Combi boiler alongside extra (unused showers)

Jamesdeer001

Hi

I was wondering if I can ask a potentially silly question please.

We currently have a combi boiler that serves one shower very well. We're going to get an extension that means that an en-suite and a downstairs bathroom will be added to the system. Radiators may also be added to the new rooms but we're exploring the effect using hot/cold air conditioning will have instead (acting as an air source heat pump).
We don't plan for the extra showers to be used all at the same time (the downstairs one is for guests/should we ever find stairs too much of a challenge and en-suite is unlikely to be used at the same time as the kids simply because my wife, myself and the kids currently don't have a routine where they overlap, so 99% of the time this shouldn't be a problem).
My understanding is that as long as multiple showers are never run at the same time, then individual shower performance won't be impacted but I'm getting conflicting information on this.
Our combi boiler is perfectly good for our needs and we don't want to change to an unvented system boiler/tank just because of the extra showers (feels an unnecessary waste of money, resources and space).

Thanks in advance...

rach_k

We have a combi boiler that was installed as suitable for a 3 bed, 1 bathroom house.  We didn't really think ahead when we got it.  We later added two extra bedrooms and a bathroom.  I asked the plumber about changing the boiler and he advised to see how we got on with it... and I'm glad we didn't waste the money. 

We sound similar to you in that shower/bath times don't tend to overlap.  Although two bathrooms are often used at once, it tends to be one is used for bath/shower while the other is used for the loo or changing after a shower, or one child will run a bath and then the other will go for a shower in the other bathroom once they've finished filling the bath.  

If we do run two showers at once, both are a bit rubbish but I've had worse - you can still wash your hair, it's just not a 'nice' shower.  Both showers are normal when they're run on their own.  

Jamesdeer001

rach_k said:

We have a combi boiler that was installed as suitable for a 3 bed, 1 bathroom house.  We didn't really think ahead when we got it.  We later added two extra bedrooms and a bathroom.  I asked the plumber about changing the boiler and he advised to see how we got on with it... and I'm glad we didn't waste the money. 

We sound similar to you in that shower/bath times don't tend to overlap.  Although two bathrooms are often used at once, it tends to be one is used for bath/shower while the other is used for the loo or changing after a shower, or one child will run a bath and then the other will go for a shower in the other bathroom once they've finished filling the bath.  

If we do run two showers at once, both are a bit rubbish but I've had worse - you can still wash your hair, it's just not a 'nice' shower.  Both showers are normal when they're run on their own.  

Thank you and is just as I thought.

Bendy_House

Just to confirm what Rach said, only the largest of combis - usually 40kW+ - coupled with a strong mains water supply, will power more than one shower at the same time. But, provided your showering times don't overlap, your current combi will continue to provide one excellent shower anywhere in the house or extension.

So, since you are confident that showers/baths won't be required at the same time, your existing setup is good to go, and it wouldn't make financial sense to try and alter it.

I presume your existing shower is thermostatically-controlled, so that any fluctuation in the water demand - someone flushing a loo, or even turning on another shower - won't actually affect the shower temperature? Ie, there is no risk of scalding or freezing?! 'All' that'd happen if someone were to make another demand on the hot water at the same time, is that the overall water flow from the shower head would drop noticeably, as the reduced hot water availability would be matched by a reduction in the cold supply to the shower, in order to keep the temp stable.

You've probably noticed that already, when someone flushes a loo or even fills a kettle with water! A brief 'ooh!', before the temp stabilises again?

On a completely different note, I'm intrigued by you considering an ASHP for just this new extension. Unless this extension was intended to become your main living space, so in constant use over winter, I cannot see the expense of this additional system being cost-effective. How would it heat the extension - directly by air fans, or via underfloor heating? The former - in my experience - could be ergonomically unpleasant - the sound and the sensation of air flow. Have you considered wet UFH instead? This would very likely be powered from the existing boiler, and should be more efficient that your existing rads, as the boiler could be tweaked down to supply a cooler water temp, tho' that might mean it would heat the rest of the house's rads as effectively - ie, not so hot.

What make and model of boiler do you have?

Apodemus

It sounds to me that you are hoping for an unlikely alignment of the planets if you think that a family of four (plus or minus guests) is not going to adapt its bathroom usage to the new situation where there is no longer competition for bathroom access.  You will face times when fights break out over who ruined the others' showers.  I'd install a 10kW electric shower in the guest/ensuite so that it is separated from the domestic system and giving you the added benefit of a separate back-up for when the boiler is out of action for any reason. 

Jamesdeer001

Bendy_House said:

Just to confirm what Rach said, only the largest of combis - usually 40kW+ - coupled with a strong mains water supply, will power more than one shower at the same time. But, provided your showering times don't overlap, your current combi will continue to provide one excellent shower anywhere in the house or extension.

So, since you are confident that showers/baths won't be required at the same time, your existing setup is good to go, and it wouldn't make financial sense to try and alter it.

I presume your existing shower is thermostatically-controlled, so that any fluctuation in the water demand - someone flushing a loo, or even turning on another shower - won't actually affect the shower temperature? Ie, there is no risk of scalding or freezing?! 'All' that'd happen if someone were to make another demand on the hot water at the same time, is that the overall water flow from the shower head would drop noticeably, as the reduced hot water availability would be matched by a reduction in the cold supply to the shower, in order to keep the temp stable.

You've probably noticed that already, when someone flushes a loo or even fills a kettle with water! A brief 'ooh!', before the temp stabilises again?

On a completely different note, I'm intrigued by you considering an ASHP for just this new extension. Unless this extension was intended to become your main living space, so in constant use over winter, I cannot see the expense of this additional system being cost-effective. How would it heat the extension - directly by air fans, or via underfloor heating? The former - in my experience - could be ergonomically unpleasant - the sound and the sensation of air flow. Have you considered wet UFH instead? This would very likely be powered from the existing boiler, and should be more efficient that your existing rads, as the boiler could be tweaked down to supply a cooler water temp, tho' that might mean it would heat the rest of the house's rads as effectively - ie, not so hot.

What make and model of boiler do you have?

Our boiler is Ideal Vogue C32 GEN2 

We're having solar panels installed on our roofs (16 panels) and are getting air conditioning installed in all upstairs living spaces (4 bedrooms). We're getting air conditioning that can also heat rooms

https://www.midlandaircon.co.uk/can-using-air-conditioning-to-heat-my-home-save-money/

https://efficientcoolheat.com/air-conditioning-more-efficient-than-radiators/

Bendy_House

Jamesdeer001 said:

Our boiler is Ideal Vogue C32 GEN2 

We're having solar panels installed on our roofs (16 panels) and are getting air conditioning installed in all upstairs living spaces (4 bedrooms). We're getting air conditioning that can also heat rooms

https://www.midlandaircon.co.uk/can-using-air-conditioning-to-heat-my-home-save-money/

https://efficientcoolheat.com/air-conditioning-more-efficient-than-radiators/

I'm not very informed about air-con units - except that there had been one installed in an extension in our house when we bought it, which I tried out a couple of times, hated (because of the air current and fan noise, even tho' this was an installed-under-the-floor-and-fed-via-floor-grills type), left alone for more a decade, and finally ripped it out when we added another extension.

In theory, at a 3:1 energy whatsit (ie 1kW of leccy in will provide 3kW heat out), they are getting close to the cost of gas; I understand that electricity is around 4 times the cost of gas per kW?

I trust you are carrying out some impartial research, rather than relying on the info provided by the manufacturers? For instance, I understand that the efficiency of ASHPs - which use the same principle - can vary quite a lot depending on the outside temp. Sadly, when it's cold outside - when its use will be greater - its efficiency is less.

Certainly, it could be useful in hot spells like we have just experienced (still pretty balmy around here...), but won't your upstairs rooms already have decent through-ventilation? What works really well for this is to allow a good draught up from the floor below, and this chimney effect can really cool down rooms quite effectively.

I fear, between the two settings, you are going to run up a sizeable leccy bill. Yes, this will be mitigated at times by the PV panels, but I think there are better ways to make use of this free electricity.

Will you be fitting batteries?

If you had the space for a Thermal Store - a type of high-quality hot cylinder - this can be charged up a good amount (sometimes it'll be all that's required) by the spare electricity produced by your PVs. This could then provide you with DHW, and also supply wet UFH in your new extension.

Personally, I'd be looking to continue to use gas over electricity due to it being much cheaper, but would also be looking to employ these PV panels as much as possible.

I could be completely wrong as I don't know much about these A/C units you are considering, but I suspect they won't provide the savings you hope for, and could also be unpleasant in use. BUT, I don't know.

Thanks for the info on the boiler - that's a good size. One more check, please - do you know what the cold incoming mains flow is like? That can make a large difference to how well a shower is affected by taps being turned on! If you don't know, then you can place a large pan under your cold kitchen tap (or garden tap), run it on full for exactly 10 seconds, measure the number of litres produced, and 'times' that by 6 to get litres-per-minute.

Apodemus

Bendy_House said:

Jamesdeer001 said:

Our boiler is Ideal Vogue C32 GEN2 

We're having solar panels installed on our roofs (16 panels) and are getting air conditioning installed in all upstairs living spaces (4 bedrooms). We're getting air conditioning that can also heat rooms

https://www.midlandaircon.co.uk/can-using-air-conditioning-to-heat-my-home-save-money/

https://efficientcoolheat.com/air-conditioning-more-efficient-than-radiators/

... you can place a large pan under your cold kitchen tap (or garden tap), run it on full for exactly 10 seconds, measure the number of litres produced, and 'times' that by 6 to get litres-per-minute.

Better to do the full minute with a bucket!  It could take 3 or 4 seconds to fully open and fully close the tap!  

Bendy_House

Apodemus said:

Better to do the full minute with a bucket!  It could take 3 or 4 seconds to fully open and fully close the tap!  

Pffft - you haven't seen my wrist-action.

(did I say that out loud)

Section62

Jamesdeer001 said:

Our combi boiler is perfectly good for our needs and we don't want to change to an unvented system boiler/tank just because of the extra showers (feels an unnecessary waste of money, resources and space).

The government has plans to phase out the use of gas for domestic heating.  Whilst there's no immediate threat to the availability of gas boilers, it isn't unreasonable to assume that the future of domestic water heating in larger homes will involve some amount of hot water storage - the alternatives may be limited to instantaneous electric water heaters using (peak-rate) electricity.  Some people see hydrogen as a replacement for 'gas' but personally I'm not optimistic about that.

Even if you don't install a vented/unvented hot water cylinder now, I think you may eventually regret not including space for one within your overall extension plans - the space won't be wasted if you use it for storage of other stuff now, and would save additional costs and upheaval in the future if you found it necessary to switch to a stored system.  Adding an extra 1m2 to a planned extension is not that expensive - creating a 1m2 space within an existing building envelope can be very expensive.

Furthermore, if you are getting solar PV installed now you should have a think about whether a good use of the excess electricity you generate  would be to sink it into a hot water storage tank - this is what a lot of people with PV find economic.

And if you are looking at heat pump solutions for your space heating needs, then the logic of retaining a gas combi boiler for (only) heating your water on demand starts to look flawed.

Plasticman

We have two bathrooms (3 toilets) in a house of 4, sometimes 5, people and we've had no issues with our Combi boiler. As others have said, if you run two showers at the same time it's "OK" rather than "great" but perfectly workable. It's surprising how infrequently that happens though!