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Hot water cylinder in unheated space

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(@peter269)
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I have a question about where it is possible to put a hot water cylinder.

My house is a 3-bed semi with loft insulation, and an uninsulated slate roof. A company has quoted for a 7kW Vaillant Arotherm Plus, and a 190l Unitower cylinder. My present cylinder is 100l, but MCS says I need at least 180l and there is no room in the airing cupboard for a larger cylinder, also there is nowhere in the house for a 'plant room'. The company's suggestion is to mount the heat pump on the side of the house, at 1st floor level over a flat-roof extension, and the Unitower and associated components (and possibly a buffer tank) in the loft.

I have some concerns about the siting - poor access, possible noise transmitted through the wall, and weight on the ceiling joists. An alternative position for the heat pump would be the back garden, with the cylinder etc. at the back of the garage. The garage is brick-built with a concrete slab roof, and is detached from the house by about 3m. You can't get a car in the garage since the extension was built, so it's really just a man-shed. Access for installation and servicing would be much better in this position, though the company has said that you can't put the Unitower in an outbuilding - is that true? I would think that under sub-zero conditions, the loft probably gets colder than the garage, though I've not checked that.

Either way, I am concerned about having the hot water cylinder in an unheated space. The company has said they will box it in with insulation board - is that acceptable, and would it apply equally whether the cylinder is in the loft or the garage?

Some calculations:- the Unitower spec requires an 'environmental temperature' of 7 to 35 deg, and standby loss is given as 1.2kWh/24h (presumably at 7deg). So that's a loss rate of 50W. If I assume the loft (or garage) can go down to -3, then my box will have a 10 degree drop across the insulation. Assuming the cylinder etc. (and possible buffer) is cased in a box 2mx2mx1m, and that I use insulation board for the four sides and the top, it will have an exposed area of 14m2. So the r-value needs to be at least (10x14/50) = 2.8 K.m2/W. I think that would need at least 75mm Kingspan or similar, which is quite thick.

Do you have experience with this? Is it 'normal practice', is it even doable? If not, then I don't think I can have a heat pump.


   
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(@bontwoody)
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Hi Peter

My first thought is why if you have managed with a 100l cylinder are they insisting you have a larger one? At the end of the day it is your decision. Surely the MCS is a guideline?

That said there are a couple of options to reduce the size you need. Mixergy cylinders heat the water from the top down and give more usable hot water than a standard cylinder so you can manage with a smaller one. I have one of these and think its very good. The other way to go is not to use a cylinder but a thermal storage device like Sunamp Thermino which are much smaller. Both of these options are more expensive than a standard cylinder however.

Like you I would probably prefer the garage option if sticking with the standard cylinder, i cant see why the pipework could not be run underground in say an insulated sewer pipe, but would be extra to do and it might be easier just to go with the suggestions above.

If the company are being very inflexible I would try and get some other quotes. you could try Octopus or any heatgeek installers in your area.

House-2 bed partial stone bungalow, 5kW Samsung Gen 6 ASHP (Self install)
6.9 kWp of PV
5kWh DC coupled battery
Blog: https://thegreeningofrosecottage.weebly.com/
Heatpump Stats: http://heatpumpmonitor.org/system/view?id=60


   
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(@peter269)
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@bontwoody Thanks for the reply. My understanding is that MCS is not just a guideline but a requirement. If you don't meet the requirement you don't get a certificate, and without a certificate you don't get the £5K grant.

I have looked at the mixergy tank, and I think it is a great idea. However, even the slimline version won't fit in my airing cupboard which is only 520 mm wide.

I intend to get a second quote, but I just want to check that what I'm asking for is legit so that I know what I'm talking about before I meet them

Peter


   
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(@bontwoody)
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@peter269 I havent gone down the MCS route Peter, so Ive pretty much done as I want. Are they really stipulating the size of the cylinder based on the house rather than the occupancy or usage? If that is the case, then there are companies who will build a custom cylinder to your specifications (Newark for instance). It still might be cheaper than paying for the extra work involved in siting the cylinder elsewhere and then any loses are heating your house rather than being lost to the outside.

A Thermino is a definite possiblity but i dont have any experience of how well they work with heat pumps, but Im sure someone on here must have. If you have solar panels, they could be used to raise the Thermino to a higher temperature via an electrical heater.

Mark

House-2 bed partial stone bungalow, 5kW Samsung Gen 6 ASHP (Self install)
6.9 kWp of PV
5kWh DC coupled battery
Blog: https://thegreeningofrosecottage.weebly.com/
Heatpump Stats: http://heatpumpmonitor.org/system/view?id=60


   
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(@jamespa)
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@peter269 

The MCS spec has some wiggle room, also it says 'should' not 'shall' and 'guidelines'.  An out of the box thinking MCS installer will work with this to deliver what you reasonably want.  I found two at least that were willing simply to duplicate the size of my 150l cylinder even though its way below the guidelines

 

 

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This post was modified 6 months ago by JamesPa

   
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(@derek-m)
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Posted by: @bontwoody

@peter269 I havent gone down the MCS route Peter, so Ive pretty much done as I want. Are they really stipulating the size of the cylinder based on the house rather than the occupancy or usage? If that is the case, then there are companies who will build a custom cylinder to your specifications (Newark for instance). It still might be cheaper than paying for the extra work involved in siting the cylinder elsewhere and then any loses are heating your house rather than being lost to the outside.

A Thermino is a definite possiblity but i dont have any experience of how well they work with heat pumps, but Im sure someone on here must have. If you have solar panels, they could be used to raise the Thermino to a higher temperature via an electrical heater.

Mark

I think that the water cylinder size is based upon the number of bedrooms and hence possible occupancy. You could try claiming that one of the bedrooms is an office or study and see if that gives a better result.

 


   
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Toodles
(@toodles)
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@derek-m I have some experience in this area as I have what is classed as four bedrooms; it is actually two plus an office and a ‘media room’ - it is just that they are all on the first floor. I feel it is time that the MCS recognised that such arrangements are possible. As per MCS requirements, 4 bedrooms requires that we have a larger tank than we could accommodate, 210 litres (our previous gas boiler powered CH and DHW featured a 99 litre tank which we never ever exhausted). Neither a slimline (too tall) tank or a shorter but larger diameter tank would fit in our airing cupboard. The loft space doesn’t seem to be favoured by installers and would have involved much extra pipework to place any tank up there (and the unwanted losses as mentioned); putting a tank out in the garage was not a practical option either!!! We decided to go with a Sunamp Thermino system and I’m glad we did. Having had to reject a Vaillant ASHP as impractical due to the gas they use and having windows and doors in the area we wished to site the ASHP, we decided on the Daikin but… there LT systems do not run hot enough to feed heat to a Sunamp system as they require 60 degrees to work well. Hence, we went for the ePV210 unit which works from electricity only; we have solar PV tp provide most of the energy and need not resort to the grid very often. The Sunamp is in our airing cupboard and so any losses (under 0.75 kW/h per day) are ‘in the right place’ anyway. As I say, we looked at a loft mounted DHW possibility but it really wasn’t for us. We have the blending valve on the Thermino’s oulet and we dispensed with the shower pump at the time of installation and all is fine. Regards, Toodles.

Toodles, 76 years young and hoping to see 100 and make some ROI on my renewable energy investment!


   
Derek M reacted
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(@jamespa)
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Posted by: @derek-m

I think that the water cylinder size is based upon the number of bedrooms and hence possible occupancy. You could try claiming that one of the bedrooms is an office or study and see if that gives a better result.

 

You are correct, its the greater of (bedrooms +1 or actual occupancy)*45l.  See however my comments above about the flexibility in MIS3005-D, although doubtless only some installers will be prepared to invoke this.  There is a get out if the calculated cylinder size doesn't fit and the requirement is anyway only a 'should', which MCS define as 'intended to be complied with unless reasonable justification can be given'.  Customer refuses, is adamant about his requirements, and has taken into account the characteristics of heat pumps seems to me to be 'reasonable justification'!

@peter269  just bear in mind that the MCS requirements, and the fact that, in order to get what you know works for you, you may have to 'persuade' installers or claim a bedroom is a study, are all in justified by MCS on the grounds of 'consumer protection'. 

Hopefully you are grateful to MCS for protecting you in this way, and to the government for making MCS mandatory if a heat pump is to be installed under permitted development rights and thus forcing the MCS requirements on people in most cases even if they wish to install without a grant.  

This post was modified 6 months ago 5 times by JamesPa

   
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(@bontwoody)
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@peter269 as luck would have it a new post with underground insulated pipes, presumabley MCS installed as its a new build.

https://renewableheatinghub.co.uk/forums/renewable-heating-air-source-heap-pumps-ashps/sensors-positioning-for-ecodan-8-5kw-ashp-with-pre-plumbed-cylinder-and-buffer

looks like you have several different options to explore now 🙂

House-2 bed partial stone bungalow, 5kW Samsung Gen 6 ASHP (Self install)
6.9 kWp of PV
5kWh DC coupled battery
Blog: https://thegreeningofrosecottage.weebly.com/
Heatpump Stats: http://heatpumpmonitor.org/system/view?id=60


   
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(@peter269)
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Joined: 6 months ago
Posts: 4
Topic starter  

Thanks to all who have contributed to this thread, it has been very useful.

So it seems that MCS has some wriggle room (though 180 down to 100 litres might be a wriggle too far). That's good.

The Thermino seems to be the ideal solution. I have checked, and even the 210 hp (equiv size 192 litre) would just fit in the existing airing cupboard if necessary, though the smaller 150 hp (128 equiv) would be adequate as there are only two of us in the house.

It greatly simplifies the plumbing - if I have the heat pump in the garden next to the garage, it would just need two pipes under the patio from the heat pump to the kitchen where it can join to the existing boiler flow and return. Nothing at all needed in the garage. The two pressure vessels could go in the loft or at the top of the airing cupboard. I might go for a small buffer tank (don't ask me to explain why - that requires a separate thread!), but that could comfortably go in the loft in place of the two existing header tanks, and not weigh much more. Another advantage is that I won't need a secondary return, which I would have needed with a tank either in the loft or in the garage.

So I now feel I have enough information to go back to the original company for a requote, and also to get second and third quotes.

Thanks again, great job.


   
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(@peter269)
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Topic starter  

I'm re-opening this thread which I thought was closed, because I've now been studying the Thermino spec. The Thermino uses a phase change material (PCM) which has to melt, so it requires an incoming flow temp of 65-80 degrees. In fact, the spec for the Vaillant compatible model (150 hp-VT) says to set the target DHW temperature to 70' on the Vaillant controller - a lot higher than if I used a regular cylinder.

My concern is - what SCOP will I get at that high flow temperature? My Vaillant documentation only gives SCOP for flow temperatures up to 55 degrees, but if I extrapolate to 70 degrees, I estimate a SCOP of 2.68 for the 7kW Arotherm Plus - that sounds quite optimistic.

Does anyone have the Thermino/Arotherm combination, and do you know what SCOP you get for DHW?


   
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