ᐅ Challenges for MEP planners: underfloor heating flow temperature and wastewater ventilation
Created on: 15 Jul 2022 10:22
P
Pacmansh
Hello,
we are at the beginning of the construction phase for our development project with the builder, and I am having some disagreements with the MEP planner. To be better prepared for the discussion, I would appreciate your assessment.
Point 1) Supply temperature of underfloor heating, new building, KfW55 standard, air-to-water heat pump
The supply temperature of the underfloor heating (end-terrace house on both floors) was stated to me as 40°C (104°F) after inquiry. This seems absurdly high to me. Additionally, I was informed that the surface temperature is designed to a maximum of 27°C (81°F) due to the flooring materials. Somehow, this does not seem consistent. When I asked about lowering the supply temperature, the response was: "A general reduction is not feasible with the underfloor heating without reducing the pipe spacing to an unacceptable level."
Do you have any ideas how I can respond to this in a reasonably professional way? Are there any documents or sources I could refer to, or information I should request?
Point 2) Wastewater venting
Contrary to earlier agreements, this has been planned in a rather unfavorable location. The reason given is "because the wastewater vent and the residential ventilation (exhaust air) must be routed over the roof with a certain separation according to flat roof guidelines." What distance should be maintained here? A quick online search only showed a 30cm (12 inches) distance to other building components. Basically, this is about the roof penetrations and their distance from each other, correct?
we are at the beginning of the construction phase for our development project with the builder, and I am having some disagreements with the MEP planner. To be better prepared for the discussion, I would appreciate your assessment.
Point 1) Supply temperature of underfloor heating, new building, KfW55 standard, air-to-water heat pump
The supply temperature of the underfloor heating (end-terrace house on both floors) was stated to me as 40°C (104°F) after inquiry. This seems absurdly high to me. Additionally, I was informed that the surface temperature is designed to a maximum of 27°C (81°F) due to the flooring materials. Somehow, this does not seem consistent. When I asked about lowering the supply temperature, the response was: "A general reduction is not feasible with the underfloor heating without reducing the pipe spacing to an unacceptable level."
Do you have any ideas how I can respond to this in a reasonably professional way? Are there any documents or sources I could refer to, or information I should request?
Point 2) Wastewater venting
Contrary to earlier agreements, this has been planned in a rather unfavorable location. The reason given is "because the wastewater vent and the residential ventilation (exhaust air) must be routed over the roof with a certain separation according to flat roof guidelines." What distance should be maintained here? A quick online search only showed a 30cm (12 inches) distance to other building components. Basically, this is about the roof penetrations and their distance from each other, correct?
It will also be tiled, but a shower element suitable for tiling will be installed. This will be made of polystyrene, and as with the bathtub, no underfloor heating will be laid there.
It might be possible to install something under the tub and shower, but it apparently won’t affect the calculation.
It might be possible to install something under the tub and shower, but it apparently won’t affect the calculation.
Yes, that would be the goal.
Is my idea of changing the calculation basis to reduce the laying spacing in the rest of the house reasonable? Basically, it’s a bit of a silly approach, but somehow I feel more confident if the ventilation air is lower. After all, this is something that will remain for a few decades.
Is my idea of changing the calculation basis to reduce the laying spacing in the rest of the house reasonable? Basically, it’s a bit of a silly approach, but somehow I feel more confident if the ventilation air is lower. After all, this is something that will remain for a few decades.
Pacmansh schrieb:
- Keep the pipe spacing in the bathroom at 5cm (2 inches), if possible add a bit more wall area (let’s see what’s feasible), and lower the target temperature in the calculation to 22/23°C (72/73°F).
- This should reduce the supply temperature and therefore allow for closer pipe spacing in the rest of the house (designed for 20°C (68°F) in living spaces).I would suggest the same: use 20cm (8 inches) spacing throughout the house. The argument that you need 40°C (104°F) supply temperature just to keep the bathroom warm is simply not convincing. This limits all options for optimization later on, or the possibility of being satisfied with 21°C (70°F) in the bathroom, possibly supplemented by an infrared mirror or bathroom radiator.
Regarding efficiency: the 40°C (104°F) supply temperature is the design temperature under maximum load—i.e., at outside temperatures of -12/-14/-16°C (10/7/3°F). This temperature is not maintained constantly, but only 1-2 weeks per year. Therefore, the electricity consumption will not double but only increase slightly.
A recent Swiss study (Quality control of small heat pumps and statistical analysis 2018 – Mick Eschmann, Interstate University of Technology NTB) measured operating small heat pumps with design temperatures of 35°C (95°F) for underfloor heating and 55°C (131°F) for radiator systems. It found average Seasonal Coefficients of Performance (SCOP) of 4.2 at 35°C and 3.2 at 55°C.
Assuming a linear relationship, 40°C (104°F) supply temperature would correspond to a SCOP of about 3.95, which means roughly 6–7% higher electricity consumption for the same heat demand compared to 35°C (95°F) design.
In reality, the relationship is likely not linear since the physical process (compression) does not consume energy linearly. Temperature changes at the lower end matter less than at the higher end—for example, reducing from 55°C (131°F) to 50°C (122°F) has a bigger effect than reducing from 40°C (104°F) to 35°C (95°F).
Much more important than the last degree in supply temperature is a proper balancing and operation of the system, so the heat pump can run smoothly without short-cycling or needing a buffer tank (which requires higher supply temperatures). Still, the general approach of minimizing the required supply temperature as much as possible is the right one.
dertill schrieb:
That really eliminates all options for optimization afterward,Thanks, that’s also my idea. In the bathroom, there are electric towel warmers anyway. dertill schrieb:
Regarding efficiency: the 40°C (104°F) is the flow temperature under full load, i.e. at outdoor temperatures of -12/14/16 °C (10/7/61°F).I think it’s -14°C (7°F) for us. She also used that argument to justify her planning. Actually, the flow temperature is usually much lower because it’s mostly warmer outside. dertill schrieb:
That means temperature changes at the lower end are less important than at the higher end. -> Reducing from 55°C (131°F) to 50°C (122°F) is more significant than from 40°C (104°F) to 35°C (95°F).Thanks for the explanations. Makes sense. Of course, you always have to consider how far you want to push efficiency. I’m happy to give up 2–3 degrees if it would be expensive, complicated, or prone to errors. But I won’t skip it out of laziness. dertill schrieb:
Much more important than the last degree in flow temperature is a proper balancing and operation of the system, to run the heat pump smoothly without short cycling and without a buffer tank (which would require a higher flow temperature).Well, you can probably imagine how someone plans who works like my planner. “A buffer tank is a must, otherwise the heat pump will just short cycle.” It was quite a task to explain that I don’t need an electric auxiliary heater in the dressing room, which, due to wardrobes, has a heated area of only 2.5 m² (27 ft²). The dressing room doesn’t even have a door separating it. At least the heat pump is rather small compared to the heating load, which I understand is better than oversizing.
As for the balancing, I’m curious about the plumbers. So far, the contractors have done a pretty good job.
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