Heat pump for a big bath!

[Jonnyc]

Newby here...

 

We're building a treehouse - but one with a ground floor. It's all insulated heavily and airtight with a MVHR so plan is not to put a lot of heating in it. But we do have a very large bath.

 

So are thinking of using an ASHP to heat one UFH heating panel (about 15m2) using 12mm pipes and then a large (300l) water tank to provide the hot water. 

 

So just looking for thoughts on sizing of the heat pump. Was hoping to get away with 5kw but I'm pretty certain someone is going to tell me that's not enough!!  

 

The unit needs to have a cooling phase as we are using a new Zhender MVHR with has a cooling loop.  Does anything spring to mind anyone??  is there a perfect ASHP/ cylinder combo?

 

Many thanks.

 

[ProDave]

5kW will be plenty.  Almost certainly way more than you need for heating and cooling the building and adequate for heating a 300L water tank.  I and many others have that size of ASHP for heating a whole house.

[Dave Jones]

5kw will be more than adequate. 

 

forget cooling with MVHR we have exactly same zehnder unit with battery and its pointless wouldn't bother again. Instead get  a wall mounted fan coil (Panasonic do nice ones) unit and insulate pipe to it with a condensate drain. You can then use the ASHP at 10c and the fancoil will essentially be aircon. We have 7 of them in bedrooms and they are fantastic. our 5kw panasonic ASHP will heat the 300L cylinder from mains temp to 55c in 20 mins in the current nice weather.

[JohnMo]

10 hours ago, Jonnyc said:

Was hoping to get away with 5kw

5kW is perfectly adequate to heat any volume of water. It just a matter of how long it takes. Would suspect the reheat times will be around an hour, from a fully depleted cylinder.

 

UFH just use 16mm, output of 16mm and pressure drop on 12mm isn't helpful. Or dump the idea of UFH.

 

To make your heating work the way you describe during the heating season your UFH will need to be close to the room temps you need all the time as it will only be putting out around 0.5kW. that will never reheat the place if it goes cold.

 

As @Dave Jones says fan coil, that's what we use after dumping the UFH in our garden room.  You will need a volumiser (about 40 to 50L) to stop the heat pump short cycling with only one or two fan coils as system volume will be too small without.

Edited June 30 by JohnMo

[Dave Jones]

use ufh pipes as volumizer! put them as tight as you can!

[JohnMo]

2 minutes ago, Dave Jones said:

use ufh pipes as volumizer! put them as tight as you can!

Did think that also, but the 16m2 area and 100m of pipe is only around 12L, which isn't enough.

[Beelbeebub]

I'm sure I saw some manufacturer makes an invented cylinder heated by a refrigerant loop that you hook up as one of the head units to a mumtisplit.  The cylinder has the necessary electronics in to interface with the outdoor unit.

 

So get a 2 or 3 head 5kw multi split and have one head as the cylinder and the other 2 as heating or cooking as required.

 

Probably cost less than the a2w setup.

[Beelbeebub]

Here it is

 

Daikin multi+

 

 

120l tanks so might need 2!

[Beelbeebub]

12 hours ago, Jonnyc said:

then a large (300l) water

How big is this bath?

 

 

[sharpener]

2 hours ago, Dave Jones said:

our 5kw panasonic ASHP will heat the 300L cylinder from mains temp to 55c in 20 mins in the current nice weather.

 

2 hours ago, JohnMo said:

5kW is perfectly adequate to heat any volume of water. It just a matter of how long it takes. Would suspect the reheat times will be around an hour.

 

5kWh is 4305 litre-degrees. So 5kW will heat a 300 litre cyl at 14.35 deg per hour. So about 3 hrs from stone cold. Reheat from tepid obviously quicker.

[SteamyTea]

How large is this big bath, 200, 300, 500lt?

How often is it used?

How long is the wallow?

Is it emptied every time, or like a spa bath, just reheated.

Will it have, like a spa bath, aeration in it?

How much do you want to spend on it?

Edited June 30 by SteamyTea

[JohnMo]

26 minutes ago, sharpener said:

 

 

5kWh is 4305 litre-degrees. So 5kW will heat a 300 litre cyl at 14.35 deg per hour. So about 3 hrs from stone cold. Reheat from tepid obviously quicker.

I based it my reheat time from call for heat to stopping heating and allowed for the extra volume of the OP.

[Dave Jones]

2 hours ago, sharpener said:

 

 

5kWh is 4305 litre-degrees. So 5kW will heat a 300 litre cyl at 14.35 deg per hour. So about 3 hrs from stone cold. Reheat from tepid obviously quicker.

 

your forgetting COP. 20 mins tops this time of year and it depends on the outside temp.

[sharpener]

1 hour ago, Dave Jones said:

your forgetting COP

 

Don't think so. Quoted ratings of HPs are for thermal output not electrical input unless your Panasonic is different. Yes it will be a bit better in warm weather, but not by a factor of 3.

 

But many ppl will reheat when it gets down to 40 as that is only slightly above body temp. So are not exercising it over the range 10 - 40C which accounts for 2/3 of the total stored heat input.

 

Is O level physics. For comparison a 3kW immersion heater takes the best part of 3 hrs to heat the usual 210 l cyl from cold to 55C. Entirely consistent with the above.

 

 

[JohnMo]

8 minutes ago, sharpener said:

210 l cyl from cold to 55C

But you wouldn't do that in real life. A heat pump cylinder generally has a thermostat within a foot of the bottom of the cylinder. So reheat times are generally manageable in a normal house without the house cooling down to much. Then the reheat volume and time is purely based on thermostat hysterisis and cylinder shape and volume from where the thermocline sits (not total capacity), which could be quite low as it may well be in @Dave Jones case my 6kW reheat times the last couple of days have been around 30 minutes, but the water has to travel about 20m each way so I do loose some efficiency.

[Beelbeebub]

As your storage volume goes up, the required input power of your heating device trends to whatever the daily power requirement is. Eg a 1000l tank might only need a 2kw heater as it can produce 48kwh a day.

 

The smaller the tank the closer your input power needs to be to the maximum instantaneous power requirement. A combi boiler might need to be 28kw to produce a decent flow.

 

Unless the OP planned to use more than 300l in a sitting, then pretty much any HP should do.

 

[SteamyTea]

45 minutes ago, sharpener said:

Is O level physics. For comparison a 3kW immersion heater takes the best part of 3 hrs to heat the usual 210 l cyl from cold to 55C. Entirely consistent with the above

Not quite true in reality as the formula E [J]= kg x c [J/kg.K] x ∆T is a linear formula, so does not account for thermal losses.

The formula Q = h . A .(T(t) - T{env}) is closer.

Where:

Q = rate of heat transfer out of the body

h = heat transfer coefficient

A = heat transfer surface area

T = temperature of the object's surface

T_{env} = temperature of the environment

T(t) = time-dependent temperature

 

 

[sharpener]

All very true but the linear approximation is good enough for everyday calcs.

 

The custom 270 litre tank I have got on order has a loss of 1.49 kWh per 24h which is small in comparison with the actual usage.

 

This standing loss of about 60W is only 1% of the OP's proposed 5 kW HP output so can safely be ignored in the heating time calculation.

 

[ReedRichards]

8 hours ago, sharpener said:

The custom 270 litre tank I have got on order has a loss of 1.49 kWh per 24h

 

Wandering off-topic but I have a horrible feeling that these quoted losses are for an unconnected tank and that the loss goes up considerably when you connect copper pipes to a tank.  Copper is a superb conductor of heat and the pipes must wick the heat away.  

[JohnMo]

9 minutes ago, ReedRichards said:

 

Wandering off-topic but I have a horrible feeling that these quoted losses are for an unconnected tank and that the loss goes up considerably when you connect copper pipes to a tank.  Copper is a superb conductor of heat and the pipes must wick the heat away.  

Probably is, maybe one of the reasons a lot of States in the US mandate heat trap, orientation pipe work. i.e. limited length upwards, horizontal and then drop down vertically on all cylinder pipe connections, as soon as practical, to stop thermal gradients moving along piping, leaching the heat away.

[sharpener]

57 minutes ago, ReedRichards said:

I have a horrible feeling that these quoted losses are for an unconnected tank

 

Possibly. It is as ever a balance between capital cost, running cost and practicality.

 

I decided to pay extra to increase the insulation on the tank itself from 50 to 75mm. The thermal store will be in a utility room so everything is inside the thermal envelope of the house anyway. The 22mm outlets will have heat traps, but for the 28mm primary connections it would serve no point anyway as there is flow all the time through the buffer section at the bottom of the tank.

 

The tank will be at max temp for only a short time because we will use the heat again as soon as the Cosy cheap periods finish at 0700 and 1600. Octopus have just added an extra two hours from 2200 to 2400, which is welcome but has changed the planned usage pattern slightly e.g. we could charge it up for a third time and use it for background heat on the landings.

 

The concept enables different flow temps for rads and UFH. In an ideal world I would add a further two-port valve so when the tank has got too cold for the rads we can use the residual heat in the UFH, but the design has been finalised with Vaillant now and there are no spare control outputs to do it.

 

 

Edited July 1 by sharpener

[ReedRichards]

4 hours ago, JohnMo said:

Probably is, maybe one of the reasons a lot of States in the US mandate heat trap, orientation pipe work. i.e. limited length upwards, horizontal and then drop down vertically on all cylinder pipe connections, as soon as practical, to stop thermal gradients moving along piping, leaching the heat away.

 

I think a heat trap works because hot water rises.  And I'm sure that they help overall, but I'm not at all sure how they influence heat conduction in the copper pipes.  

[JohnMo]

46 minutes ago, ReedRichards said:

 

I think a heat trap works because hot water rises.  And I'm sure that they help overall, but I'm not at all sure how they influence heat conduction in the copper pipes.  

I have one at the top of my cylinder (top upwards exit) goes up about 50mm and across cylinder and then down. The downwards pipe is stone cold the top pipe hot.

[SteamyTea]

I seem to remember that water storage cylinders have an odd methodology when measuring thermally losses.

Something like bringing the water up to temperature, then using that heated water i.e. a bath, then letting the residual heat reduce in temperature for the remains of the 24 hour period.

That is not the same as having a cylinder of heated water and measuring the temperature drops over 24 hours.

The methodology worked against Sunamp which had very low standing losses apart from  around the designed operating temperature.

[Jonnyc]

Hi All,

 

Thanks for all this wonderful help.  I had a feeling I would be able to rely on you all.  So maybe a little more detail might help.  But I think I'm going to be fine which is very reassuring!!  

 

The bath in question is a timber bath we have made from Douglas FIr.  You can see it in the foreground of the attached pic (assuming I manage to attach it) internally it measures 1670x700x390 but it has some space saving blocks inside so the total volume is 390 lites .. Assuming a person of say 75 kilos- and possibly a second one of a similar size means the Total volume comes down to around 240-300 litres per bath.  As long as the water is actually hot - is 55% unreasonable ? - then we should get away with only needing about 200 litres of hot water at a time. The bath is designed, to overflow onto the floor where it drains away so creating a sort of infinity edge.  We're assuming therefore it will generally need all that water.  The treehouse is a commercial holiday let aimed at couples so I don't think anyone will need the bath again super fast.  There are also other showers and it will actually  accommodate up to 4- 5 people.  So on balance they might run out once in a while but I think we're almost there... 

 

I guess the next thing is to choose a ASHP unit. But I have no idea on that...  do I just plump for a 5kw affordable HP and a heat pump cylinder and allow the guys at REDD to work it all out for me?  (or is that a really stupid question?)

 

Or should I just go up one size to make sure???

 

Many thanks for all you patience!!