Posted by: @alva

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The local installer proposed:

• 12kw Chaffoteaux heat pump
• Separate 500L DHW tank
• 50L Thermal flywheel / buffer tank
• Thermostats/zones for effectively each individual room

Quite possibly all wrong.  Get other quotes or regret

Can you tell us more about the house, construction, location, floor area, climate where you are

Do you have any figures for current fuel consumption?

You are thinking along the right lines but would need more detail to be certain. 

Does the family of 8 visit at coldest time of year?

Why not 2 zones one for grandmother one for family, 2 dhw tanks independently switched, all run off perhaps an 8kW pump.  You may need a volumiser, you won't need a buffer. Something like that is probably where I would start but you need to do the actual numbers for the two scenarios separately then apply a bit of thought because of the two distinct regimes 

@jamespa Thanks for the feedback! I'll try to answer your questions.

House is on a single floor. We are completely renovating floor, ceiling, walls, windows for improved isolation and adding some extra space. I used the NRG calculator for heat loss, as I know all the materials and their uvalues. Unfortunately I don't have access to previous fuel usage.

The total UFH area is 140 m2 (I guess ~120L volume): 100 m2 primary living, 40 m2 for guest rooms. Heat loss of 4,650W and 1,150W respectively. The climate is mediterranean, with a 99th percentile low of ~3c.

The guest bedrooms are are on opposite sides of the house, so I didn't create a single zone for them. But not sure if the rooms distant from each other actually matters for the UFH zoning.

It might also help to have fewer thermostats so there's less chance for someone to mess with the settings thinking it works like the radiator system back home.

When family visits the max total amount of people in the house is 8. This is mostly during summer, except for 1-2 weeks during Christmas.

Hope this helps clarify!

Did you count heat loss due to ventilation?

About that... if there are bathroom(s) in the intermittent occupation rooms, could be useful to be able to reduce ventilation in these when unoccupied, at least in winter, to throw away less heated air.

If your UFH is a concrete slab, you have literally tons of heat storage, making a 50l "thermal flywheel" absolutely useless in comparison.

If you know the length and pipe internal diameter (or type of pipe) of your UFH loops please post, I'll run a flow/pressure calculation.

Please give more details about your UFH. If it is a concrete slab, zoning with on/off manifold actuators controlled by thermostats can produce large temperature swings, because it takes a while for the heat to pass through the slab, and by the time it reaches the thermostat and it decides to stop heating, too much heat has been stored in the slab, which then releases it into the room over the rest of the day, making it too hot. Strength of this effect depends on flow water temperature, if it's low enough (25-28°C) then it's possible to control a UFH slab with a thermostat, but higher than that (35°C) it will most likely overshoot a lot.

If it's not a concrete slab then it reacts much faster so you can actually use an on/off thermostat without this kind of overshoot problems, but then... you also get temperature swings due to it turning on/off.

If you insulate the exterior walls well enough, then heat leaking between rooms through the partition walls tends to keep everything at roughly the same temperature anyway, which makes zoning not that useful. Honestly if you heat 100m² constantly and 40m² intermittently, you probably won't save much compared to heating the whole house.

Another issue with thermostats/zoning is, if you run the heat pump open loop to the main UFH and optimize flow temperature so it heats "just right", then... the hottest temperature you can get in the guest bedroom with the valves fully open will probably be about the same as in the rest of the house.

Now consider a guest who likes their bedroom very warm.

To get the guest bedroom warmer, you'd need hotter flow temperature, so shift the weather compensation, but then it will overheat the main area, so reduce the flow through the main UFH to avoid overheating, but not too much otherwise the HP will give flow errors... quite annoying... Or set the whole system to run with higher temperature and lower flow all the time, which costs more due to lower COP...

And you can't have the heat pump heat only the guest bedroom because flow won't be enough. Adding a buffer tank would fix the flow via hydraulic decoupling, but then you get distortion, lower COP, and cycling.

Basically a well optimized heat pump system means flow temperature is as low as possible. It tends to deliver a constant comfy temperature at very low running cost but the drawback is you can't adjust per-room temperature that much, as you would be able to do with a boiler which delivers super hot water.

Separate heat pump water heater is very expensive and only worth it if you use a lot of hot water to recoup the costs during the life time of the equipment. Which is not the case, because occupation is intermittent. So, waste of money.

JamesPA's suggestion is much better: just put two water tanks instead of one, with a manual three way valve to enable the intermittent one. This has another advantage: you can put the two tanks next to the corresponding bathrooms, to make hot water pipe runs very short. Everyone loves when hot water comes instantly in the shower...

Note losses from a hot water tank aren't proportional to volume, but to surface area of the tank. It also depends on how thick the insulation is. With the same volume, a short, squat, fat tank has less surface area than a tall thin one. So you need to consider the actual difference in heat loss between the 500l tank and the 200l tank and compare with the cost and complexity of using two tanks.

By the way, if the house has air conditioning, is it reversible? Does it provide heating too?

I agree with pretty all of what @bobflux says.  With 140sq m well insulated and the guest rooms not all together zoning is going to be a waste of time. And as @bobflux says don't expect thermostatic control to work if the ufh is in a slab, you essentially have a storage heater without controllable vents so you need to decide 12hrs in advance how much heat to put in because you have no control how much comes out; fortunately weather compensation more or less does that.

It sounds very much to me like a case for one zone, run on pure weather compensation, balance the loops manually and leave.

140sqm sounds well insulated in a climate where it doesn't go below zero is nowhere near 12kW and so you are right to think it's oversized.

The one thought I had about dhw is reheat time.  If you do need 500l when the family is present that's 24kWh to heat the DHW.  You need to think about pattern and quantity of usage and maybe consider either waste water heat recovery if it's not too late in the refurb process, or supplementary dhw heating, eg a 6kW immersion (if your electric supply can stand it), for when the family is present.  The value/need for this will depend on the usage pattern though.  If they all want a long shower first thing in the morning you may need something, if use is distributed through the day less so.  Remember that the 24kWh going to DHW deprives the house of that energy so you will need to factor this in in system sizing for Christmas.  

Hope that helps.

@bobflux Thanks for the feedback! I'll try to answer your questions below.

Did you count heat loss due to ventilation?

In the NRG heat loss calculator I set the number of extraction fans for each individual room. I think it uses the default room ventilation rate of 0.5 as a base for houses following recent standards.

About that... if there are bathroom(s) in the intermittent occupation rooms, could be useful to be able to reduce ventilation in these when unoccupied, at least in winter, to throw away less heated air.

Great idea. Bathrooms will have distributed extraction fans with an off switch. We were thinking of fans with heat exchange. Though not sure if worth the extra cost as winters are quite mild.

If your UFH is a concrete slab, you have literally tons of heat storage, making a 50l "thermal flywheel" absolutely useless in comparison.

If you know the length and pipe internal diameter (or type of pipe) of your UFH loops please post, I'll run a flow/pressure calculation.

The UFH hasn't been placed yet, but our current plan is to use UFH pipe spacing of 150mm with standard 16mm pipes following the NRG calculator recs.

The points you make regarding the zoning are great! The airconditioners in the bedrooms have a heating function. 

My grandma likes the house cold (18c!), so maybe UFH with a single zone at an 18c base temperature plus individual bedroom control through airconditioners could work?

JamesPA's suggestion is much better: just put two water tanks instead of one, with a manual three way valve to enable the intermittent one. This has another advantage: you can put the two tanks next to the corresponding bathrooms, to make hot water pipe runs very short. Everyone loves when hot water comes instantly in the shower...

That sounds like it could work! How would you build such a configuration?

I'm a bit confused as we were thinking a system with integrated internal tank like the Ecodan or Altherma 3 RF. Could these then also heat an additional separate water tank?

Posted by: @alva

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The points you make regarding the zoning are great! The airconditioners in the bedrooms have a heating function. 

My grandma likes the house cold (18c!), so maybe UFH with a single zone at an 18c base temperature plus individual bedroom control through airconditioners could work?

That would be a very nice solution, leverage the existing aircon, without adding extra complication to the air to water heat pump.

But if the aircons also do heating, why bother installing a heat pump and UFH?

Is it the type of multisplit aircon that also does hot water if you add a tank to it? Some do, you might want to check, to open up your options.

You haven't answered whether the UFH is in a big slab (high thermal mass) or not.

Regarding ventilation, beware of small extraction fans, they tend to be quite noisy. To move air, a big fan spinning slowly is much quieter than a small fan spinning fast. Central ventilation unit is basically a big silent centrifugal fan spinning in a box, plus a bunch of ducts, and vents mounted on the walls/ceilng in the kitchen/bathrooms. It's important to have quiet ventilation, because it needs to run continuously. It takes quite a while to dry a bathroom, plus the towels etc after someone washes, and the common little extractor fan on a 10 minute timer is completely inadequate for that. Besides, if you insulate the house, you're probably going to plug most of the air leaks, otherwise insulation is useless. A draft-free house is a lot more comfy. But then once you get rid of the drafts, you have also removed all the natural ventilation, so you need to add mechanical ventilation otherwise it you can get problems like mold. Even better some units have a humidity sensor and switch into higher speed when they detect humidity from a shower for example. 

If you insulate the attic, you can probably run the ducts before blowing in the insulation. There are vents for intermittent use rooms, they can be set to fully open or almost closed. There are also vents that react to ambient humidity.

I have a strong personal dislike for heat pump indoor units with integrated tank, regarding maintenance. It's a big mystery box and you're married to it. If the tank leaks what will you do? Change the whole thing, if it is still made?