How large a boiler do I really need?

[Garald]

 

[Preface to this and several other questions I am asking:

 

Well, the planning stage is over, and we should start working soon. And guess what, my architect has just presented me with a huge bill courtesy of her contractor for me to approve - much larger than the budget I had originally fixed. So, we now have to sit down and negotiate - and go through every unimportant, expensive detail that can be taken off. ]

 

I am doing a pretty thorough renovation of a place I bought recently (as some people here already know). The architect is proposing a 300L hot water heater. However, this is a 110m^2 place (well, 184m^2 if you count both the places that are not currently inhabitable as well as the places you cannot stand in), meant for myself, my girlfriend, my parents when they visit, a cat, the occasional wandering mathematician - not a large family of amphibians.

 

A friend who is helping me go through the details says that 120L should be enough, and 150L largely enough - anything above that is an unnecessary expense and also makes matters less efficient.

 

Is he right? What size would be reasonable for my sort of situation?

 

(This is a heater I am getting in conjunction with a heat pump - not sure how the two relate to each other, but that's not the main point of the question.)

 

[MJNewton]

The floor area doesn't really play into it, more how many showers and baths you'd expect to be having per day (and when). For example, if a shower gives a flow of say 10-15 L/min combined hot+cold then you might expect the hot portion to be just over half so perhaps 6-8 L/min. A 120L cylinder would therefore only give around 15 minutes worth before needing reheating. Of course, this could be happening whilst the shower is drawing but I'm sure you'd end up  cursing your mate with the family round and you're trying to keep up with demand, particularly if the cat suddenly decides it wants a bath.

 

Edited September 10, 2022 by MJNewton

[JohnMo]

So heat pump heated, so storage temperature is around 48 degrees.  Not the same as a gas boiler where storage would be 60 to 65. Average usage according to SAP for a 2 person is just under 5kWh of hot water.  At around 4.5kWh usage the water would be coming out of the tap at around 35 degs.  So you need 300l, any less your taps would be running cold, assuming one heat charge per day.

[Conor]

Yep, we've managed to empty our 300l cylinder before when we had a couple people visiting. It's only at 45-48c so little or no mixing at the tap. You could get away with 200l at a minimum.

[Kelvin]

Our ASHP installer specced our system with a 250 litre cylinder but I’ll get that increased to 300 litre minimum as we regularly have people stay. We had a 300 litre tank and ASHP in our last place it is was fine even with a house full. 120 litre tank is far too small. 

[RobLe]

4 or 5 of us here, but all light shower users, 220l tank heated to 45C generally, 60C every couple of weeks.  Seems fine, rarely lacking oomph.  People sometimes check the temp on a display prior to showering - I think if it were more effortlessly capable it might encourage longer and longer showers 🤔

[Garald]

I should have made clear that we are working under the assumption of a *high-temperature* heat pump. Whether it would be wise to switch the entire network to low-temperature is a different, important question, which I will ask separately.

[Garald]

11 hours ago, MJNewton said:

 particularly if the cat suddenly decides it wants a bath.

 

 

Wait, that happens? It has got a bath only on the very rare occasions it has had an, um, accident.

[Iceverge]

On 10/09/2022 at 23:29, Garald said:

And guess what, my architect has just presented me with a huge bill courtesy of her contractor for me to approve - much larger than the budget I had originally fixed.

 

Well if you had fixed a price you're architect needs reminding on the meaning of fixed. I'd get on top of this sharpish and part company unless it's sorted out. 

 

On the cylinder issue. Bigger is always better. And the price step is small. Only about £80 to go from 200l to 300l.

 

The larger it is the more off peak electricity you can use. The more excess PV solar you can store. The lower temp you can get away with for the ASHP and the better efficiency it'll run at. If you have too much water you can turn down the storage temperature and you'll reduce the storage heat losses. 

[Nickfromwales]

Are you going with radiators instead of UFH? Why has a high temp ASHP been spec'd? They're a PITA wit a noisy obtrusive internal unit ( internal as in indoors!! ) and efficiencies will drop like a rock if the HP is married to a system that isn't sympathetic to the favored input temp / operating conditions.

A HP will also not recover ( reheat ) the cylinder as quickly as a boiler, so that is another reason to go for the 300L cylinder ( 120L would be getting ripped out in a few months, after you realised it was an epic fail btw ). High-flow gas combis have a theoretical matched capacity to a 200L UVC, for eg ).

Can you not suitably engineer the build fabric ( insulation and draught-proofing ) and therefore reduce the heating system so as to be able to accept low temp heat from a monoblock ASHP? You buy insulation once, for the lifetime of the dwelling, but energy input ( running costs ) to mitigate against any poor decisions made now will be ever increasing.....

 

[Garald]

28 minutes ago, Nickfromwales said:

Are you going with radiators instead of UFH? Why has a high temp ASHP been spec'd?

 

 

Because this is a renovation job, and I do not want to rip out every single thing. Of course, now it turns out that some work *will* be needed on the radiators and heating ducts, so I am asking myself whether it wouldn't be better to shift to high-temperature. See the thread

 

31 minutes ago, Nickfromwales said:

Can you not suitably engineer the build fabric ( insulation and draught-proofing ) and therefore reduce the heating system so as to be able to accept low temp heat from a monoblock ASHP? You buy insulation once, for the lifetime of the dwelling, but energy input ( running costs ) to mitigate against any poor decisions made now will be ever increasing.....

 

 

Right - let's discuss this in the other thread. My architect estimates the short-term net costs (work - (the savings that come from the fact that low-temperature heat pumps are actually cheaper)) would be in the order of 5k eur. Not trivial given the circumstances.

33 minutes ago, Nickfromwales said:

They're a PITA wit a noisy obtrusive internal unit ( internal as in indoors!! )

 

 

I wouldn't be too worried about that.  I think I (still) trust her with sound insulation. The heat pump will be in its own tiny room (half of an outhouse I just bought from the co-op).

[Garald]

New input from the architect on the low-temperature possibility:

 

Long story short, we are seriously considering switching to low-temp. (This is a dramatic-intervention-of-a-guest-who-decided-not-to-remain-silent-forever-at-the-wedding sort of thing; work on the site is scheduled to start this week.)

 

The difference in price between 200L and 300L is 620€ for the low-temperature arrangement we are considering. (It is 520€ for high-temperature.) So: is 300L worth it?

 

Notice that, in low-temperature, it's all monobloc - the tank is part of the heat-pump cabinet. The question is really: is 300L more efficient - enough to justify the difference in price? (I don't think I'll ever be short of water with 200L, not unless I invite many particularly cold-blooded amphibian guests for a sleepover.)

 

Here's the link to the heat pump's specs: https://www.amzair.eu/wp-content/uploads/dlm_uploads/2015/04/pac-amzair-optimduo-dossier-technique-20211125.pdf

 

Edited September 12, 2022 by Garald

[Garald]

(I don't yet know whether I'll install PV panels, at least for now, given that my budget is strained.)

[SteamyTea]

12 hours ago, Garald said:

(I don't yet know whether I'll install PV panels, at least for now, given that my budget is strained.)

Hasn't your government (well the French one) limited the price of domestic energy at a low level?

They need to keep their investment of EDF alive somehow. The French will riot if a kWh costs more than a small glass of wine.

 

Also not sure how the PV industry works over there. I think you still have quite small domestic connections, which could limit array size.

Edited September 13, 2022 by SteamyTea

[Garald]

3 minutes ago, SteamyTea said:

Hasn't your government (well the French one) limited the price of domestic energy at a low level?

 

With all the work I'm doing in insulation, and a low-temperature heat-pump, I suspect PV would be mostly an eco-gesture of my part at the end, if I have any money left (hint: I won't). Perhaps I can leave it for later? Or is it the sort of thing I should take care of now, or rather at the end of the project?

 

3 minutes ago, SteamyTea said:

Also not sure how the PV industry works over there. I think you still have quite small domestic connections, which could limit array size.

 

I've been messaging a local PV company repeatedly, but they haven't got back to me yet.

 

[SteamyTea]

8 minutes ago, Garald said:

I've been messaging a local PV company repeatedly, but they haven't got back to me yet.

They are having lunch.

[Dave Jones]

bear in mind a decent shower is 12-17L/M, the female of the species tend not to take 5 minute showers i find. If everyone wants a shower each morning plan accordingly. Nothing worse than a cold shower on a cold morning!

Edited September 13, 2022 by Dave Jones

[SteamyTea]

5 minutes ago, Dave Jones said:

bear in mind a decent shower is 12-17L/M, the female of the species tend not to take 5 minute showers i find.

Garald's girlfriend may be French.

Have you never wondered why they make perfume.

 

"home in three days, don't wash"

Napoleon 

[Garald]

She's American, actually!

 

The 200L tank isn't just cheaper - it can come in monobloc with the heatpump. Floor space gained counts for something, even in the bike shed.

[Garald]

Ah, it can't, says the architect - but it can fit in a closet in the entryway.

 

In any event, I will have so much space in the garage, that, if my calculations are right, one will be able to fit a Citroën 2CV in it! That won't happen in practice, though - it will be a bike shed and storage space (since I do not have and cannot have a basement). But, who knows, perhaps a Microlino some day...