Garald Posted February 2, 2023 Share Posted February 2, 2023 I mentioned the following issue yesterday. I just double-checked my calculations (and translated them into French, since I'll send them to the heat-pump installer who suggested the problematic setup; hope this isn't a problem - math is math). What situation do I have in mind? A garage like mine - it's in a rowhouse, so surrounded by house (mostly mine) on all sides except one, and the external wall will be mostly a solid wooden door (since the garage is becoming a bike shed/machine room). [OK, I'm neglecting the floor, which is a concrete slab, covered up a bit.] Even though I've already put some insulation between the rest of the house and the garage, the garage will presumably never get quite as cold as the street in winter - it will presumably get about as cold as a coop corridor with a wooden door. Let's say that, before the installation of a water heater, the temperature T of the garage is (1-eta) T_0 + eta T_1, where T_0 is the outside temperature and T_1 is the temperature inside the house. Then the calculation here shows that, if you install a fancy water heater with its own heat pump that extracts heat from the ambient air in the garage, then a proportion eta of the heat "gained" by the water heater is in fact being sucked from your house (yes, I mean it would not have been lost by the house if the heat pump were not there). Really not nice if eta is not so small! For example, if the temperature of the garage (with no water heater) is at midpoint between the outside and inside temperature (say, 9C when it's 20C indoors and -2 outdoors), then eta = 0.5, and so half of the heat "gained" is heat sucked from the house. Now, if the house is being heated by a good heat pump, this *may* still be better than a conventional electric water heater, but it is still ridiculous: it would be much better to just let the heat pump that heats the house produce domestic hot water! (If it's summer, though, you are happy, since you *want* to lose heat - but in that case, you'd be better off just letting the heat-pump extract heat directly from the inside of the house, instead of working on the other side of the garage-house separation wall.) Another concern was raised in the original thread: that the water heater could cause the garage to become extremely cold. The model here does show that the temperature could drop beneath the outside temperature. However, in practice, there's a limit to this, since the heat pump in the water heater is likely to be weak, and stop working at a certain temperature (5 C?), beneath which it just becomes a conventional electric water heater. Thus, the garage wouldn't get colder than 5C or the outside temperature, whichever is lower. Link to comment Share on other sites More sharing options...
SteamyTea Posted February 2, 2023 Share Posted February 2, 2023 Two things. Will the temperature drop go below the dewpoint, this will cause condensation. And It is maths, not math. 1 Link to comment Share on other sites More sharing options...
Garald Posted February 2, 2023 Author Share Posted February 2, 2023 1 hour ago, SteamyTea said: Two things. Will the temperature drop go below the dewpoint, this will cause condensation. The temperature of the machine? I'm not sure how that would happen - the air in the garage could get cold (as cold as the minimum of the outside temperature and the temperature at which the water heater switches to being just a plain electrical water heater (5C perhaps?)) but the machine itself would be warmer on the inside, at least when running, no? 1 hour ago, SteamyTea said: And It is maths, not math. Sorry... Link to comment Share on other sites More sharing options...
SteamyTea Posted February 2, 2023 Share Posted February 2, 2023 13 minutes ago, Garald said: The temperature of the machine? No, the air temperature. If it drops below the dew point, condensation will form, so you end up with a damp, cold garage. And a rusty bicyclette. https://en.wikipedia.org/wiki/Dew_point Link to comment Share on other sites More sharing options...
Garald Posted February 2, 2023 Author Share Posted February 2, 2023 30 minutes ago, SteamyTea said: No, the air temperature. If it drops below the dew point, condensation will form, so you end up with a damp, cold garage. And a rusty bicyclette. https://en.wikipedia.org/wiki/Dew_point Right. Yes, this may be an issue, depending on the exact point at which the machine switches over to being a conventional electric water heater. That provides a bit of a safety mechanism: the garage won't get colder than the minimum of the outside temperature and the temperature at which that shift happens. Can also be a problem if one makes holes in the garage door or wall to keep things from happening (as then the garage would be about as cold as the outside world). Link to comment Share on other sites More sharing options...
Nickfromwales Posted February 2, 2023 Share Posted February 2, 2023 The garage temp wouldn't drop if you vent the 'very cold' exhaust air to atmosphere. You'd need a transfer grille in the external wall of the equivalent size eg to input the equal volume of 'fresh' air as the exhaust expels ( I'd recommend 20% bigger to stave off resistance and risk air being pulled from the house ), and this would then only allow air as cold / damp as outdoors to arrive at the garage interior. To be perfectly honest, I think the heatpump will suck the available heat energy out of the garage in a matter of minutes, and the exercise is flawed / pointless as the CoP will then drop like a rock. 1 Link to comment Share on other sites More sharing options...
Garald Posted February 2, 2023 Author Share Posted February 2, 2023 5 minutes ago, Nickfromwales said: M 5 minutes ago, Nickfromwales said: The garage temp wouldn't drop if you vent the 'very cold' exhaust air to atmosphere. Right - I suggested it (in fact venting directly from or to the street) and the installer advised against it. 5 minutes ago, Nickfromwales said: To be perfectly honest, I think the heatpump will suck the available heat energy out of the garage in a matter of minutes, and the exercise is flawed / pointless as the CoP will then drop like a rock. Can also happen, depending on volume and power. (700W is not *that* much.) Information in French suggests that water heater installers believe that this happens when the room volume is less than 20m3 (this is obviously an empirical rule, and very rough, even if it turns out to have any validity). Link to comment Share on other sites More sharing options...
SteamyTea Posted February 2, 2023 Share Posted February 2, 2023 43 minutes ago, Garald said: 700W is not *that* much Well it is 700 J/s which is enough to cool 0.56 of a kilogram of air by 1⁰C every second. So 20m³ would cool by 1⁰C in 36 seconds. I think. I hate doing arithmetic without a spreadsheet. Link to comment Share on other sites More sharing options...
JamesPa Posted February 2, 2023 Share Posted February 2, 2023 I confess to not really understanding what the valid, practical, use case is for this type of water heater in a typical domestic situation. Fundamentally you are taking heat energy from somewhere, so unless you don't need that heat energy and it renews itself from somewhere which you are happy gets cooler, you are inevitably robbing energy from yourself. In summer, if you use it to cool the house and warm hot water then yes, it might make sense (bus so does PV and a solar diverter), but in winter its more difficult to see, unless you have a source of genuinely waste heat from some other process which is otherwise not contributing to warming your building/stopping it cooling. Most of us don't! Hopefully someone can put me right and convince me that these are not just exploitative marketing. 2 Link to comment Share on other sites More sharing options...
billt Posted February 2, 2023 Share Posted February 2, 2023 I guess the main use would be in systems without wet heat distribution, e.g. if you use air to air heat pumps. More efficient than an immersion system. It's also potentially useful if you want to dissociate hot water heating from room heating. I'm changing my heating to an ASHP system and it would have made installation easier to install a heat pump hot water cylinder separately from the main heating system. I decided against it because of the noise being unwanted where it would have been installed. Link to comment Share on other sites More sharing options...
Garald Posted February 2, 2023 Author Share Posted February 2, 2023 Well, it would make sense if the water heater were directly connected to outside air. Then in effect you would just have a second, smaller ASHP setup - dissociating the two can be useful. (The original rationale of the seller was: this way you will not run your main ASHP in the summer and annoy your neighbors. This may make sense, but (a) how noisy is this water heater? I could be annoying myself, (b) would the main ASHP be working that hard just for DHW, if programmed correctly? And if it isn't, shouldn't it be pretty quiet?) Link to comment Share on other sites More sharing options...
SteamyTea Posted February 2, 2023 Share Posted February 2, 2023 (edited) 40 minutes ago, Garald said: how noisy is this water heater? I could be annoying myself How noisy is your home already. May be worth finding out, you may be worrying unnecessarily about it. My experience of Paris is that it is noisy. Now I know you are not in the centre (the last place I stayed was in Rue Beautreillis, Jim Morrison fame), so not the busiest place, but noisy. Edited February 2, 2023 by SteamyTea Link to comment Share on other sites More sharing options...
Nickfromwales Posted February 2, 2023 Share Posted February 2, 2023 3 hours ago, Garald said: The original rationale of the seller was: ….that he was a salesman. They won’t tell you that there are better / more cost effective / more efficient / less obtrusive solutions to the same problem, as their job depends on sales to feed their kids. Ergo, their product is a good idea. Link to comment Share on other sites More sharing options...
Garald Posted February 2, 2023 Author Share Posted February 2, 2023 1 hour ago, SteamyTea said: How noisy is your home already. May be worth finding out, you may be worrying unnecessarily about it. My experience of Paris is that it is noisy. Now I know you are not in the centre (the last place I stayed was in Rue Beautreillis, Jim Morrison fame), so not the busiest place, but noisy. Well, I've already spent a pretty penny on making it quiet: new double-glazing on the street side (the old double-glazing was OK thermically, but you could hear every car going down the street), 18cm of cellulose-based-insulation (hemp/linen/cotton) with particularly good sonic properties (yes, it's also nice thermically, but I could have got away with using up less space by using a combination of cellulose and reflective materials as on the other side)... So I really don't want to sabotage myself in this way. The old garage (bike-shed-and-machine-room-to-be) is right below one end of my library/music room (which is also where I'll eat when I don't just deal with that necessity in the kitchen), so silence is important. Link to comment Share on other sites More sharing options...
Garald Posted February 2, 2023 Author Share Posted February 2, 2023 (edited) 3 hours ago, JamesPa said: In summer, if you use it to cool the house and warm hot water then yes, it might make sense (bus so does PV and a solar diverter), Right, it might make particular sense to put such a device in the top floor (an attic that I'm afraid might get uncomfortably hot during heat waves) to be used in summer only, so as to kill two birds with one stone: (a) I wouldn't be bothering the neighbors by making the main heat pump work in the shared courtyard precisely in the season when they may want to have their windows open, (b) I would heat water at the same time that I cool my attic! If, however, I suggest such a change in plans at this point, when the attic is already partly redone, the architect may finally have reasons to want to kill me. This is more something to keep in mind if I ever have the roof in the attic raised - I can make this kind of reforms then. Should I talk it over with the head builder so that he takes this hypothetical plan into account now that he's working on the pipes? Or is that not necessary? Edited February 2, 2023 by Garald Link to comment Share on other sites More sharing options...
SteamyTea Posted February 2, 2023 Share Posted February 2, 2023 18 minutes ago, Garald said: an attic that I'm afraid might get uncomfortably hot during heat waves That can be insulated the same as any other area, energy transfer is agnostic on direction. 19 minutes ago, Garald said: the architect may finally have reasons to want to kill me. She should have thought about all this before, what they are meant to do. Why on this forum, many of are very sceptical of architects. Link to comment Share on other sites More sharing options...
Garald Posted February 2, 2023 Author Share Posted February 2, 2023 She's a ventilation specialist but has zero previous experience with heat pumps (and used to be highly skeptical of them for whatever reason). The contractor knows about radiators, and his dad has a heat pump somewhere in the countryside, so at least it's two against one. Link to comment Share on other sites More sharing options...
Garald Posted February 2, 2023 Author Share Posted February 2, 2023 7 minutes ago, SteamyTea said: That can be insulated the same as any other area, energy transfer is agnostic on direction. Well, sure, but there's convection, so... 7 minutes ago, SteamyTea said: Link to comment Share on other sites More sharing options...
Iceverge Posted February 2, 2023 Share Posted February 2, 2023 How are you providing space heating again? Link to comment Share on other sites More sharing options...
Garald Posted February 2, 2023 Author Share Posted February 2, 2023 3 minutes ago, Iceverge said: How are you providing space heating again? Air-to-water heat pump. Link to comment Share on other sites More sharing options...
Iceverge Posted February 2, 2023 Share Posted February 2, 2023 2 hours ago, Garald said: Air-to-water heat pump. Why not use this for the DHW so? Link to comment Share on other sites More sharing options...
Garald Posted February 2, 2023 Author Share Posted February 2, 2023 8 minutes ago, Iceverge said: Why not use this for the DHW so? Yes, this is what I have been saying all along! In winter or mid-season, it's clearly superior to what the salesman was proposing. His argument for why this is an inferior solution in the summer: you'll have to run the heat pump even then, the neighbors will have their windows open because of the heat, they will be annoyed, come and vandalize your external unit... Link to comment Share on other sites More sharing options...
SteamyTea Posted February 2, 2023 Share Posted February 2, 2023 1 hour ago, Garald said: they will be annoyed, come and vandalize your external unit Will they be wearing yellow vests, and carrying pitchforks. When it is 34°C in the summer, heating 200 lt of water by 25°C is not going to take much energy, or time. You can limit the rate that it heats up to keep the stress off the system. Link to comment Share on other sites More sharing options...
Garald Posted February 2, 2023 Author Share Posted February 2, 2023 3 minutes ago, SteamyTea said: Will they be wearing yellow vests, and carrying pitchforks. When it is 34°C in the summer, heating 200 lt of water by 25°C is not going to take much energy, or time. You can limit the rate that it heats up to keep the stress off the system. Right. The Daikin unit that he's trying to sell me is a good product, with "quiet" and "very quiet" modes. (Well, those are just 3dB and 5dB less than the peak setting, but every dB helps.) Unfortunately, I can't seem to find online what the top wattage of a 11kW unit would be under those settings. How do I find that out? (Yes, 11kW is too much, but the next one in Daikin is 8kW (which is really 7kW under some conditions), and, while that should certainly be fine given my calculations (and he seemed inclined to agree in person), he's just not willing to run the risk. That's a pattern among all installers, btw; no exceptions. I should already be thankful that another installer wants to sell me an Atlantic 10 model (which is really 9kW, and yes, he's the first to say that).) Link to comment Share on other sites More sharing options...
Nickfromwales Posted February 2, 2023 Share Posted February 2, 2023 44 minutes ago, Garald said: he's trying to sell me is a good product, with "quiet" and "very quiet" modes. So basically a 2-speed system Where do you think the “noise” goes? Link to comment Share on other sites More sharing options...
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