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Heating & hot water for temporary accommodation


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Suggestions please on the best/most economic way to provide hot water and heating without using gas in the following situation:

 

A 7m x 7m x 2.4m, well insulated, temporary accommodation for us while we build (max 1 year hopefully), which will ultimately be used intermittently as part craft workshop/part gym.

 

  • size (approx 7m x 7m) which will ultimately be split into two rooms
    • Walls are concrete block dry lined with 100mm insulation and OSB board
    • Floating floor - 50mm insulation & board, insulated ceiling
    • There will be a simple MVHR system in place to provide ventilation
  • Living area ( to become a craft room), bedroom, shower room, small office (to become gym/workshop)
  • Living area will have a log burning stove in
  • 12kw solar panels on the roof ( for use with house also ultimately)
  • Eventual use will require hot water  - preferably instantaneous, as it will be used occasionally and often for shortish periods . Heat from the log burner in the craft workshop will be sufficient.
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Fit the WBS and use that. Plug in electric heaters on other rooms if needed.  Fitting a WBS to our static caravan was one of the best things we did to see us through a particularly bad winter.

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6 minutes ago, ProDave said:

Fit the WBS and use that. Plug in electric heaters on other rooms if needed.  Fitting a WBS to our static caravan was one of the best things we did to see us through a particularly bad winter.

What is a WBS please?! 

 

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23 minutes ago, SuesieG said:

What is a WBS please?! 

 

An air pollution device that prematurely ends millions of peoples lives every year.

 

WBS = Will be Buried Soon.

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WBS is an expensive and inconvenient solution in a building with such low heat demand. 

 

Your heating load will almost certainty be less than a 2kW resistance heater. I dragged one home from the dump for free and it heats out passive house just fine. A stove would cost £1000 minimum and wouldn't be as healthy or safe or convenient. 

 

A cheap electric shower and an undersink water heater for the hot taps. 

 

 

 

 

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31 minutes ago, Iceverge said:

cheap electric shower and an undersink water heater for the hot taps. 

But 

 

5 hours ago, SuesieG said:

12kw solar panels on the roof

Unless it is not going to be connected until the house is finished.

200 Lt cylinder best and a 3 kW element at the base.

With that much PV, probably get DHW all year round.

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Thanks... We have a wood burning stove so that's no cost & don't see the point of storing hot water for future intermittent use so we had come to the conclusion too that either a single powerful electric water heater to do sink & shower or seperate ones is best. Then add electric resistance heaters where necessary.. At least one in the shower room! As we have no experience of these we were looking for reassurance that this would be best. 

 

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Buy and temporarily install the unvented hot water cylinder ( UVC ) that will end up in the house, and spend once.

Electricity may be an issue with all this resistive heating, so what supply do you have? Fuse board aka consumer unit ( CU ) already in? You’ll need a 60a supply minimum, plus any resistive cooking will be on top. 
A single powerful instant water heater will be 12-16kW, and need an enormous amount of electricity. Over a year you’ll probably pay for the UVC, certainly when you add up buying and installing all this kit, plus you’ll be able to pee faster than the water comes out of an instant water heater, unless it’s 3-phase.
Just buy the UVC and put it in a 2nd hand shed / temp outdoor structure.

You should have a really long think about what will end up in this outbuilding and install the long-term kit now. If it’s to be a gym, you’ll likely be fit from A/C, so maybe fit some split units now for heating ( as these units will provide heating and cooling ) and, again, purchase once. PV will run cooling and divert to domestic hot water ( DHW ) IF it is a storage device and NOT instantaneous. 
How far from the house to this building? 

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Posted (edited)

Couple thoughts:

You mentioned it will be well insulated, but the quoted figures (100mm / 50mm) don't match that. Around here you'd need 200mm on floor walls and roof to be well insulated.

 

Depending what degree of activity the gym needs, in my experience you need lots of ventilation and cooling. This means making sure the WBS is not on when the gym is being used, and an MVHR alone is not sufficient for a gym.

Generally if either the wbs or gym are in use I expect you'll need a window or 2 open. And wbs probably means low degree of airtightness anyway. Overall I'm doubtful you'll get much benefit from an MVHR over the long run. 

 

With that much solar I'd also consider batteries and an instant water boiler 

Edited by joth
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39 minutes ago, Nickfromwales said:

Heater or cylinder? 

Well if serious about using the wbs for DHW it would have to be a cylinder, but it all depends on the expected usage patterns. If it's less than daily use I wouldn't have thought a cylinder would have much value. Certainly in summer you won't want to fire up the wbs just to get some hot water so some sort of electrical source DHW is needed 

 

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A couple of questions.

 

Is the garage built already?

 

Are you planning on using it year round in the future?

 

Like Nick said how far is it from the house? 
 

What is your planned heat source for the new build? 

 

Do you know the heating load for the new house?

 

Do you have a free source of timber and enjoy the labour of processing it and cleaning ashes etc?

 

Does the garage have a chimney already?

 

 

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12 hours ago, SuesieG said:

We have a wood burning stove so that's no cost & don't see the point of storing hot water for future intermittent use so we had come to the conclusion too that either a single powerful electric water heater to do sink & shower or seperate ones is best.

12kWp of solar would give you a huge amount of excess, and if your DHW is instantaneous you would defo still be buying electricity even on a bright sunny day for all times where the irradiance isn’t at max / other base loads are not satisfied. With a cylinder and an immersion you could store all the excess PV generation as DHW ( plus other things ) and save a LOT of money each year as that setup would provide free DHW for prob north of 6 months of the year to both the home AND the annex. That 12kWp would diminish to sub 4kWp for the 3 months of winter, so will not even scratch the surface for space heating with resistive heaters. Used a heat pump ( which will run the split A/C, and that 4kWp will be equivalent to 12kWp again because of the CoP of the heat pump. Please ask for an explanation if you do not fully understand any of my blabbering ;) 

 

The reason I ask about how far the annex is from the house is because it should be quite easy to tether the annex to a centralised plant in the house. My current clients want a WC and wash basin ( + DHW ) in the detached garage, so I have ducted between the house plant ( UVC location ) and the garage to run plumbing inside. I intend pulling 2x10mm pipes ( hot and hot return ) together, mummified with Armorflex neoprene insulation, to give the few sporadic handfuls of DHW p/a that the garage requires. A PIR sensor ( aka occupancy switch ) will trigger a pump which circulates DHW in a return loop to give instant DHW to the WC basin hot tap, and when the room is vacated the pipes will just cool and go cold again. The hot return will be required to stave off the “dead leg” that would be created if it was fed with just a single hot ‘leg’, but only really a requirement if the hot supply pipe run exceeds 25m ( risk of legionella ). Upsize that hot feed to a 15mm DHW supply and a 15mm hot return and feed it from the house and that’s your all of your DHW to that’s annex done for a few hundred quid of pipe and insulation.

 

Space heating could be completely via A2A split A/C units afaic, and I would save the cost of the WBS install and use those funds to pay for the A2A system. A/C gives cooling in summer, essential for a gym(?), and space heating in the winter. The summer A/C will run from excess PV generation.

 

The insulation levels are sub BRegs so are by no means admirable, and floating floors are cold-ventilated, so space heating in the winter wouldl benefit from the excess heat from the WBS if you do fit it as the floors will be quite a significant cold bridge during the worst of the winter, but it will likely give off too much heat for all other times, with that heat contained to the room that the WBS is in. MVHR will not distribute that heat btw. Another issue is, the chimney of the WBS will be a cold-inducing ventilation heat loss demon, which will be constantly reducing the room temps for all the times that it’s not lit. Going for a room sealed appliance would resolve this, but it sounds as though you already have a WBS?

 

The MVHR is a bone of contention, because if you don’t get the air tightness detailed, AND tested, and get a score of <1 ACH ( air changes p/hour ) the MVHR will do next to zilch or less.

 

Use this time and the advice available here to make some informed decisions ;) 

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10 hours ago, joth said:

Well if serious about using the wbs for DHW it would have to be a cylinder, but it all depends on the expected usage patterns. If it's less than daily use I wouldn't have thought a cylinder would have much value. Certainly in summer you won't want to fire up the wbs just to get some hot water so some sort of electrical source DHW is needed 

 

No mention of DHW via WBS?

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Minisplit for heat / cool and small 50/80 litre unvented cylinder with small immersion mounted at high level in bathroom if not in a cupboard.

 

PLenty for a single shower. Small immersion helps keep it all on pv. Gives you a redundant system Vs main house for some resilience.

 

Avoid burying pipes IMO unless the shower is stupidly powerful. Avoid electric shower as that won't make good use of pv. Wood burning stove is a pita and pollution.

 

 

 

 

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Posted (edited)
22 hours ago, Nickfromwales said:

No mention of DHW via WBS?

I'm afraid I don't understand your question 

Edited by joth
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10 hours ago, joth said:

I'm afraid I don't understand your question 

My comment was prompted by yours;

On 02/07/2022 at 12:04, joth said:

Well if serious about using the wbs for DHW it would have to be a cylinder,

AFAIK there is no mention here about the WBS providing DHW. It would be a hugely complex installation for an annex, and require a lot of components / plumbing / header tanks etc.

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On 02/07/2022 at 10:23, joth said:

Couple thoughts:

You mentioned it will be well insulated, but the quoted figures (100mm / 50mm) don't match that. Around here you'd need 200mm on floor walls and roof to be well insulated.

 

Depending what degree of activity the gym needs, in my experience you need lots of ventilation and cooling. This means making sure the WBS is not on when the gym is being used, and an MVHR 

On 02/07/2022 at 18:04, Iceverge said:

A couple of questions.

 

Is the garage built already?

 

Are you planning on using it year round in the future?

 

Like Nick said how far is it from the house? 
 

What is your planned heat source for the new build? 

 

Do you know the heating load for the new house?

 

Do you have a free source of timber and enjoy the labour of processing it and cleaning ashes etc?

 

Does the garage have a chimney already?

 

 

Yes garage is existing, built from big "figure of 8 shape " concrete blocks with a screeded floor & high cement fibre pitched roof. 

We intend dry lining with 75mm PIR insulation and OSB board. Floor to be similar with mdf flooring, ceiling to be put in with about 30mm Rock wool. Putting in the small mvhr which is in the existing house which I know does help even in a leaky old house - especially in bathrooms.but I understand its limitations. There will also be opening windows. 

No chimney but have existing twin walled flue and logburner to be reused and a free supply of timber which we are well used to using - farm! 

Garage is 30m from house. 

We have 3 phase available at the fuse box. 

Heating load for new house will be low as building to roughly passivhaus standard.

My craft workshop and the gym will be used year round

 

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6 hours ago, Nickfromwales said:
On 02/07/2022 at 12:04, joth said:

Well if serious about using the wbs for DHW it would have to be a cylinder,

AFAIK there is no mention here about the WBS providing DHW. It would be a hugely complex installation for an annex, and require a lot of components / plumbing / header tanks etc.

 

 

Ahh I was going on this comment in the OP:

 

  • Eventual use will require hot water  - preferably instantaneous, as it will be used occasionally and often for shortish periods . Heat from the log burner in the craft workshop will be sufficient.
 
 
I thought that was saying Heat energy from the WBS would be sufficient to supply the DHW. Now I see that second sentence is probably about space heating and nothing to do with DHW?
 
If so, great. That matches what I was driving towards too.
 
yes, 12kW PV + battery plus an instance water heater sized appropriately for whatever it is delivering to. (Shower? utility sink? hand basin?). A cylinder is not a good choice for something only used occasionally 
 
 
 
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On second thoughts (the cost of those 50/80 litre cylinders in the UK) then subject to distances what @Nickfromwalesdescribes with a buried DHW recirc loop from the main house that isn't recirculated unless the room is occupied would make more sense. You're buying the power cable and cold water feed anyway so you might as well bung a pair of pipes with closed cell insulation on them in a duct in the same hole. Provided that the usage truly will be intermittent that is.

 

I wouldn't plumb in an unvented cylinder on a "temporary" basis though. Lots of faff if you're paying somebody else to do that. I'd chuck in a cheap electric shower instead even if it's just for handwash / dishwash / toolwash use.

 

In ongoing operation you PV > Heatpump > Cylinder in house > Undergound pipe OR PV > Minisplit.

 

 

If it's a long way from this outbuilding to the house then I'd still chuck in an 80 litre wall hung cylinder.

 

https://www.waterheater.shop/en/products/electricwaterheaters/80-liter/eldom-spectra-80-liter-boiler-manual-control/

https://www.modernheat.co.uk/product/80-litre-tesy-bi-light-electric-hot-water-cylinder/

 

If you were a European / American you'd rely on the safety valve to discharge a bit of cold water as the tank heated up instead of installing an expansion vessel to take up that water. Not legal in the UK but...for a shed on a building site... 😇

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3 hours ago, markocosic said:

How high is the roof? What will the finished ceiling be? Can you make it a cold roof and put 30 cm of insulation on top of that not 30 mm?

Yes I meant 30cm 🙈🙈 cold high roof hence the lower insulated ceiling at 8ft so we can use whole osb boards! 

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Try to avoid compromising your decision making by bending the design to shoehorn an existing (or planned) bit of equipment into the plans. The solar PV and the WBS are pushing you towards this here.  It took me a long time to get out of this mindset as we're in a similar position re timber. However living with a a stove in a cottage was a pain in the end. Too hot when lit or too cold when it wasn't, messy and labour intensive. Putting one in our new passive house would have been a list of compromises as long as my arm. 

 

As the building will be used long term

(and longer than you think for the build) you really need to get the fabric right. 

 

I'd bump to 100mm PIR in 2 layers with the joints staggered and taped in the top layer. Put a membrane under the roof joists and tape that too. If done well

(use a DIY blowerdoor fan to check) your heating requirement will be tiny. Probably nothing with 2 bodies in residence. A 5kw minimum WBS would be unusable without opening all the windows and letting a howling gale blow through. 

 

As you're going to be using it as a gym long term a mini split with cooling is ideal. Sell the WBS and the flue to pay for it.  However of you don't want to bother with the cost initially an electric heater is about £30. Sell the farm timber and it'll more than cover your electricity bill. 

 

As for the DHW. I lived with an instant shower for most of my life and it was perfectly fine. An instant 10.5kW shower will give you about 5l/m for £70. You'll have a useable independent system if the main house ever breaks. Put a small undersink water heater (£140) under the kitchen sink and run a 10mm Hep2O feed to the wash basin. 

 

No need to complicate anything with solar PV. No G3, expansion vessels or discharge pipework. You'll have a fully functioning DHW and Heating system for £250.  

 

 

 

 

 

 

 

 

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