ASHP Sizing

[JulianB]

6 hours ago, joth said:

 

Why do you want the UVC to benefit from the stored energy in the buffer tank - for speed of heating the UVC, or feeling it will be 'wasted' energy if not used?

To the former, the buffer tank is lower temperature so not sure it will make much of a dent in heating the UVC unless it has been totally drained and is stone cold. To the latter, so long as the buffer tank is inside the thermal envelope of the building, the energy in it should not go to waste: this  escaping energy is usefully heating the house. This is how I'm reasoning about it anyway ? 

 

 

Thanks Joth for the input; I would probably say both points you raised play a part. For example if you could somehow use the heated water from the buffer tank as the cold water inlet to the UVC, surely that would mean both quicker heating time and less energy needed to hit the set point? Not sure if something like this is even possible, potentially if you had an indirect buffer tank which had 2 coils?  With regards to the heat loss, my slight issue is that the tank will be in the loft which will currently be un-insulated... I could insulate the loft which I guess would also benefit the MVHR which will be located in the same area?

 

4 hours ago, Stones said:

 

@Andrew  @JulianB  Some additional reading for you both in respect of ASHP feeding UFH, and a 300 litre UVC using a preplumbed Mitsubishi Ecodan:

 

 

 

 

Thanks Stones, that was really informative and reassuring, and actually quite applicable to my situation as we are looking at the ecodan and live in an exposed area where strong winds are a regular occurrence! With our gable ends to the south and  west being primarily glazing, I too am somewhat concerned re. overheating; I'm hoping to combat it by taking advantage of the constant wind strength, together with potentially circulating the UFH when not demanding heat as that will bring into play the old part of the house and the lower ground floor level which is part subterranean. Could I ask if you're running solely the pre-plumbed UVC or incorporating a buffer tank as I didn't spot any mention of a buffer on the blog? Also did you DIY install the tank & ASHP in the end? I don't suppose you have any trade contact details at secon solar - did the trade price save you quite a bit? Great blog btw, slowly making my way through it.

[JulianB]

9 hours ago, joth said:

 

Why do you want the UVC to benefit from the stored energy in the buffer tank - for speed of heating the UVC, or feeling it will be 'wasted' energy if not used?

To the former, the buffer tank is lower temperature so not sure it will make much of a dent in heating the UVC unless it has been totally drained and is stone cold. To the latter, so long as the buffer tank is inside the thermal envelope of the building, the energy in it should not go to waste: this  escaping energy is usefully heating the house. This is how I'm reasoning about it anyway ? 

 

 

Come to think of my previous response - my argument of it wasting energy might be muted if the same levels of COP are achieved when demanding the heat from either CH or DHW...so really it's just the speed of heating and whether the fact that the buffer tank remains in series for both the CH and DHW improves the COP of the ASHP?

[joth]

9 hours ago, JulianB said:

 

Come to think of my previous response - my argument of it wasting energy might be muted if the same levels of COP are achieved when demanding the heat from either CH or DHW...so really it's just the speed of heating and whether the fact that the buffer tank remains in series for both the CH and DHW improves the COP of the ASHP?

A few more unorganized thoughts on this:

- the key point of a buffer tank is to simplify commissioning, making differing flow rates compatible. Do anything clever and you've added rather than removed complexity. (Tripley so is using SA as buffer tank)

- I've understood  the uvc is in the loft, but does the buffer tank need to be there too? Putting it nearer the place that needs heating would be more intuitive to avoid heatloss. This will probably payback much faster (I'm reduced heatloss) than incremental gains on COP for the DHW)

- if ubc cold-start reheat times are critical, what are you going to do in summer when the buffer tank is not in use? Maybe consider a larger uvc and don't let it get empty?

- with uvc in the cold loft, insulation on it will be important. This maybe of interest: https://www.osohotwater.com/en/domestic/optima-geocoil A rated with "tank in tank" integrated buffer tank. 

- will you be using a monobloc heat pump? We're putting in a warm loft, but went off putting the UVC there because of the pipe run length to get to it. 

 

Edited May 24, 2020 by joth

[Stones]

20 hours ago, JulianB said:

 

Thanks Stones, that was really informative and reassuring, and actually quite applicable to my situation as we are looking at the ecodan and live in an exposed area where strong winds are a regular occurrence! With our gable ends to the south and  west being primarily glazing, I too am somewhat concerned re. overheating; I'm hoping to combat it by taking advantage of the constant wind strength, together with potentially circulating the UFH when not demanding heat as that will bring into play the old part of the house and the lower ground floor level which is part subterranean. Could I ask if you're running solely the pre-plumbed UVC or incorporating a buffer tank as I didn't spot any mention of a buffer on the blog? Also did you DIY install the tank & ASHP in the end? I don't suppose you have any trade contact details at secon solar - did the trade price save you quite a bit? Great blog btw, slowly making my way through it.

 

No buffer tank, didn't see the need or point as we have UFH covering 150 sq m of slab, which is of course a huge buffer in of itself, and we've not had any issues with short cycling which was one of the main reasons why we might have fitted a buffer.    Plumber fitted the cylinder and ASHP, but as per the blog entries, simple and quick for them to do.  No specific contacts - just give them a ring.  The package was circa £800 or so cheaper than the plumber quoted via the local merchant here.  

[DenkiJidousha]

Resurrecting an old thread, but if heat loss calculations or historic usage puts you on the cusp of two size models of ASHP (in this case 7kW or 10kW), is the rule of thumb always go large?

 

Sadly the larger capacity unit in this case would be significantly larger in terms of the outdoor unit (two fans vs one, I think), noisier, and adds about 10% to the capital costs (combi replacement), but is in theory more efficient. Aesthetics and wallet say go small, but is this a bad idea?

[Nickfromwales]

11 minutes ago, DenkiJidousha said:

Resurrecting an old thread, but if heat loss calculations or historic usage puts you on the cusp of two size models of ASHP (in this case 7kW or 10kW), is the rule of thumb always go large?

 

Sadly the larger capacity unit in this case would be significantly larger in terms of the outdoor unit (two fans vs one, I think), noisier, and adds about 10% to the capital costs (combi replacement), but is in theory more efficient. Aesthetics and wallet say go small, but is this a bad idea?

All depends if you want a high-revving 2-stroke, or a smooth running 4-stroke.... Both will work, one will work full time, the other part time. Longevity has to come into the equation too so it's a little multi-faceted.

[DenkiJidousha]

Thanks - and good analogy. If the installer agrees (they are going to recheck their numbers), I'm leaning to the physically smaller quieter unit. It is going to on the front of the house so the aesthetics are a real factor, sadly nowhere else to tuck it away out of sight.

[Nickfromwales]

19 minutes ago, DenkiJidousha said:

Thanks - and good analogy. If the installer agrees (they are going to recheck their numbers), I'm leaning to the physically smaller quieter unit. It is going to on the front of the house so the aesthetics are a real factor, sadly nowhere else to tuck it away out of sight.

There are now "silent" / super quiet models available too.

Random example

[Dan F]

2 hours ago, DenkiJidousha said:

Resurrecting an old thread, but if heat loss calculations or historic usage puts you on the cusp of two size models of ASHP (in this case 7kW or 10kW), is the rule of thumb always go large?

 

Sadly the larger capacity unit in this case would be significantly larger in terms of the outdoor unit (two fans vs one, I think), noisier, and adds about 10% to the capital costs (combi replacement), but is in theory more efficient. Aesthetics and wallet say go small, but is this a bad idea?

 

I was looking at exactly this same issue, with exactly the same ASHP model a while ago.  Still not decided which one to go for though.  What I would say, is look at the power table, as the 7kW is only 7kW at -5C/55C.   I'm not sure where you are based, but we are in the south-east and don't expect -5C.

 

 

 

The other difference is that:

- 10kW version has a "scroll compressor" instead of a "rotary piston".

- 10kW version I assume has a higher minimum power

 

Edited October 20, 2020 by Dan F

[DenkiJidousha]

We're in Scotland, near Dundee. Sustained cold spells below zero are rare but have happened, like the winter of 2010/2011:

 

https://en.wikipedia.org/wiki/Winter_of_2010–11_in_Great_Britain_and_Ireland

 

Good point that only in those extremes (when you need most heat) does the Vaillant areoTherm plus' output fall to 7kW, or below if making even hotter water.

 

At power level A7/W55, and 1m away, they quote 47 dB(A) for the 7kW and 52 dB(A) for the 10kW - neither is as good as the 42kW for that Mitsubishi Electric Ultra Quiet Ecodan example (assuming this is a fair comparison). I don't have a feel for how loud these actually would be in situ.

[Dan F]

11 hours ago, DenkiJidousha said:

Nneither is as good as the 42kW for that Mitsubishi Electric Ultra Quiet Ecodan example (assuming this is a fair comparison). I don't have a feel for how loud these actually would be in situ.

 

Not sure how the "1m away" calculation works.  If you compare the sound power of each, they doesn't seem to be too much in it:

aroTHERM plus 7kW:     55dB. 
aroTHERM plus 10kW:   60dB

Ecodan R32 8.5kW:         58dB

Ecodan R32 11.2kW:       58dB

[SteamyTea]

9 hours ago, Dan F said:

If you compare the sound power of each, they doesn't seem to be too much in it

The dB sound scale is non linear though.

https://www.britannica.com/science/sound-physics/The-decibel-scale

[Nickfromwales]

5 minutes ago, SteamyTea said:

The dB sound scale is non linear though.

https://www.britannica.com/science/sound-physics/The-decibel-scale

You can also strategically mitigate against noise, or at least not make things any worse, by avoiding sound traps. 
Two opposing faces have always been a natural form of amplification ( that why people put both hands to their mouth to call someone ) so “free dB” which, in this instance of course, you don’t want.

Siting the ASHP perpendicular to the wall for eg would be one easy way of reducing the problem, and another would be to introduce a living wall opposite it to absorb the sound waves. 
Small measures will have a significant impact, as as stated above the dB scale is not linear ( so small improvements will be largely worthwhile ). 

[SteamyTea]

46 minutes ago, Nickfromwales said:

that why people put both hands to their mouth to call someone

That is to do with reducing attenuation.  Which is another factor in sound propagation.

It is why there is a large reducing in sound levels between 1 m and 2 m away, the surface area of the 'sphere of sound' is greatly increased, which reduces the energy per unit area.

 

Realistically, any half decent ASHP is not going to make much noise.  @joe90's air blower for his sewage was a bigger problem.

 

[SuperJohnG]

On 20/10/2020 at 22:04, Dan F said:

 

I was looking at exactly this same issue, with exactly the same ASHP model a while ago.  Still not decided which one to go for though.  What I would say, is look at the power table, as the 7kW is only 7kW at -5C/55C.   I'm not sure where you are based, but we are in the south-east and don't expect -5C.

 

 

 

The other difference is that:

- 10kW version has a "scroll compressor" instead of a "rotary piston".

- 10kW version I assume has a higher minimum power

 

@Dan F what did you get in the end? In reality the 7kW unit will be more than enough. But unsure if a rotary piston is better than a scroll type? 

[Dan F]

2 hours ago, SuperJohnG said:

@Dan F what did you get in the end? In reality the 7kW unit will be more than enough. But unsure if a rotary piston is better than a scroll type? 

 

7kw.  The rotary piston may technically be better, but the minimum output on the 10kw unit is a significantly higher and wouldn't have been suitable.

[SuperJohnG]

20 hours ago, Dan F said:

 

7kw.  The rotary piston may technically be better, but the minimum output on the 10kw unit is a significantly higher and wouldn't have been suitable.

Thanks. Think we are going to go with the 7kW unit.