Heat loss, and the result

[Big Jimbo]

I'm building a new house of 2500 square ft. Basically an oblong, internally, 10.2m x 12m with a 2.5m wide glazed porch on the front. The "u" values as designed are, floor .09, walls .16 roof .11 windows and doors .96  Windows and doors are just under 25% of floor area.             So, a bit better than building regs, but not a huge improvement.  I have done some detailed heat loss calcs on a room by room basis, using the following. Design temp for bedrooms 18 Degrees. bathrooms 22 degrees. all other areas 21 degrees. I used an outside air temp of -3 degrees, and i used an air change rate of 3 per hour. (I will aim for less air changes, but wanted to be safe for my calculations)

I got a total house heat loss of 12125 watts. Would anybody have any idea if that sounds reasonable for a 2023 new build of 2500 Square ft ?

Thanks in advance as always.

[ProDave]

What calculation software did you use?  Was it Jeremy's spreadsheet on this forum?

 

I have broadly similar insulation values to you but only a 150 square metre house, and my max heat loss is about 2200W with inside at 20 degrees and outside at -10

[S2D2]

The air changes per hour will make a huge difference and you should be nowhere near 3 in a new build. Try 1 as a conservative starting point.

[Big Jimbo]

I did try Jeremy's, @ProDave but noticed when i finished that he had the air changes set at .5 I know people on here try and get as low as they can, but .5 seems very low, and if you did a decent job and got 1.5 my worry is that you would not have enough energy being provided to heat the house in cold weather.

[Big Jimbo]

8 minutes ago, S2D2 said:

The air changes per hour will make a huge difference and you should be nowhere near 3 in a new build. Try 1 as a conservative starting point.

Most of the losses seemed to come from the air changes, but my concern is how realistic is 1 ? All very well calculating it at that, but if you then score 2 ?

[JohnMo]

58 minutes ago, Big Jimbo said:

air change rate of 3 per hour

Even if your airtight test shows you have a leakage rate of that, that isn't the figure you would used in a heat loss calculation. The 50Pa used during the test is the equivalent of extremely high winds, on all sides of the house - so you have to have the lowest temp and extremely high winds from all directions at the same time, which is rare or impossible. You would be fine using 0.5. It's only really going to be that if you have mechanical ventilation running 24/7. Otherwise the infiltration would less than 0.2. If you have a SAP report the infiltration rate will be in there.

 

 

[SteamyTea]

I just quickly modelled your house as a simple cube. 12 m long, 10 m wide and 5 m high.

Came out, as 292 W/°C at 1 ACH.

 

So at a delta of 20°C, that would be 5831 W, at 23°C, 6705 W, or 6.7 kW.

 

Have you mixed up some units somewhere i.e. metres and feet.

 

Edited September 14, 2023 by SteamyTea

[Big Jimbo]

@SteamyTea I was using 3 ach. I am now redoing it at .5       I will report back. Thanks everybody inc @JohnMo

[Thorfun]

1 hour ago, Big Jimbo said:

Most of the losses seemed to come from the air changes, but my concern is how realistic is 1 ? All very well calculating it at that, but if you then score 2 ?

our target was 1ACH. our house is much more complicated that a basic rectangle and we spent a lot of time taping windows and membrane around attic trusses and fitting Tony trays etc. we ended up with a score of 0.98ACH@50Pa and were very happy. we know we could've got it lower if we had the time (and a diy blower to keep testing with) but as we'd already hit my target i was happy to leave it there. so 1ACH is very achievable doing it DIY.

[SteamyTea]

48 minutes ago, Big Jimbo said:

was using 3 ach.

That would only be another 600 W.

 

[S2D2]

20 minutes ago, SteamyTea said:

That would only be another 600 W.

 

An extra 208x2x23=9.568kW surely? ACH assumptions make a huge difference to heat loss calculations, particularly in well insulated large houses. @SteamyTea's worked example shows that clearly, 84W for fabric and 208W for 1ACH.

 

As mentioned above, it's the realistic value you want to use not the @50Pa result.

[Big Jimbo]

So, happy with my fabric losses as calculated. Re-doing my ACH using .5      Would i be right in thinking that my calculation would be......Volume (LxWXH) X ACH (using0.5) X 0.33 X mytemp for that area added to my outside temp at -3 ? (so for 21 it would be 24 ?

[Dave Jones]

@Big Jimbo try https://www.changeplan.co.uk/heat_loss_calculator.php

 

It's a lot easier to use, i just entered ours as a box, similar U values to yours and at -15C out 21 in im getting 6.6kw for 211m2

 

[SteamyTea]

1 hour ago, S2D2 said:

An extra 208x2x23=9.568kW surely

Yes.

I should have said it was per degree difference.

[Big Jimbo]

Just to let all the kind people who have contributed to my plight. 

At an outside temp of -2, inside target temps of 18 degrees bedrooms, 22 degrees bathrooms, and 21 degrees all other areas, i will need 6509.52 watts of energy to heat my house on a bloody cold day. (OK, not a scottish cold day, but those lads and lasses are proper hard)

So 6.5kW. So should i now be looking for an ASHP of that size ?

My next task will be to try and calculate how much water, and at what temp i will require to store it at, in order to provide enough to run 4 showers at the same time. My thoughts are that, (and i'm not saying i'm right) I don't see the point of storing hot water at much higher temp than i actually need it at the outlets, just to blend it down by then adding cold water. Again with my badly educated mind my thoughts are that the lower temp i store the water at, then the larger the tank will need to be ?

[JohnMo]

Is this any use, found it in a manual for my heat pump cylinder.

 

Is 4 showers at the same time an every day event or once in a blue moon? As if a rare event you can always increase capacity by switching on the immersion and heating to a hotter temp. Just add a mixing valve to the UVC hot water outlet.

 

[joth]

13 minutes ago, JohnMo said:

Just add a mixing valve to the UVC hot water outlet.

Word of warning from someone badly scolded by this: 

- putting a mixing valve on the UVC output is liable to cause issues if you have secondary return.

- the mixing valve doesn't work at all if it's too close to the UVC outlet and gets heat washed by it (e.g. in the obvious place directly above the cylinder).

- And it  really doesn't work if you have both of the above

- (and, it's even worse again if your installer forgets the check valve on the secondary return pump).

 

In the end I decided easier to put a mixing valve on every tap / appliance that needs one than try and do this centrally. In practice this is just the bathtub for me, as everything else already had one.

 

 

Edited September 14, 2023 by joth

[joth]

30 minutes ago, Big Jimbo said:

So 6.5kW. So should i now be looking for an ASHP of that size ?

You might want to go a bit larger, as you want it running less than 24/7 on heating on coldest day as it'll also need some time to do the DHW reheat.

Also going larger means the DHW reheat is faster, meaning you can get away with a smaller tank.

 

31 minutes ago, Big Jimbo said:

Again with my badly educated mind my thoughts are that the lower temp i store the water at, then the larger the tank will need to be ?

Spot on. You increase the stored energy by storing more volume, or higher temp, or both. 

Generally a larger volume at lower temp is more efficient (better COP to heat it, and lower standing losses due to lower delta T and the better surface area to volume ratio on the tank. I think 😅)

 

 

[Dave Jones]

im looking at a 9kw panasonic aquera as running it at 4-6kw is its most efficient apparently.

[SteamyTea]

52 minutes ago, Big Jimbo said:

So 6.5kW. So should i now be looking for an ASHP of that size

Probably an 8 kW model is needed.

Most of the time it will be fine as it will not be -2⁰C outside, more like 5 to 10⁰C, then the heating is off.

 

Are you fitting MVHR, if so, your air change losses will be quite low, as long as you get your airtightness good.

 

As others have said, how often will you really be running 4 showers at the same time?

At 10 litres a minute, with 4 showers running, a 300 litre cylinder would be empty in about 10 minutes. Actually if it was stored evenly at shower temperature, the 7.5 minutes.

 

[Big Jimbo]

@SteamyTea How blinking close were you at my heat loss calcs. Gold star fella.

[SteamyTea]

10 minutes ago, Big Jimbo said:

@SteamyTea How blinking close were you at my heat loss calcs. Gold star fella.

I don't know, how close?

 

It is always worth doing a simple model as a sanity check. Then do the detailed one to set emitter sizes for different room temperatures.

[Big Jimbo]

@SteamyTea within 200 watts. 

[SteamyTea]

Shows I can bull shit with the best of them still.

 

I was thinking that it may be worthwhile knocking up a very simple spreadsheet that does a sanity check.

Or if there is a clever person on here, a web based one.

[JohnMo]

54 minutes ago, joth said:

Word of warning from someone badly scolded by this: 

- putting a mixing valve on the UVC output is liable to cause issues if you have secondary return.

- the mixing valve doesn't work at all if it's too close to the UVC outlet and gets heat washed by it (e.g. in the obvious place directly above the cylinder).

- And it  really doesn't work if you have both of the above

- (and, it's even worse again if your installer forgets the check valve on the secondary return pump).

Not sure why a mixing valve would not work at the top if a cylinder. It is supposed to be first thing hot water goes through.

 

https://forum.buildhub.org.uk/topic/21490-thermostatic-basin-mixer-taps/

 

Are you sure the issue wasn't the plumber not installing the check valve on the secondary return. As then if the secondary return pump is off the easiest route for the hot to go would be to reverse flow through the secondary loop to the hot water outlet.