycle starts - so the 'loss' isn't in heating a 'huge tank of water' every time, it is just in the need to replenish the heat lost since the last heating cycle.

The advantage of having a (largish) hot water cylinder is in the ability to heat it either a) at times of day when energy is cheap or b) using heating methods that don't have the capacity to provide 'instant' hot water.  E.g. Domestic PV solar won't have the heating capacity to provide instant hot water when it is needed, but PV can be used to (slowly) heat water when solar energy is available, to be stored for use when required.

The OP's question is almost impossible to answer because everyone's circumstances differ.  We can talk about generalisations - such as the future probably involving more expensive energy supply at peak times of day, and/or gas becoming more expensive (relatively) than it is today.  But absolute answers in specific individual cases don't come easy.

My own view is the most cost-effective systems in the future will involve energy storage, therefore removing an existing storage system and replacing it with an on-demand one is not likely to be a wise move other than in very specific circumstances.

kittennose

ThisIsWeird said:

kittennose said:

We need a new boiler and have been considering a combi. On paper it gives us lots of benefits over our old gravity fed boiler:

✅ Better shower pressure
✅ Instant hot water

✅ More storage
However I have found plumbers appear to either love combos or hate them. Depending on who we've had round, it's either 'combis are the way to go' or 'you're mad to get a combi mate'.
So I am looking for real life experiences from families and kids in 4 bed homes with a combi.
Note: our water pressure is bog standard average: 11.8 Litres per min, 1.6 Bar pressure.

Your options:

1) Keep your vented hot cylinder, and replace your 'heat-only' boiler with a ditto. That is your cheapest option, and should be around £1.5-2k. Your new boiler will almost certainly be more efficient, so expect some energy cost savings. Nothing else will change; your tap and shower flows will be as they currently are.

2) Keep your vented hot cylinder, and replace your 'heat-only' boiler with a 'system' type. That will remove the small tank in the loft, and give you a simpler and more reliable setup. Your new boiler will almost certainly be more efficient, so expect some energy cost savings. Nothing else will change; your tap and shower flows will be as they currently are.

3) Either of these^ two options, but also addressing any hot water flow issues you may have. Can the CWS be raised? Can the pipe runs be simplified? Could pumps be added?

4) Change to a combi. This will free up cupboard space, lose the tanks in the loft, be a more efficient boiler (like the other new ones above), possibly save you hot water heating costs - but likely minimal - and provide you with adequate showers. But only one at a time.

5) Swap your existing 'vented' hot cylinder for an 'unvented' type. Unvented are supplied by the mains, and deliver hot water at mains pressure and flow. But, since yours are carp in both regards, that is the last thing I'd do. Cost - along with the required new boiler - £4k upwards?

6) Either of 4 or 5, but with the addition of a pumped accumulator. These are plumbed into the cold mains as it comes into your house. It's a storage cylinder with a pump, so takes your pitiful 1.6bar pressure and boosts it to ~3bar (some to 4.5) within this tank. That will then blow your mind. It'll provide whatever flow you could possibly want, at whatever pressure. If you use it with a combi, you could fit even a 38kW jobbie, and have stupendous 'rain' showers, even two at the same time. If you use it with an unvented hot cylinder, you will definitely have stupendous showers - as many as you like at the same time (until the hot cylinder runs cold, which could be quite soon...)

Cost of accum? Around the £1k+ mark, but you need to have room for it. An attached garage could be used, for example.

Thanks very helpful.

Our shower is 3L/min and bathtub is 6L/min. It's appalling. Kids refuse to shower becasue it's just not nice. The top of the shower head to the bottom of the water tanks is about 0.5m. I can't see room to move these up significantly and we've been told a pump would be difficult for our setup.

 did consider digital showers, but these are £1k a piece.

The motivation for a combi was because If we had 10L/min shower this would be huge. I was hoping a combi would do that and along with the storage increase it would easily justify the extra £500 being quoted.

Our shower is right above the combi, so I was hoping this would help but to be honest I have no idea how they would plumb a combi in, whether it needs new pipes or not etc.

That was the temptation with a combi.

Keeping what we have is an option but clearly a harder one as we spend £3,500 and gain nothing in return other than maybe 20% off our gas bills.

Our Annual gas usage is 20,000Kwh - and we are very tight with the heating (max 18 and only boost for 1-2 hours per night in the week). so actually I expect better than 20% savings.

PS: We did get quite for a pumped accumulator, that was just under £5k alone.

As for heatpumps, we can get one for the same price as a combi, plus all new rads but unfortunately we have 8mm microbore and this counts us out as the whole lot would need repiping apparently.

FreeBear

Section62 said:

Postik said:

I do find the idea of heating a huge tank of water very wasteful.

The problem with the above is people imagining the heat from a hot water cylinder vanishing into thin air at a rapid rate.  The reality is a well-insulated hot water cylinder kept in a space like an airing cupboard is not losing heat at anything like the rate people might imagine.

80W to 90W per hour is typical for a modern insulated tank - That equates to around 2KWh per day to maintain the water temperature. If you are using a lot of hot water and/or have the heating on all the time, a DHW tank works quite well. Changing to an unvented tank would improve the water flow & pressure, but there are a limited number of plumbers qualified to install & service them.

As mentioned earlier, I had my combi installed close to the bathroom - In the hallway below to be exact. Cold water feed was already in place (part of the reason for installing in the hallway), and it was a very simple task to run the new hot water pipe straight up the wall. getting under the bath to hook up the pipes, not so easy, so I'll leave that tail for another day.