Jimlad

Large property, 10 bathroom ASHP Advice

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Hi all,

 

I'd like some advice please - i've spoken to two installers and had two completely conflicting opinions so some impartial advice would be really helpful.

 

The property is 309 m2 and is a large shared house with 10 ensuite bedrooms, essentially a small hotel I suppose. Building fabric will be at the following U-Values:

 

Ground Floor slab - 0.12 - with wet underfloor heating - 50m2 approx

External walls - 0.21

Roof - 0.13

 

So far, the two options suggested are:

 

Option 1

Steibel Eltron WPL 25 AS ASHP

100l Buffer tank

2 x three-phase Heat Interface Units - just for DHW when demand outstrips ASHP capability

 

As i understand it the ASHP pre heats the HIU heat exchangers which pass through DHW and only kick in when the flow rate or overall demand necessitates the extra electric power to maintain temp.

This gives us a benefit in not having to store large amounts of hot water. I'm concerned that the heat pump isn't man enough to tackle the floor area in question however. No in depth heat calcs done yet but assumed at 45w/m2 = 13.9KW as a minimum.

 

Option 2

Vaillant Hybrid system with ASHP and Gas System boiler

Unvented cylinder/s -  size not defined yet

 

Explained to me as better than option 1 as the boiler will always be there to pick up any slack for either DHW or Heating.

 

We have to use renewables as a condition of planning and an ASHP specifically. I'm keen to claim against the RHI as well.

 

I've no experience with renewable technology on any previous projects so any nuggets of wisdom greatfully received!

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Are the bathrooms fitted with a bath, or just showers?

It does also depend how creative you want to be and how much you want to spend.  Nothing to stop you having 10 smaller ASHPs, but it gets expensive.

It also depends if you can put up with the occasional cold bath/shower.

Also have a look at commercial CO2 heat pumps, they can deliver to a higher temperature.

Are you fitting any other renewables like PV, if so, how much.

May be worth looking at solar thermal for this sort of thing.

Edited by SteamyTea

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What is this "heat interface unit"?

 

I have a Stiebel Eltron DHC-E 8/10 modulating instant 10KW water heater in the output from my UVC.  It s there to cover multiple showers / baths in quick succession faster than the ASHP can re heat the tank, to prevent "no hot water" issues.

 

Normally it sits there and does nothing as it's temperature is set slightly lower than the UVC temperature but if the water starts to run cool it will heat it up to maintain a hot water source.

 

I have no way of knowing how many times it has "saved the day" but there were at least 3 "ran out of hot water" incidents before I installed it.  not a single complaint since it was installed.

 

Firstly you are going to need to know the worst case space heating requirements for the building to size the ASHP then think about how you are going to deal with the hot water needs. One very large tank or several smaller ones or something other than the ASHP for DHW.

 

More details on the building. You only mention 50 square metres of UFH but the building is 309 square metres so 6 floors?

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1 hour ago, SteamyTea said:

Are the bathrooms fitted with a bath, or just showers?

It does also depend how creative you want to be and how much you want to spend.  Nothing to stop you having 10 smaller ASHPs, but it gets expensive.

It also depends if you can put up with the occasional cold bath/shower.

Also have a look at commercial CO2 heat pumps, they can deliver to a higher temperature.

Are you fitting any other renewables like PV, if so, how much.

May be worth looking at solar thermal for this sort of thing.

There will be 10 showers, no baths and running out of hot water is a definite no-no!

We are restricted by the design of the roof so cannot fit any PV, so presumably solar thermal is also out of the question. Site also precludes GSHP.

Thanks for the mention of CO2 heat pumps, i've not read up about those so will do some reading around that option.

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Are your showers all planned to be gas?

 

The other challenge you have here will be water flow and pressure, and there are I think showers which have their own tank and are electric, which would make it more resilient overall.

 

When I looked at an 8 bed HMO I was planning to use cubicles of the sort that come complete with shower and whb, and potentially loo, which you carry in and just connect to the pipes and the power.

 

Might be worth a look into how eg Travelodge do this. 

 

F

Edited by Ferdinand

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36 minutes ago, ProDave said:

What is this "heat interface unit"?

 

I have a Stiebel Eltron DHC-E 8/10 modulating instant 10KW water heater in the output from my UVC.  It s there to cover multiple showers / baths in quick succession faster than the ASHP can re heat the tank, to prevent "no hot water" issues.

 

Normally it sits there and does nothing as it's temperature is set slightly lower than the UVC temperature but if the water starts to run cool it will heat it up to maintain a hot water source.

 

I have no way of knowing how many times it has "saved the day" but there were at least 3 "ran out of hot water" incidents before I installed it.  not a single complaint since it was installed.

 

Firstly you are going to need to know the worst case space heating requirements for the building to size the ASHP then think about how you are going to deal with the hot water needs. One very large tank or several smaller ones or something other than the ASHP for DHW.

 

More details on the building. You only mention 50 square metres of UFH but the building is 309 square metres so 6 floors?

The heat interface unit is a heat exchanger and instantaneous heater contained in one unit. It uses the ASHP to heat the DHW via the exchanger and then the instantaneous heater kicks in when necessary. (or at least as a layperson that's how I understand it.) Info attached if you're interested. Model in question is the WSG-2-DUO.

 

I've attached the plans so you can see the layout. It's actually 3 floors but the 1st and 2nd extend over a shop so the building is larger at those levels than the ground floor.

1st floor.JPG

Ground Floor.JPG

STE_TB_Heat_Interface_Units_en_INT.pdf

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Two things that occurred to me this morning.

Can you get a large enough electricity supply?

Worth splitting the DHW and the Space Heating systems as they do different things, at different times and at different temperatures.

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You could fit a natural gas ‘hybrid’ air source HP and a pair of size 12 HW+I Sunamps, with the SA’s doing hot water only. The SA’s are fitted with 3kW immersions so you can heat the SA’s off cheap rate grid electricity plus you will have boost / failsafe options to boot ( eg full DHW capability still available even if the ASHP goes offline ) plus you can also heat them off the high temp HP via the hydraulic heat exchanger. The 2x SA units will give you the equivalent DHW capacity of a single 600L UVC but with no G3 requirements or annual maintenance schedule ( and related annual inspection costs ) plus very low standing losses compared to traditional cylinders.


Have the SA’s set to heat once a night on economy rate electricity ( E7 / 10 ) via the immersions to deal with the bulk DHW requirement with near zero system fatigue / maximised overall system longevity ( as the ASHP will then be moth-balled most of the year ) and just use the HP to top up DHW during the daytime if required. 
The SA’s ( 2x ) will hold around 26-28kWh of heat energy so will provide probably 8-9 of the daily showers, with gas+hybrid bridging the gaps only. As it’s basically a “commercial” installation in what appears to be an HMO, system longevity ( and increased reliability ) will be  an important factor. Remember this will attract ‘non-domestic’ RHI which is a 20 year period but at a lower £rate, so I would look closely at preserving the capital expenditure by increasing the lifespan of the system vs trying to squeeze every ounce out of the RHI as replacing the equipment early will neutralise any benefits thereof. 

 

Great that you have wet UFH aka low temp emitters on the ground floor, as ideally you’d have the HP provide space heating at as low a flow temp as possible ( to avoid the gas element of the hybrid kicking in and therefore maximise the CoP ). What about your intended solution for 1st & 2nd floors space heating emitters ? If rads then they need to be over sized to give useful heat at lower flow temps.


Alternatively, you could fit 10x 8.5kW electric showers and upgrade the electricity supply ( if there isn’t already 3-phase on site that is ), and then just fit a small 150L HP UVC to provide the rest of the DHW needs ( sinks and basins ). 

Regardless of the way you provide DHW you will definitely need to boost the cold mains with a single or pair of cold mains accumulators to get anywhere near running a few of these showers simultaneously ( plus allowing for WC’s being flushed / basins being run / utility taking cold water etc ) or, which you may already be planning, each self contained dwelling would need its own cold mains supply ( & meter ) to keep up with peak cold mains demand. 
Early system planning, and consideration to everything else that will be needed to fortify your base question will be of paramount importance ;)  
Pointless having a whopper of a DHW system if there’s nothing to force that water out of the outlets ( particularly at the 2nd floor height ).

 

Another option is separate the DHW and heating, as above, and fit 3x 12’s and a pair of low temp ( regular ) HP’s for space heating only. Really needs some numbers crunching tbh. 
 

Confirmation on the situation WRT electricity supply 1 / 3 ph would help. 3-ph HP’s are available. 👍

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