Twin coil HW cylinder - worth it?

[d438a1]

I'm just finalising my new ASHP system to replace existing LPG boiler. I'm also about to get solar installed, with 12kw generation and 16kw battery.

 

It's a larger 18kw 3 phase ASHP as it will be powering heat and hot water to main house, garage annexe and a separate workshop. He's specified 300ltr HW cylinder and 500 litre powertank and pump as our mains pressure is terrible

 

My ASHP installer doesn't seem hugely on board of specifying a twin coil HW cylinder and seems to think a heat pump cylinder would be fine with normal immersion.

 

What is consensus here?

 

Thanks

[JohnMo]

23 minutes ago, d438a1 said:

twin coil HW cylinder

What would you be doing with the second coil? A normal heat pump cylinder is perfectly adequate with a single coil. Just get one with around 3m² coil. Job done.

[d438a1]

Maybe it's my simple misunderstanding! The ones with twin coil seem marketed at using excess solar to heat up water?

[JohnMo]

If it's solar PV you heat via the immersion, using a diverter. Wouldn't bother with solar thermal these days, it's an expensive one trick pony.

 

But if you are getting an MCS solar PV then just export excess and get paid for it. If a self install divert to hot water via immersion.

 

Either way a 2 coil cylinder is for solar thermal. It also takes away a chuck of cylinder volume dedicated to solar thermal.

[d438a1]

Ah I see. That was me being dumb on that front. 
 

Yeah it’s MCS install. I’m hoping to get the full 11-12 export given we are moving to three phase. 
 

I haven’t done full maths yet to compare export vs HW divert yet.

 

On the HW front. Is there any guidance to help on tank size? In the house we will likely have max of 2 bathrooms. In the annex for rental it was also likely have 2 bathrooms. Is 300ltr too small for this?

 

Thanks 

[Nickfromwales]

34 minutes ago, d438a1 said:

Ah I see. That was me being dumb on that front. 
 

Yeah it’s MCS install. I’m hoping to get the full 11-12 export given we are moving to three phase. 
 

I haven’t done full maths yet to compare export vs HW divert yet.

 

On the HW front. Is there any guidance to help on tank size? In the house we will likely have max of 2 bathrooms. In the annex for rental it was also likely have 2 bathrooms. Is 300ltr too small for this?

 

Thanks 

I'd go 500L without a second thought. 300L is defo not big enough here. Each property would want a 250-300L to itself. What is heating the annex? 

 

An ASHP likes to give hot water at around 55^oC, the immersion will boost this to a possible 85^oC, but you should only use the 85oC maths for useful DHW capacity if you are not going to export everything to line your pocketbook.

 

You should focus on as close to 100% self-consumption as is possible, so house(s) base loads 1st, DHW priority 2nd, space heating 3rd, charging batteries 4th, and then export.

 

Focus on things that have sporadic use outside of your control, such as your tenants DHW use, to best economise. A bigger cylinder heated when it is advantageous to do so will ensure a reserve capacity that is nearly never heated under duress (peak rate periods etc).

[JohnMo]

9 hours ago, d438a1 said:

annex

Are you paying the utility bills for a rental?

[d438a1]

Yeah I will be paying bills for rental for air bnb.

 

the garage annex will be heated from the ASHP as well. 

 

the annex is about 20-30 metres from house. The idea is plant room will be in annex. So HW cylinder and power tank will be in there with ASHP nearby outside 

 

 

[Nickfromwales]

2 minutes ago, d438a1 said:

Yeah I will be paying bills for rental for air bnb.

 

the garage annex will be heated from the ASHP as well. 

 

the annex is about 20-30 metres from house. The idea is plant room will be in annex. So HW cylinder and power tank will be in there with ASHP nearby outside 

 

 

Is this somewhere you'll be staying long-term?

 

[d438a1]

Yes I think so. Why do you ask?

[Nickfromwales]

1 hour ago, d438a1 said:

Yes I think so. Why do you ask?

Then I’d ditch the idea of the umbilical. It’ll have significant heat loss for life, and the costs will only go up over time. 
 

I would get a Panasonic mono block heat pump (sub £3k) and fit one to the annex for heating and DHW. Then choose another for the house, and run them as 2 separate systems.

 

If you can swallow the costs for losses then fine, but be aware that it needs factoring in.

 

Having a cold water booster out there that creates an artificial cold mains for the 2 dwellings to share is fine, just know that you’ll need at least a 32mm cold mains pipe from the pump and accumulator to the house to not suffer losses of the stored energy over distance. I’ve done a good few of these with great results, and seen others that fail for a proper lack of understanding as to how the pipework needs to be upsized to suit.

 

No good trying to empty a 4 pint pitcher with a tiny straw.

 

I’d seriously reconsider your options here, for heating and hot water, is my 2 cents. 

[d438a1]

Thanks for the insight. 

 

We are fighting a few competing things in coming up with solution. One of the main things is we may knock down and rebuild house in the near distant future. So by housing all ASHP, HW, CW as well as new electric meter, etc in garage annex it somewhat helps with that. 

 

If it helps, we are looking at using insulated pipe like this? https://www.flowsupplies.co.uk/shop/if-bq32-32-25-20-isoflow-32-32-25-20-quadruple-heating-sanitary-dhw-160mm-duct-2067?category=21#attribute_values=350,388,454,456

 

If I wasn't clear, the new workshop/studio is a further 15m to other side of house which will also make use of ASHP with UFH in the slab. 

 

 

[JohnMo]

3 hours ago, d438a1 said:

we are looking at using insulated pipe like this

So reality check - spending the same price on piping as an ASHP? If you are knocking down why are spending a fortune on ASHP and heating system?

 

Pipes you linked to are pretty small dia. The pipe sizes will not support your huge heat pump, they are too small for you cold water main with the booster. I have a long run from ASHP to house and use 28mm pipe for that, your 18kW will have to support 3x the flow! You really would need a secondary return loop for DHW to make hot water usable in the house. So you need an additional pipe.

 

If you are knocking down and rebuilding, your heat pump will be huge and no longer appropriate for new house. A new build to slightly better that building regs is unlikely to need any more than 6kW. While your house is being built the 18kW ASHP will not support your rental and workshop - it will be too big and short cycle to an early death.

 

Suspect some more thinking time is needed, your current plan doesn't work!

[d438a1]

I'm really appreciating the feedback, thanks.

 

Knocking down maybe in 15 years earliest. Also a possibility we won't.  I think regardless of 1 or 2 ASHP, we like the idea of locating heat pump away from house.

 

But yes, it is a lot of money, but idea is we are spending as little as possible in actual house, as for whatever reason the previous owner really over specc'd on radiators.

 

The cold water will likely be mdpe or insulated mlcp as it will be deep enough in trench. The hot water return is therefore in that quad. 

 

I've attached a photo to show. So green line is heat pump(s) location, and other side of that wall is where plant room will be. That garage will be converted in to a 2 storey living block, with UFH on ground floor and rads on 2nd floor. It's approx 12x6m.

 

Pink line is trench to the old boiler outhouse where it can easily connect to existing pipework for CH, H and C. It then connects in that outhouse in yellow line out to workshop which will be around 8x5m.

[Nickfromwales]

Can't the ASHP and plant go into the outhouse, ASHP to the rear/side elevations for convenience, so you have the shortest umbilical possible?

 

For the annex, defo a stand alone ASHP there and a simple UVC for DHW, in isolation, and job done. You're adding a huge run there for UG pipework, which if the DHW HRC is in there too will be recirculating at 60^oC+!

 

This is less than great if it's just lower grade heat or a very short run, but at the temps and losses you'll have I think it's one to avoid.

 

I'd seriously look at having the UVC's in the respective spaces that they'll service, and even look at oversizing them so they can be heated via off peak electricity directly via the immersions, as that will massively reduce the labour on the heat pump(s) and the losses from the HRC etc.

 

With modern insulated UVC's and their very low standing losses, I'd go for DHW from electricity, and local ASHP's for space heating, and drop the HRC and UG pipework to the very minimum requirement.

 

Put the accumulators and pump set (if you need a pump) in the annex, no probs there.

 

Any need for heating or hot water in the outbuilding (orange)?

[d438a1]

Well the idea is to have zero equipment in the house so when it's likely knocked down everything can still operate. We did originally consider the ASHP on that side elevation but it's going to be a patio area as well as it gets evening sun. 

 

In the outbuilding, yep it will have UFH in the concrete slab and also hot water. 

 

A lot of different options to consider...

[JohnMo]

29 minutes ago, d438a1 said:

Knocking down maybe in 15 years earliest

So not really on the agenda, not even worth discussing it so far in to the future, so much can change in that time. 12 years ago I moved in to my forever home - since built a house elsewhere and sold the forever home. All your equipment could be ready for scrap heap if you don't install well in 15 years time.

 

Have actually sized the heat loads you need? No idea the insulation levels of the additional building or the house? The heat pump could be big or it could be way undersized?

[Nickfromwales]

1 hour ago, d438a1 said:

Well the idea is to have zero equipment in the house so when it's likely knocked down everything can still operate. We did originally consider the ASHP on that side elevation but it's going to be a patio area as well as it gets evening sun. 

 

In the outbuilding, yep it will have UFH in the concrete slab and also hot water. 

 

A lot of different options to consider...

Fit the UVC in the house, remove and reinstate when (if) you rebuild. £1500 or so isn’t huge and the cost of the pipework and running costs of the UG multi core pipework will be 4x that. 

[d438a1]

Yeah the MCS installer did the heat loss calcs. I’ve asked him for the detail. Bit rough maths was 12kw for house, annex 4kw and workshop 2kw.

 

I think I’m coming round to idea of UVC in house outhouse. If it’s to be purely electric fed would I not need a heat pump type? I think he had spec’d the uk cylinder elite heat pump one. 
 

What abput one ASHP for house and workshop and 1 for annex? But both located at garage…

[JohnMo]

20 minutes ago, d438a1 said:

heat pump type

Thread here on them

 

24 minutes ago, d438a1 said:

If it’s to be purely electric fed would I not need a heat pump type

To get grant, heating and DHW have to be driven by an ASHP according to BUS rules. 

[d438a1]

Oh I see…there goes that idea then…

[d438a1]

Does it count if UVC for annex is still powered by heat pump?

[JohnMo]

All it says it has to heated by a heat pump, it doesn't say from only one heat pump, or from multiple heat pumps. So a heat pump cylinder is fine, an immersion only cylinder not fine.

[d438a1]

So just so I'm clear on options...

 

Option 1: -

• 18kw 3ph ASHP and UVC in annex supplying heating and hot water for all 3 buildings (annex, house and workshop)

 

Option 2: -

• 18kw ASHP in annex powering heating for all 3 buildings
• UVC in garage connected to heatpump supplying annex only
• UVC in house powered by electric only for hot water in house and workshop

 

Option 3: -

• 12-16kw ASHP powering house and workshop located at annex
• 4-6kw ASHP powering annex located at annex
• UVC in garage connected to heatpump supplying annex only
• UVC in house powered by electric only for hot water in house and workshop

 

I think that captures it? Question is...does option 2 and 3 still allow for grant?

 

Thanks

[Nickfromwales]

3 hours ago, d438a1 said:

Oh I see…there goes that idea then…

Not quite 

 

You leave them do the grant job, and then turn DHW off, and heat vis the immersion. The HP will possibly want to do a legionella purge, weekly iirc, but the temp being above that means the HP should ignore it. May need to be programmed to do that but easy enough to achieve.

 

4 hours ago, d438a1 said:

I think I’m coming round to idea of UVC in house outhouse. If it’s to be purely electric fed would I not need a heat pump type? I think he had spec’d the uk cylinder elite heat pump one. 

I should have been clearer, sorry, the UVC would always be connected to the HP for the grant. My point was, you can still just use electricity. It would be good to see if your installer will upsize the cylinder slightly, as then you are guaranteed to go the whole day without needing to top it up, after a midnight > 4-5am recharge.

 

You'd still need the UG pipe but it would be largely redundant, and you wouldn't be paying for those losses 24/7/365.