Jump to content


ebuild is sad to announce its closure - it has become too time and resource intensive to develop, manage and maintain.

However, ebuild will remain on-line in archive mode (ie no posting facilties) for several weeks so that users can use it as an information resource.

Hot Water For Annexe In Bedfordshire


  • Please log in to reply
38 replies to this topic

#1 Natattef

Natattef

    New Member

  • Members
  • Pip
  • 3 posts

Posted 27 March 2016 - 09:03 PM

Hi,

I am trying to find out information and advice for the hot water provision for the small annexe we are about to build. I've left it a bit late as it is not a self-build and work and family demands meant that I thought I could leave these decisions to "later".


Some details about the annexe, then my question at the end:

The annexe is only one storey. It is roughly 4.5m wide and 16m long (64 square meters inside floor area), with the long side south-facing. It will be build to near-passiv standards, but won't meet the passiv criteria as it is too small. Design is nothing special due to conservation area restrictions, so no large windows.

Following good reviews about MBC on this website we have chosen them to do the foundations and timber frame, and the insulation. A local builder will put on the roof, install the windows and do the external and internal finishes.

Our architect (local small firm specialising in eco projects) recommended we use an MVHR system for ventilation. I haven't ordered it yet, but have receive three (very varying) quotes with some initial design.

The annexe will have a single bedroom, small bathroom with shower, "kitchen / dining" room, and a slightly larger room (roughly 4.5mx5m) which we will use as an office for two people (both my husband and I work from home).

In terms of use of the annexe, most of the time it will be at most 2 people working in the office. Occasionally we will have guests (the bedroom has space for a double bed and a bunk bed for friends with children) and the shower will be needed.

The main house (draughty medieval, hence our obsession with passiv-standards) is not connected to gas and the oil boiler is over 20m away from the annexe. Hence we thought to heat the annexe electrically as and when required.

A while ago I had read a comment on this site (I think from J Harris) that he would prefer to use in-line water heaters instead of water cylinders (can't find that comment now :-( ), so I had in my mind that is how we would run the hot water in the annexe. However, looking at flow rates for the shower we now feel that an in-line heater won't be sufficient. Water pressure locally is good, but we don't have 3-phase electricity so I'm worried that showers in the winter may be rather pathetic.

My husband suggested we install a small cylinder (mega-flow, like we have in the house itself), just heated via an immersion heater as and when we have people staying in the annexe).

Without going into the quantitative details, does anybody have any ideas / advice around this? I have seen a few comments about hindsight being a useful thing, and hope to learn from people's experience here.

Many thanks,
Nathalie

#2 Nickfromwales

Nickfromwales

    Short cuts take three times longer.....Fact

  • Moderators
  • 8,182 posts
  • LocationSouth Wales

Posted 27 March 2016 - 10:20 PM

Hi, and welcome.
Firstly, how do you expect to deal with the conflict between the two parts of your home with a draughty section and an uber constructed new part with mvhr? Have you thoughts of integrating the mvhr into the existing part of the house also. Where do you envisage all the ducting and the main unit will go?
Anyhoo.
An electric shower seems the quickest and best for this situation. Occasional use most definitely doesn't warrant fitting an unvented hot water cylinder ( UVC ), and the basin can be done off a small 2kw instant, hidden in a vanity unit. Instant hot water, near 100% efficient! no standing losses and next to no capital expenditure. Also, an UVC will need a G3 sign off, installation by a competent person, and annual inspection and maintenance.
Who's asked about 3-phase electricity :huh: ?! An electric shower can be as low as 7.5kw, with 8.5kw as a happy medium. The flow rate with any device will be dependant on the cold mains pressure coming into the property so don't be under any disillusion about either per front better than the other as it's down to each individual scenario. These will all happily run off the incoming single phase supply btw ;)
Have you had an electrician do a survey of your current loading and the proposed additions?
It seems madness to heat the annex electrically if you have an existing heating system which you could extend relatively easily. If your at PH levels on just the annex, then a zoned circuit for the annex would be easily achievable via a low temp solution like rads.
20m for the dhw to get to the shower wouldn't be problematic either tbh, so I'd really rethink abandoning your existing heating and water system if I were you. ;). If you can't fit an electric shower then go instant for the basin and connect the shower to the boiler. :)
Regards, nick.

#3 SteamyTea

SteamyTea

    Advanced Member

  • Members
  • PipPipPipPip
  • 4,322 posts
  • LocationCornwall

Posted 28 March 2016 - 06:39 AM

Just for a change I tend to agree with Nick, you already have a heating system, use it.
This does depend on the occupancy though.

What does concern me is the use of MVHR in the new extension (which is nearly twice the total size of my house). If it is not atmospherically isolated from the main house, you are running the risk of having a dynamically unbalanced system. Be a bit like leaving a window open all the time.

You can easily work out the flow rate for a shower and then from that the power for an instantaneous heater needed. There is no harm is going larger, as long as your electrical system can handle the extra load.

#4 Nickfromwales

Nickfromwales

    Short cuts take three times longer.....Fact

  • Moderators
  • 8,182 posts
  • LocationSouth Wales

Posted 28 March 2016 - 06:51 AM

http://groceries.asd...es/910000424873
Just an example of how you can better use the money you'll save from not over engineering the dhw. :)
;)

#5 Crofter

Crofter

    Regular Member

  • Member Blogger
  • PipPipPip
  • 592 posts

Posted 28 March 2016 - 12:59 PM

These requirements are quite similar to my own project.
I'm building a c.42m2 cottage, freestanding though so no chance of connecting to my main house (which is electric anyway). Insulation/airtightness not quite at PH levels but down around 0.15, with TG and MVHR. It will principally be for holiday letting so intermittent usage. I've received all sorts of different advice- a mate who is a plumber insists that a UVC is the best thing to fit and that instataneous water heaters are never as good as advertised. It would certainly be more straightforward to just fit one device (UVC) and then pipes from that, rather than a number of different heaters. But the lack of standing losses makes some sort of instant system very attractive.

#6 Nickfromwales

Nickfromwales

    Short cuts take three times longer.....Fact

  • Moderators
  • 8,182 posts
  • LocationSouth Wales

Posted 28 March 2016 - 01:31 PM

View PostCrofter, on 28 March 2016 - 12:59 PM, said:

These requirements are quite similar to my own project.
I'm building a c.42m2 cottage, freestanding though so no chance of connecting to my main house (which is electric anyway). Insulation/airtightness not quite at PH levels but down around 0.15, with TG and MVHR. It will principally be for holiday letting so intermittent usage. I've received all sorts of different advice- a mate who is a plumber insists that a UVC is the best thing to fit and that instataneous water heaters are never as good as advertised. It would certainly be more straightforward to just fit one device (UVC) and then pipes from that, rather than a number of different heaters. But the lack of standing losses makes some sort of instant system very attractive.
Instant for you, for so many reasons.
;)

#7 jonathon916

jonathon916

    Member

  • Members
  • PipPip
  • 48 posts

Posted 28 March 2016 - 06:01 PM

Hi I have used a small Megaflow water heater in the past, works very well, with the immersion heater on night time electric. Plenty of hot water when you need it all at mains pressure and no useless electric type shower.

#8 Nickfromwales

Nickfromwales

    Short cuts take three times longer.....Fact

  • Moderators
  • 8,182 posts
  • LocationSouth Wales

Posted 28 March 2016 - 10:14 PM

An UVC is still a cold mains dependant device ;).
When extending a property, and adding to the occupancy, then you need to take extra caution which devices you specify as no cold in = no hot out :). People frequently ask how to get the extra hot water, but the point of where the additional cold supply / head will come from is more important.
Also, as much as I dislike electric showers, saying they're useless is not right. I had one for 9 years ( an industry standard Trition T80si 8.5kw ) and as long as no one else opened a hot / cold outlet whilst I was in there, it performed more than adequately. In the OP's situation, regardless of thermostatic or electric shower, the question will remain....how is it going to be fed with sufficient water to make it operate as the manufacturer states?
Fitting an UVC will NOT guarantee that the shower / basin performs well, and it's madness that some plumbers instantly value bulk stored hot water to the extent that they'll suggest spending north of ££££'s to run a shower and a basin without any survey, and evidence that it will ACTUALLY do so. :(
For my two penneth, I think the OP would be better off with an electric shower + instant hot water heater for the basin, or a single large instant inline to do the basin and to feed a thermostatic mixer shower, ( if the electric is available ), and reinforce this ( and the existing cold / hot system ) with a medium sized cold mains accumulator, if so required. As the OP states this is for occasional use, I'd bet this would work just fine :)
Regards, nick.


#9 Triassic

Triassic

    Advanced Member

  • Members
  • PipPipPipPip
  • 1,239 posts
  • LocationCumbria

Posted 29 March 2016 - 06:35 AM

A quick search for inline and instant water heaters brings up pages of the things, so which make would you recommend?

Edited by Triassic, 29 March 2016 - 06:38 AM.


#10 jsharris

jsharris

    Please ignore all posts by me, some are erroneous

  • Member Blogger
  • PipPipPipPip
  • 11,461 posts
  • LocationWiltshire/Dorset border

Posted 29 March 2016 - 08:20 AM

We have a Steibel Eltron DHC-E, which is 9.6 kW (they do a 12 kW version too) and has the advantage of being electronically controlled, so only uses as much electricity as is needed to meet the flow and temperature requirement. It's thermostatically controlled, and can have the output temperature locked to stop people fiddling with it.

Something like this may well run all the hot water needs for an annex, provided a relatively low flow rate shower head was fitted (one the same as used on an electric shower).

We lived with an electric shower for around 8 years, until I had the old system boiler changed to a combi (before there was only a head of around 1.5m above the shower on the hot water, so a shower wouldn't work). TBH, I don't think the shower we have now is significantly better than the electric one, even though it uses a fair bit more water. I remember being disappointed when I fitted it, as I'd thought that, with the much better flow rate, the shower would be a lot better. It wasn't really, but it's perfectly OK.

#11 Natattef

Natattef

    New Member

  • Members
  • Pip
  • 3 posts

Posted 29 March 2016 - 11:31 AM

Thank you all very much for your responses, really helpful!

To be clear, the new annexe is over 5m away from the house, it is not an extension, completely self contained. Mains water will come directly from the main supply to the house as the annexe is closer to that than to where the water enters the main house. Mains pressure is very good.

Moreover, the boiler we currently use is located in a shed outside the main house, over 20m away from the new annexe (the house is between the boiler house and the annexe). Connecting the house's heating system to the annexe is really not going to be (cost) efficient, unfortunately.

This online flow-rate calculator (https://www.aquahot....heaters?cat=306) tells me that if I have a 10kW electric shower, needing 40degrees increase in water temperature (in winter that seems quite realistic) I'd get a flow rate of 3.6 litres/minute. That is significantly lower than our eco shower head which (theoretically) gives 4.5 litres/min.

Would it be practical to have a single inline heater for the shower, as well as the bathroom basin and the kitchen tap (which is located the other side of the wall from the shower?) Or would we need a separate heater for the kitchen one for the bathroom sink?

So far the electrician seems to say the electric is sufficient to run all hot water electrically, but I haven't discussed this in detail with him. Will do so armed with my new knowledge!

Thanks,
Nathalie

#12 jsharris

jsharris

    Please ignore all posts by me, some are erroneous

  • Member Blogger
  • PipPipPipPip
  • 11,461 posts
  • LocationWiltshire/Dorset border

Posted 29 March 2016 - 11:58 AM

You'll never need a 40 deg increase in water temperature.

Main cold water is almost always around deep ground temperature, so around 7 or 8 deg C all year around. Ours comes in at about 8 deg C, as I measured it, together with the temperature at our shower head (and we like fairly hot showers) which was 38 deg C at the head, so the temperature increase is going to be around 30 deg, rather than 40 deg, which makes a useful difference to the flow rate.

Few electric showers are over around 10 kW, and yet many of them work very effectively, which pretty much proves that, in practice, this sort of power level works. The inline heater we have is this one: http://www.stiebel-e...e-tested/dhc-e/ . Seems to work fine with an incoming water temperature of 8 deg C, as long as the flow rate is kept down a bit.

#13 Nickfromwales

Nickfromwales

    Short cuts take three times longer.....Fact

  • Moderators
  • 8,182 posts
  • LocationSouth Wales

Posted 29 March 2016 - 07:48 PM

The higher in kW you go with the electric shower, the more susceptible it will be to shutting down if there's insufficient cold pressure, eg when someone in the house flushes a loo or a washing machine / dishwasher fills / someone is using bathing facilities etc so be very wary of jumping the kW up too high. Likelihood is that it'll spend it's most of it's working life in the low or medium setting just to be useable if it's being used at the same time that the house occupants are active.
My 8.5kw gave a more than satisfactory shower, and best to remember that this is an occasional use scenario not the main shower in the house so therefore can be engineered around that remit. Pointless owning a Ferrari if you only drive it once every 3 months, and then are stuck to 30mph ;).
Also good to remember that electric instant units of whatever guise are affected massively by seasonal rises and falls in the incoming cold mains temperature ( the temp in which the water arrives from the network ). If you have the room for a small / medium UVC then it may well be preferable if those spikes / performance swings aren't tolerable, but I'd recommend against it and consider the following instead.
I would suggest that you make provision for adding ( should it be required ) a small cold mains accumulator vessel, equivalent in size to the most average 'nuisance' cold mains consumption of the other water fed devices in the house ( mentioned above ). This would compensate, temporarily, for any cold mains deficit whilst using said instant water heater and would make a HUGE difference to the way, and characteristics of how, this / these devices all perform. More info on that if required.
Regards, nick.

#14 jsharris

jsharris

    Please ignore all posts by me, some are erroneous

  • Member Blogger
  • PipPipPipPip
  • 11,461 posts
  • LocationWiltshire/Dorset border

Posted 29 March 2016 - 08:22 PM

Nick, the Stiebel Eltron DHC-E heaters don't ever shut down if there's a pressure drop, they just modulate the power down to maintain the set temperature at the output. It's one reason they are a far better bet that a "dumb" electric shower than runs the element at full power all the time and only modulates the water flow rate to control temperature.

#15 Nickfromwales

Nickfromwales

    Short cuts take three times longer.....Fact

  • Moderators
  • 8,182 posts
  • LocationSouth Wales

Posted 29 March 2016 - 08:42 PM

View Postjsharris, on 29 March 2016 - 08:22 PM, said:

Nick, the Stiebel Eltron DHC-E heaters don't ever shut down if there's a pressure drop, they just modulate the power down to maintain the set temperature at the output. It's one reason they are a far better bet that a "dumb" electric shower than runs the element at full power all the time and only modulates the water flow rate to control temperature.
To clarify upon that, the reduction you refer to will ultimately reduce the available litres per minute, will it not ? Every instant hot water heater ( electric / gas combi ) is 100% dependant upon incoming cold water pressure, flow rate, and most importantly the stability of the delivery of that supply. My point is / was that with the annex as a stand alone situation may not be so problematic to solve, but when you add in the fact that there is an existing ( occupied ) dwelling sharing the same cold supply, the question of cold water is equally important as the one for how to provide hot water, but often overlooked.
The instant you mention will be a much better unit in terms of water delivery when the mains is fluctuating, but will still have to reduce performance to compensate. It makes it the better of the two evils, but an accumulator would still be the weapon of choice here for me, possibly in conjunction with the modulating instant if the sink and basin need hot water too.
Regards, nick.

#16 SteamyTea

SteamyTea

    Advanced Member

  • Members
  • PipPipPipPip
  • 4,322 posts
  • LocationCornwall

Posted 29 March 2016 - 08:48 PM

Does Bedfordshire have hard water, I think it does (Bucks does), so you will need a water softener, if you don't already have one.

#17 jsharris

jsharris

    Please ignore all posts by me, some are erroneous

  • Member Blogger
  • PipPipPipPip
  • 11,461 posts
  • LocationWiltshire/Dorset border

Posted 29 March 2016 - 08:51 PM

The DHC-E will vary the power as much as it can to sustain the set temperature with varying flow, from a trickle up to the maximum flow that the power level will allow it will deliver a constant temperature, as set on the dial on the front. Once you exceed the maximum flow, then the temperature gradually drops with the power at maximum. It will continue to heat the water right up to the maximum flow level of around 25 litres/min, but by that stage the water will be luke warm rather than hot.

It has a temperature lock internally, that can be set if the unit is to be fitted in a nursing home, hotel or whatever, where the hot water must not exceed the safe legal limit. If that link is in place then the temperature dial on the front only allows lower than the preset max to be delivered. It's pretty clever, as it will happily accept warm water at the input. If combined with an accumulator, and if the accumulator was inside the heated envelope of the house, then the water in the accumulator would warm up a bit, meaning that the instant heater would have less to do.

Edited by jsharris, 29 March 2016 - 08:51 PM.


#18 Nickfromwales

Nickfromwales

    Short cuts take three times longer.....Fact

  • Moderators
  • 8,182 posts
  • LocationSouth Wales

Posted 29 March 2016 - 08:58 PM

It does sound like a very good instant :). Could I be a pain and ask you what the unit costed please? ( to save me digging around ;) ).
Regards, nick.

#19 jsharris

jsharris

    Please ignore all posts by me, some are erroneous

  • Member Blogger
  • PipPipPipPip
  • 11,461 posts
  • LocationWiltshire/Dorset border

Posted 29 March 2016 - 09:03 PM

It was around £240 for the 9.6kW model we have. More expensive than an electric shower, but it's a multipoint that's sealed and needs no PRV or overflow, so is simple to fit.

#20 Nickfromwales

Nickfromwales

    Short cuts take three times longer.....Fact

  • Moderators
  • 8,182 posts
  • LocationSouth Wales

Posted 29 March 2016 - 09:18 PM

No PRV ( pressure relief valve ) and it's subsequent discharge pipework to outside ( usually by G3 stipulation ) is a big plus IMO. Sounds like this would be an excellent candidate for the OP's scenario, and an accumulator would ensure peak performance too so if fitting a cylinder of any sort here I'd make it an accumulator, not an UVC, as both the house and the annex would benefit from the reinforcement it would offer.
Regards. Nick.