Selecting DHW cylinder for ASHP + solar thermal

[muhrix]

Hello,

 

I am in the process of replacing the LPG tank in my property with an ASHP (Mitsubishi Ecodan 14kW).

 

I currently have 2 solar thermal panels which cover all hot water demand for 5-6 months in a year (and I'm planning to keep them).

 

It has been really difficult for me to find a suitable DHW cylinder that (in my limited understanding) would work well with ASHPs and solar thermal panels. Even harder to find options where I'd be able to monitor water temperature at different heights. Ah - the cylinder volume should be 250 litres.

 

I really like the idea of Mixergy tanks because of volumetric heating, the plate heat exchanger (more efficient), the diffusers (for better stratification) and all the data I can get from the tank. However, they are very expensive and, for the ASHP+solar thermal versions, I cannot do volumetric heating (I've been told so by Mixergy). More annoyingly, it seems there is no dedicated solar volume, so it'd be hard to make the most of the already installed solar thermal panels if I want hot water in the morning.

 

The pre-plumbed Mitsubishi cylinder ticked a lot of boxes, but it has 3 pumps and my installer warned me of audible noise (the cylinder is to be installed in a bedroom cupboard). It's also very expensive.

 

I looked at options from Gledhill, Joule, Kingspan, Heatrae Sadia, ... and neither of them looked perfect.

 

Yes, I am likely overthinking here, but would anyone have any advice or experience they could share with me? I haven't ruled out any options yet.

 

Thanks a lot!

[Iceverge]

https://www.cylinders2go.co.uk/shop/stainless-steel-unvented-cylinders/telford-tempest-300-litre-twin-coil-solar-and-heat-pump-unvented-cylinder/

[muhrix]

27 minutes ago, Iceverge said:

https://www.cylinders2go.co.uk/shop/stainless-steel-unvented-cylinders/telford-tempest-300-litre-twin-coil-solar-and-heat-pump-unvented-cylinder/

Thanks for the suggestion!

 

I came across that model, but I couldn't find any more technical details regarding dedicated solar volume or energy rating (kW) of coils (surface area is a little subjective). I'll try and contact Telford and ask for it.

[Gus Potter]

13 minutes ago, muhrix said:

Hello,

 

I am in the process of replacing the LPG tank in my property with an ASHP (Mitsubishi Ecodan 14kW).

 

I currently have 2 solar thermal panels which cover all hot water demand for 5-6 months in a year (and I'm planning to keep them).

 

It has been really difficult for me to find a suitable DHW cylinder that (in my limited understanding) would work well with ASHPs and solar thermal panels. Even harder to find options where I'd be able to monitor water temperature at different heights. Ah - the cylinder volume should be 250 litres.

 

I really like the idea of Mixergy tanks because of volumetric heating, the plate heat exchanger (more efficient), the diffusers (for better stratification) and all the data I can get from the tank. However, they are very expensive and, for the ASHP+solar thermal versions, I cannot do volumetric heating (I've been told so by Mixergy). More annoyingly, it seems there is no dedicated solar volume, so it'd be hard to make the most of the already installed solar thermal panels if I want hot water in the morning.

 

The pre-plumbed Mitsubishi cylinder ticked a lot of boxes, but it has 3 pumps and my installer warned me of audible noise (the cylinder is to be installed in a bedroom cupboard). It's also very expensive.

 

I looked at options from Gledhill, Joule, Kingspan, Heatrae Sadia, ... and neither of them looked perfect.

 

Yes, I am likely overthinking here, but would anyone have any advice or experience they could share with me? I haven't ruled out any options yet.

 

Thanks a lot!

I'm a bit of a philstine..

 

My sister lives on Tiree. She has a big house used as a B&B with lots of demand on the UF heating and hot water. The house is served by twin ASHPs all facing the prevailing wind which pretty much blows all the time. Temperatures rarely drop to a level ( say below -4.0 deg C) where the pumps really need to work / think hard. They cycle fine on the defost mode and so on.

They also have a big water cylinder and the controls are pretty simple. Any excess heat energy is contained within the building envelope any way so it's trapped inside the building.

 

Have a look at what is outside your house, sheltering effects from the wind and so on.. think / start / review from the basics.. is the location right? Am I going to get good air flow outside without the fans having to work too hard? It's easy to neglect this. Often you can't see the woods for the trees.. you know you want one but don't carry out a full review of your design.. often because you don't want bad news.  The design of ASHP's is an iterative process just like Architectural Design.

 

Yes the tank (Tiree) may be over sized and the controls simple but when you look at the long term costs in terms of maintenance it's a no brainer. If you have the environment in mind remember that if you make your system complex every year at least one part is going to go faulty.. person in a van to fix..plus often top up of inhibitor.. parts will fail just before or during the xmas period so you'll get no points here.

 

Look the simple stupid option, some times it's cheeper / more practical in the long term to over size say the HWC but keep the works inside simple. Keep the controls as simple as you can. Yes it's tempting to get carried away at this stage but once the novelty has worn off and you get back to normal life or want to sell the house on.. imagine trying to explain to a prospective buyer how the system works!

 

I would go back and force yourself to look at how simple and stupid you can make this. Compare the costs over the long term then take a view on what suits yourself.

 

 

 

 

 

 

 

 

 

 

[muhrix]

Thanks for sharing your sister's use case, Gus. You make some very good points in your comments. I appreciate your advice.

[Gus Potter]

1 minute ago, muhrix said:

Thanks for sharing your sister's use case, Gus. You make some very good points in your comments. I appreciate your advice.

Hiya muhrix.

 

You'll find lots of different takes on BH.. the great thing about BH I find is that you get loads of ideas,  food for thought and friendly support.

 

To get the best out of BH, just keep asking questions. But try if you can to provide some detail on your particular circumstances. This way you'll get the best feedback. I'm definitly not an expert on ASHPs compared with some of the other members of BH!

 

 

 

 

 

 

 

 

[Iceverge]

1 hour ago, muhrix said:

Ah - the cylinder volume should be 250 litres.

 

How did you come by this number? With low temperature heat sources like solar thermal in the shoulder seasons and ASHP (50 deg) you will need larger than your current LPG setup at 70deg, 

 

 

 

I think i’d go for the largest simplest cylinder you could fit. 

 

If you want something bespoke Newark cylinders are helpful. 

[Gus Potter]

49 minutes ago, Iceverge said:

 

How did you come by this number? With low temperature heat sources like solar thermal in the shoulder seasons and ASHP (50 deg) you will need larger than your current LPG setup at 70deg, 

 

 

 

I think i’d go for the largest simplest cylinder you could fit. 

 

If you want something bespoke Newark cylinders are helpful. 

This does my head in.. Not you Iceverge it's the formula!

 

Yes I'm a philistine but I want hot water to come out the hot water tap!

 

If you go to parts of Africa the air is that hot 40 deg C plus, I have lived there! If you have towels to sterilize, a greasy pan to clean after frying a Mars Bars you need 60 deg water!

 

Fine if you can live this way.. you can also spend money on a boiling water tap.. but it will take you an age to fill a bucket of water to clean the floor after the dog has left a message.

 

Yes I recognise there is no free lunch here but from my experience your average member of the public when buying a house is not going to be too keen on purchasing your house when the water temperature is the same as what comes out of a cold water tap in parts of Ibiza?

 

For the self builder looking to improve their asset value and make it attractive to a wider market if you wish to sell on.. keep it simple stupid and save your money. Insulate the house to death, detail the design so that you don't get condensation that compromises the structure. Find the right way for you for extracting energy from the outside.. that may be your local  gas / electricity supplier or ASHP/ ground pumps / wood fire stoves etc. Also balance this against how much heat comes in during say the summer and how how you get rid of that.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Edited November 17, 2021 by Gus Potter

[dpmiller]

As well as Telford, have a chat with the guys at World Heat cylinders, they've no problem doing bespoke and already supply some of the ASHP companies.

[ProDave]

I have my hot water in the tank at 48 degrees.  That is plenty.  That temperature was found by experiment, if i run a kitchen sink full of water using just the hot tap, I can just, and only for a short period, immerse my hands in it.  There is no point having washing up water hotter than you can put your hands in.

 

Occasionally if I want to soak something pre wash, e.g. a baking tray with something baked on then that is a case for some really hot water (as you won't be putting your hands in it)  for that I run some water from the boiling water tap.

 

I think the Mixergy tank concept is something that all tanks pretty much do anyway.  I am happy with my Telford heat pump tank, and I would have thought the twin coil one linked above would do, but go for at least 300 litres.

[JohnMo]

I like to think of it simply.

 

Coil for heat pump, coil for solar thermal.

Set ASHP Stat as suggested at 48/50 degs.

 

Base case, ASHP heat tank to 48 deg.

If solar heat availability is there, have a separate thermostat, to allow tank to max heat the cylinder will allow.

 

You can achieve this with an unvented cylinder or thermal store. 

 

Next question is do you need a buffer for your heating?  If so, this could use the volume of water within the thermal store as part of the heating circuit. A two port valve and pump controlled by your central heating thermostat.  Your heat pump only has to heat water in tank, no sanitiser program required, as DHW flows through a coil in the thermal store.

 

Why do you need to monitor water temp at different heights?

 

 

 

 

 

[Iceverge]

10 hours ago, Gus Potter said:

This does my head in.. Not you Iceverge it's the formula!

 

Yes I'm a philistine but I want hot water to come out the hot water tap!

 

If you go to parts of Africa the air is that hot 40 deg C plus, I have lived there! If you have towels to sterilize, a greasy pan to clean after frying a Mars Bars you need 60 deg water!

 

Fine if you can live this way.. you can also spend money on a boiling water tap.. but it will take you an age to fill a bucket of water to clean the floor after the dog has left a message.

 

Yes I recognise there is no free lunch here but from my experience your average member of the public when buying a house is not going to be too keen on purchasing your house when the water temperature is the same as what comes out of a cold water tap in parts of Ibiza?

 

For the self builder looking to improve their asset value and make it attractive to a wider market if you wish to sell on.. keep it simple stupid and save your money. Insulate the house to death, detail the design so that you don't get condensation that compromises the structure. Find the right way for you for extracting energy from the outside.. that may be your local  gas / electricity supplier or ASHP/ ground pumps / wood fire stoves etc. Also balance this against how much heat comes in during say the summer and how how you get rid of that.

 

 

 

Maybe I wasn't clear. I was only suggesting OP should consider water storage temperature before committing to a 250l tank. Our taps are at about 50deg (same method as @ProDave) 

 

We use about 300l of water at 40deg (baths+showers+handwashing etc) and about 20l at 50deg per day. I disregarded the lower volume and higher temperature water for simplicity in the formula.

 

In our case we get 400l of shower water from our 300l cylinder stored at 70deg but if we stored it at 50deg we would only get 200l.

 

10 hours ago, Gus Potter said:

keep it simple stupid and save your money

Agreed. We have a direct 300l E7 cylinder. 10kWh per night or €365/annum for hot water (2 adults +2 kids). Here's the complex control system and tank. Cost about €700. 

[dpmiller]

4 hours ago, JohnMo said:

Next question is do you need a buffer for your heating?  If so, this could use the volume of water within the thermal store as part of the heating circuit. A two port valve and pump controlled by your central heating thermostat.  Your heat pump only has to heat water in tank, no sanitiser program required, as DHW flows through a coil in the thermal store.

 

 

I'm doing this except with a coil to segregate, as the thermal store also has input from a boiler stove. Stove is on the open vented side (store contents) and all flow from the ASHP passes through the lower coil of the tank en-route to the UFH and rad circuits.

[markocosic]

Are you at your PV limit?

 

If not PV the roof and rely on PV plus ASHP for hot water. 

 

You'll produce more than enough hot water and the excess electricity is worth far more to you than excess heat.

[muhrix]

Thanks a lot everyone! Catching up with the helpful comments and suggestions after a long working day.

 

More context of my use case: we just moved to our new house (refurb. 1970s). It's a 4-bed house and we are 2 adults + 2 kids.

We will get a 14kW Mitsubishi Ecodan ASHP running at flow temperature of 55C. I'm already having to change 11 radiators for much bigger sizes and cost-wise (and radiator size-wise) I just couldn't go with a lower temperature even though I would have liked to.

 

The house already had thermal solar panels and we really enjoyed the free hot water between July (when we moved in) and September.

 

18 hours ago, Iceverge said:

How did you come by this number? With low temperature heat sources like solar thermal in the shoulder seasons and ASHP (50 deg) you will need larger than your current LPG setup at 70deg,

Well, I currently have a 250-litre cylinder and it seemed more than enough. I'd expect to heat the tank up to 50C with the ASHP, but you make a good point. I also checked information online and given the property size and number of occupants, a cylinder with 250-300 litres should be enough.

 

11 hours ago, JohnMo said:

Next question is do you need a buffer for your heating?

I don't think I do. My installer was going to confirm, but said if I end up needing one, it would be a small 50-litre tank. I'm going for an unvented cylinder.

 

9 hours ago, JohnMo said:

Why do you need to monitor water temp at different heights?

I am after data. Not only from the cylinder, but from the whole system. Ecodan's Melcloud and the MMSP will give me lots of data.
It's like a hobby. I enjoy DIY/electronics/software and home automation... If I can improve the overall system performance by 5% and have fun, it's a win-win.

 

Sure, there is a limit; the time and budget at my disposal as well as considerations such as those that @Gus Potter highlighted (I'd still like a system that works if I were to sell the house as he said).

 

@markocosic Unfortunately I don't have PV (there were 2 solar thermal panels already installed). I am trying to make sure I can take advantage of PV, but it's not something I'll afford in the foreseeable future.

 

I am going to check the Newark and World Heat cylinders as suggested.

 

@Iceverge would you happen to have a reference or more context regarding the equation you wrote down? I'm curious to try and understand the theory/rationale.

 

One interesting point I picked up is how people normally heat their cylinders (it's the first time I have one).

 

It seems that one method is to heat to temperature and keep topping up given some hysteresis.

The other method I came across was setting the timer to heat the water at given times and manually turn to top up when needed.

A third alternative approach is to do what Mixergy proposes to do (estimate demand) and heat accordingly.

Is there another method?
 

Depending on the method, I can see it would have an influence on the most appropriate volume.

 

Thanks again, everyone! I'm really enjoying the discussion!

 

[Iceverge]

46 minutes ago, muhrix said:

would you happen to have a reference or more context regarding the equation you wrote down

 

From experimentation I found the average temperature of my tank is 30 Deg when it's no longer providing 40 Deg water at the top.

 

After that it's just calculating the total energy in a body of water before and after. Putting the two formulas equal to each other and cancelling. 

 

Mass X deltaT X specific heat capacity of water 

[SteamyTea]

1 hour ago, muhrix said:

We will get a 14kW Mitsubishi Ecodan ASHP running at flow temperature of 55C.

That is towards the very top temperature for an ASHP.  It will kill the CoP.

 

[muhrix]

19 hours ago, SteamyTea said:

That is towards the very top temperature for an ASHP.  It will kill the CoP.

 

Yes, it is a high temperature. It is actually the highest flow temperature permitted for me to still get the RHI grant.

 

According to the MCS certified product directory, the SCOP at 55C is 3.26. If I were to reduce to 50C or 45C, the SCOP would be 3.52 or 3.77, respectively.

For the RHI, I'd have to go down to at least 42C in order to get SCOP of 3.9 and increase the amount I'd get over 7 years.

 

My rationale was: going down to 42C to get an extra £700 over 7 years would not pay off the cost of much larger radiators and the additional cost of replacing them (pipework, etc.).

I believe that, once the system is up and running, I can experiment with the weather compensation curve and lower the flow whilst keeping the house comfortable to get a better SCOP.

 

If I'm honest, the 14 kW also feels oversized to me, but the surveyor who did the heat loss calculation issued me a very thorough report, answered all of my (probably silly) questions and used a commercial software to input measurements, etc. The house is already fairly well insulated, not a lot more I can do about that.

[muhrix]

20 hours ago, Iceverge said:

 

From experimentation I found the average temperature of my tank is 30 Deg when it's no longer providing 40 Deg water at the top.

 

After that it's just calculating the total energy in a body of water before and after. Putting the two formulas equal to each other and cancelling. 

 

Mass X deltaT X specific heat capacity of water 

Thanks a lot!

[rh2205]

We have a grant aerona3 13kw in in a big refurbished house with larger rads, they have quite good SCOP and are cheaper. I’m sure there is some reason on here someone will know about why our ASHP is rubbish but it came with 7yrs installer warranty so I can’t complain really, we have a pretty basic install 300l tank (+50l buffer) & no solar.

[rh2205]

Should add our scop is 4.49 at 45C would of been 4.11 at 50C we managed to get some big rads in. Maybe we are weird but we have our water thermostat to 45C, at times I’ve thought about turning it down as it seems plenty warm (though if someone knows a reason we shouldn’t?) we do have two dishwashers though ?

Edited November 19, 2021 by rh2205

[SteamyTea]

41 minutes ago, rh2205 said:

Maybe we are weird but we have our water thermostat to 45C, at times I’ve thought about turning it down as it seems plenty warm (though if someone knows a reason we shouldn’t?) we do have two dishwashers though

Turn it down till you are not happy, nothing to worry about.

Most dishwashers take a cold feed only, some older ones may take a hot feed.  Same with washing machines.

[muhrix]

2 hours ago, rh2205 said:

We have a grant aerona3 13kw in in a big refurbished house with larger rads, they have quite good SCOP and are cheaper. I’m sure there is some reason on here someone will know about why our ASHP is rubbish but it came with 7yrs installer warranty so I can’t complain really, we have a pretty basic install 300l tank (+50l buffer) & no solar.

I don't think there is anything wrong with Grant heat pumps (the solar thermal panels I have are from Grant).

 

The Aerona3 SCOP look really impressive! I really couldn't justify the cost and disruption of installing radiators big enough to go lower than 55C (as much as I would've liked to).

That said, according to their specs, I'd need to go for the 17kW model as at 55C the ASHP would deliver 10kW where I am, while the house demands ~13.4kW.

 

Price-wise, I don't think they are too dissimilar from what I found online.

 

I too plan to experiment and keep both flow temp and DHW temp as low as possible.

[rh2205]

@SteamyTeahaha I meant we have two dishwashers so don’t really try to get hot water out of the taps & they definitely run off cold we are a bit old fashioned and have a kettle for tea though! 
 

Our install from a MCS accredited place was quoted lower so probably why I thought grant were cheaper because all the other quotes were more (including non mcs installers) & didn’t come with a buffer so assumed grant must be cheaper!! Plumbers are expensive round here regardless of their certification ?

[joe90]

I too like KISS (keep it simple stupid), a member here in the past (Jeremy) designed his own weather compensating computer controlled heating system but eventually changed it out fir one room stat. He knew that as it was bespoke if it broke nothing “off the shelf” could replace it and he admitted it did the same job. I adopted the same technique (as I also am a Luddite and don’t understand all the mathematics some here quote ?) I too have an ASHP set to 48’, 300 litre DHW tank and wet UFH downstairs only. Frankly I copied others here with similar properties and it works well, Very little to go wrong ?‍♂️.