What size Hot Water Cylinder for family of 5?

[Meabh]

Hi all

 

What size of hot water cylinder would you do? Installer has quote 250L preplumbed but ICF shell builder suggested might want bigger cylinder as heat up time is 79mins.

 

Family - 2 adults 3kids (boys currently aged 6,10,12 if that matters...). 4 bed house, 4 bathrooms (Office is same size as bedroom so could class as 5 bed).

ICF - super insulated, ACH 1, MVHR, UFH downstairs only etc

 

We are having

ASHP - Mitsubishi Ecodan 11.2KW Coastal. Heat loss is 9.6kW as we are like a bungalow with a bit on top but Mitsubishi only do the 8 or 11KW size ASHP. Local installer, well recommended for install and servicing in the area but only uses Ecodan.

 

Go bigger or is 250L enough?

 

Many thanks

 

 

[joth]

Do all 4 bathrooms have showers? With 5ppl in the house, 3-4 showers could easily be used at once (if your water pressure & flow are up to it).

The boys are approaching the ages where showers will get longer too.

I'd be looing at 300L minimum. 400L if you have space for it. A 11kW ASHP will recharge it pretty well.

An alternative / addition to oversizing tank is to store water at a higher temperature. If you get the new R290 Ecodan it will heat up to 70C, obviously at a hit to COP but an option if you're space constrained and think you'd only "sometimes" need the higher heat energy storage capacity.

 

Place it somewhere central and the heat losses will offset your heating demand in winter. (Think about mitigation if you do get overheating in summer)

 

You say it's super insulated, but 9.6kW suggests it's either >400m2 floor area, or not that thermally efficient really. Do you have an airtightness target, and did the heat loss calc take it into consideration? (The MCS protocol typically doesn't)

[JohnMo]

Heat pump cylinder size guide

 

[JohnMo]

Our 195m2 big vaulted ceiling in every room and lots of glazing bungalow, uses around 3.5kW at -9, so yours must be huge, or the heat loss calcs are wrong.

[ProDave]

We have 300L and find that okay for familly of 3. I would not go smaller.  Heat up time is slower for us with only a 5kW ASHP

 

My observations is it is the ladies, especially if they have long hair, that spend a lot of time in a shower.  It sounds like you only have one lady in your household so long showers may not be an issue.

[Dan F]

We have 300l for family of 4 and wouldn't go smaller.   Our effective capacity is increased due to use of WWHRS, also if we want to get more showers out of the tank (e.g. due to visitors) we increase tank temperature.  How many showers you get is a function of tank size + tank temp + wwhrs, and of course shower length.
 

 

 

[jayc89]

Whilst we were down to 1 bathroom and were showering "serially", 250L was fine for us. When our old ensuite was still functioning, 2x concurrent showers would drain the UVC. When we refurb the bathroom, which is where the cylinder is located, I plan to swap it out for a 400L one, joists allowing... 

[joth]

48 minutes ago, ProDave said:

My observations is it is the ladies, especially if they have long hair, that spend a lot of time in a shower.  It sounds like you only have one lady in your household so long showers may not be an issue.

FWIW I suggest this could prove a fragile way to specify requirements. A six year old child may yet grow up to enjoy growing their hair out very long, regardless of gender - who knows what the fashion will be for post-zoomer generation. A teenager of dating age may desire long shower times regardless of gender. The future owners of a 5 bed house may have any various needs for  many long showers or baths.

[jack]

If you're using an ASHP, I'd go larger than the recommendations suggest. A larger volume allows you to heat more water to a lower temperature, giving you the same useful output but allowing a higher COP.

[IanR]

1 hour ago, Meabh said:

Family - 2 adults 3kids (boys currently aged 6,10,12 if that matters...). 4 bed house, 4 bathrooms (Office is same size as bedroom so could class as 5 bed).

 

We are having

ASHP - Mitsubishi Ecodan 11.2KW Coastal. Heat loss is 9.6kW 

 

Go bigger or is 250L enough?

 

Personally I'd go larger, and with a similar set up, did.

Family of five also, but with guests or a future owner it could be more.

I went with 500l UVC, and can have all occupants showering within a 1 hour period, without running out of water, but the temp at the top will drop to 40°C from 50°C. I've not timed it, but recovery to 50°C is within the hour I allow DHW to have priority, from a 12kW ASHP.

I was happy to go larger as I planned to divert PV to the tank, which I really must get around to...

But, the 12kW ASHP can't modulate down to my space heating energy requirements, on all but the coldest day, so I have a 4 port buffer. Should I replace the HP in the future I'd likely downsize to 8kW.

[jack]

2 minutes ago, IanR said:

But, the 12kW ASHP can't modulate down to my space heating energy requirements, on all but the coldest day, so I have a 4 port buffer. Should I replace the HP in the future I'd likely downsize to 8kW.

 

If I were doing it again, I'd consider a larger DHW tank and a decent sized buffer for the UFH to allow more heat to be stored during the cheap overnight period. That would also enable a larger ASHP to be used, as it wouldn't need to modulate down so much.

[Meabh]

8 hours ago, joth said:

Do all 4 bathrooms have showers? With 5ppl in the house, 3-4 showers could easily be used at once (if your water pressure & flow are up to it).

The boys are approaching the ages where showers will get longer too.

I'd be looing at 300L minimum. 400L if you have space for it. A 11kW ASHP will recharge it pretty well.

An alternative / addition to oversizing tank is to store water at a higher temperature. If you get the new R290 Ecodan it will heat up to 70C, obviously at a hit to COP but an option if you're space constrained and think you'd only "sometimes" need the higher heat energy storage capacity.

 

Place it somewhere central and the heat losses will offset your heating demand in winter. (Think about mitigation if you do get overheating in summer)

 

You say it's super insulated, but 9.6kW suggests it's either >400m2 floor area, or not that thermally efficient really. Do you have an airtightness target, and did the heat loss calc take it into consideration? (The MCS protocol typically doesn't)

Thanks - all bathrooms have showers - boys already have to be told to get out of the shower.....

We are 330 sq m but 230 of that is a very spread out ground floor. Pre plaster/render/silicon on windows ACH was 1.3. They said they took account of the ACH but don't take account for MVHR...

Looks like I need a bigger tank!

[Meabh]

8 hours ago, JohnMo said:

Our 195m2 big vaulted ceiling in every room and lots of glazing bungalow, uses around 3.5kW at -9, so yours must be huge, or the heat loss calcs are wrong.

I've had them run the calcs x2 after getting the SAP redone (it's a 99 but we do have PV on the roof). I also had even bigger heat pumps suggested when I shopped around...

We do have high monopitched ceilings in 2 areas...and are around 330sqm.....with 230 of that a very spread out ground floor. So it is an inefficient shape?

[jack]

14 hours ago, Meabh said:

Looks like I need a bigger tank!

 

I didn't say above, but I have a 250 L tank and I think it's too small. If all four of us have a shower in the morning or evening, there's a chance we might run out of water (especially if number two child isn't physically dragged out of the shower in time).

 

We also have waste water heat recovery, which probably helps a bit, so if you don't have that it could be even worse.

 

Also, we have a PV immersion diverter. The larger your tank, the more free heat you can store. On sunny days in summer and some of the shoulder months, with a big tank you can probably store a couple of days' of heat from just one sunny day. 

 

I'd probably be looking at 400 L if I were doing this again.

 

[SteamyTea]

We should really be looking at ways to reduce water consumption, not at ways to supply it endlessly.

A smaller cylinder and frequent bollokings is the cheapest option.

[joth]

16 hours ago, Meabh said:

We are 330 sq m but 230 of that is a very spread out ground floor. Pre plaster/render/silicon on windows ACH was 1.3. They said they took account of the ACH but don't take account for MVHR...

 

So 30W/m2 - about twice the energy demand of our retrofit. (We achieved 0.5ACH) 

I'd ask them to do a calc with the MVHR recovery figured in too, so you can get an idea of the likely real-world demand. Mine was way over spec too until I did this.

 

[Meabh]

9 hours ago, joth said:

So 30W/m2 - about twice the energy demand of our retrofit. (We achieved 0.5ACH) 

I'd ask them to do a calc with the MVHR recovery figured in too, so you can get an idea of the likely real-world demand. Mine was way over spec too until I did this.

 

They say they don't include the MVHR in the MCS calcs.....but I had a go at the one on here and if I'm using it correctly it's the ventilation heat loss section that pushes us up to 9.6kw. 

 

I've not managed to get a quote from any ASHP supplier to give us a unit less than 10kw.

 

How do you convince them to change? They are already concerned that we aren't having heating upstairs (it's currently sitting at 16 degrees because a dehumidifier is running and we are in SE Scotland so it's not warm outside - that proves to me that we won't be cold!)

[Meabh]

12 hours ago, SteamyTea said:

We should really be looking at ways to reduce water consumption, not at ways to supply it endlessly.

A smaller cylinder and frequent bollokings is the cheapest option.

Fair point - we are currently in a rental with 1 bathroom and a tank that runs out quickly and the solution is to shout at the kids to get out or their siblings can't wash....but the new house is bigger with more than one bathroom

[Meabh]

12 hours ago, jack said:

 

I didn't say above, but I have a 250 L tank and I think it's too small. If all four of us have a shower in the morning or evening, there's a chance we might run out of water (especially if number two child isn't physically dragged out of the shower in time).

 

We also have waste water heat recovery, which probably helps a bit, so if you don't have that it could be even worse.

 

Also, we have a PV immersion diverter. The larger your tank, the more free heat you can store. On sunny days in summer and some of the shoulder months, with a big tank you can probably store a couple of days' of heat from just one sunny day. 

 

I'd probably be looking at 400 L if I were doing this again.

 

Thanks - we don't have WWHR but do have solar and 10kw of battery so we can store in that but I think the Ecodan immersion is inbuilt so we have the "free electric" but to turn it to hot water it has to run the ASHP and takes 79 mins to heat up the tank....

I've asked them to include 300L in the quote. 

[JohnMo]

9 hours ago, Meabh said:

had a go at the one on here and if I'm using it correctly it's the ventilation heat loss section that pushes us up to 9.6kw. 

Still not understanding how your heat loss is that big, even with MCS ignoring heat recovery. Your ventilation heat loss will be in the region of 1800W ignoring heat recovery. Are you sure they are using the correct U values for you walls, floor, roof and windows and doors. Or using a stupid design temperature.

 

Not sure the heat loss you are being quoted, would meet current min building regs?