Criticisms please

[Beelbeebub]

We managed about 16kwh/day average for heating and DHW over a year.

 

130m2 "dorma bungalow", 8" glass wool cavity fill between alight blocks. 12" same in roof with 2" of wood fiber over the top. 8" of XPS foam under floor slab with beam and block. 

 

High performance DG windows (not triple). Very air tight with MHRV.

 

Solar thermal panels for summer DHW and a (rarely used) wood stove that coukd feed the heating and DHW in winter.

 

Our winter peak use was obviously higher than 16kwh/day as some summer days we used 0kwh gas.

 

 

[waxingsatirical]

Just to be clear, 15.5kWH is the energy drawn by the heat pump not the heat loss for the house.

 

But yes, it does seem low, the house is not especially well insulated, it's a mix of filled cavity/unfilled cavity, pretty-well draughtproofed though and loft-insulation is good. It's a semi and a cube, so that helps, and only 1500sqft.

 

I'm now thinking that the heat-pump is under-estimating the energy it's generating. I don't know how it's working it out. I guess the difference between flow and return temp multiplied by the flow rate. Would explain the low heat-loss and the poor COP.

 

I put the hot water on the immersion for a few days, to take that out of the equation, and it didn't make much of a difference to the COP, still only just above 3.

 

Also, my COP figures are "energy-weighted", meaning I take a day's worth of energy out and divide it by a days worth of energy in. So this would give a lower COP than "time-weighted", where you would take multiple readings of the COP throughout a day and average them.

[SteamyTea]

2 hours ago, waxingsatirical said:

I guess the difference between flow and return temp multiplied by the flow rate

SHC of water (~4 kJ.kg^-1.K^-1) x Flow Rate (kg.day^-1) x Delta T (Flow Temp - Return Temp) x 0.00027778 = kWh.day^-1

 

  

2 hours ago, waxingsatirical said:

Also, my COP figures are "energy-weighted"

I think that is the proper way to do it, though I am not sure if that is how the Seasonal Performance is calculated.

 

 

Edited December 30, 2024 by SteamyTea

[PhilT]

18 hours ago, waxingsatirical said:

Just to be clear, 15.5kWH is the energy drawn by the heat pump not the heat loss for the house.

 

But yes, it does seem low, the house is not especially well insulated, it's a mix of filled cavity/unfilled cavity, pretty-well draughtproofed though and loft-insulation is good. It's a semi and a cube, so that helps, and only 1500sqft.

 

I'm now thinking that the heat-pump is under-estimating the energy it's generating. I don't know how it's working it out. I guess the difference between flow and return temp multiplied by the flow rate. Would explain the low heat-loss and the poor COP.

 

I put the hot water on the immersion for a few days, to take that out of the equation, and it didn't make much of a difference to the COP, still only just above 3.

 

Also, my COP figures are "energy-weighted", meaning I take a day's worth of energy out and divide it by a days worth of energy in. So this would give a lower COP than "time-weighted", where you would take multiple readings of the COP throughout a day and average them.

Do you know how much gas you used last Dec? That will give you a broad sense check. This Dec is very similar re temperature and weather conditions, in my area at least. 

[SteamyTea]

8 hours ago, PhilT said:

This Dec is very similar re temperature and weather conditions, in my area at least. 

Sometimes when I go up to Aylesbury, which is frequently, it is lovely and warm, other times, really cold.  Down here it seems to have been just wet.

[PhilT]

3 hours ago, SteamyTea said:

Sometimes when I go up to Aylesbury, which is frequently, it is lovely and warm

Yes the number of new houses that have been built is so huge, it all acts like a gigantic storage radiator

[waxingsatirical]

Another year another update.

 

Bought a CO2 monitor, as I was hoping to use it to justify closing trickle vents for the really cold months. Turns out that even with all trickle vents open CO2 gets up to ~1300 (forget the units, sorry) at night. It varies a lot with the wind I think. There is a draught coming from the spare room, which I was planning on closing up but instead I make sure I have the door to this room open at night now, which keeps C02 below 1000.

 

This is with a family of 4 in a modest 4-bed house.

 

It does make me wonder why I bothered with all that insulating, and draught proofing when I practically need to leave a window open 24/7.

 

So now looking into mechanical ventilation with heat recovery.... any shared experiences with this would be welcome 

[Nickfromwales]

https://forum.buildhub.org.uk/forum/79-mechanical-ventilation-with-heat-recovery-mvhr/

[JohnMo]

1 hour ago, waxingsatirical said:

 

So now looking into mechanical ventilation with heat recovery

Unless your actually pretty much airtight I read wouldn't. What are your current fans in wetrooms? You just need to promote a cross flow ventilation from dry rooms to wet. It doesn't need to be howling gail just a gentle flow. 

 

You also need to make sure you can close doors by have a small ventilation gap at the bottom of all doors.

 

What I have written many times

Greenwood CV2 or CV3 fans in all wetrooms including kitchen. They run all the time at a very low rate, they are almost silent. They automatically visit on rising humidity.

 

Automatic trickle vents that respond to humidity in all dry rooms. Seal all trickle vents in wet rooms.

 

Undercut all doors.

 

You then have a system that automatically responds to where people are in the building minimising ventilation. Easy to retro fit, pretty cheap implement, zero maintenance.

[Beelbeebub]

JohnMo's solution sounds nice and simple.

 

If you do decide to go for mechanical ventilation, I have used the ventaxia sentinel kentic series a few times now and been happy with the price/performance.

 

Build quality seems pretty good and (hopefully) spares & support should be OK given they are a UK company. 🤞

 

I use the radial 75mm smoothbore ducting with manifolds for ease of installation and reduced noise between rooms.

 

For some properties I have put a supply and extract in opposite corners of the room to get around the issue of undercutting doors etc. 

[waxingsatirical]

Thanks for your input guys. I've been busy with my CO2 monitor to try and figure out how many additional air changes I need in a night to keep the CO2 constant. Then I'm trying to figure out how much heating that air will cost me, to try and decide if it's worth going for mechanical ventilation with heat recovery or not.

 

Here are my back of envelope calculations.

Measured CO2 increase per hour at night: 30ppm

CO2 output from a sleeping family per hour 30L (From this website https://docs.openenergymonitor.org/heatpumps/measuring_ach_with_co2.html) 

100sqm of floor area (rest of house is closed off) x 2.2m ceiling height = 220 m3 = 220,000L

Therefore, with zero air changes would expect 220,000 * 30 /1,000,0000  = 136 ppm increase per hour (this is using the 30L family CO2 output)

Let's say that indoor is 800 and outdoor is 400, so 1 ACH results in decrease of 400ppm

I'm seeing a difference of ~100ppm between measured and calculated increase per hour, so getting 0.25 ACH

To acheive a steady state, need an additional ~0.1ACH

 

For 1 cold night:

0.1 * 220m3 * 1.2kg/m3 (air density) * 1005 (heat capacity in Joules) * 20 (temp difference on cold night) * 8 (hours in night) / 3,600,000 (convert to kWh) = 1.2kWh

1.2 /3 (COP of heat pump) = 0.4kWh

 

So what's that? about 5p on the night rate?

[waxingsatirical]

Seems a bit low, I think I'm spending about £5 a day on heating at the moment

[marshian]

I'd put a PIV unit it and just run it over night - you could probably run it on the lowest speed (2 W consumption) and at that rate it's not going to cost much at all to heat the incoming air

 

We run ours on setting 3 overnight (7W) so that's 30 Litres per sec in a 291 m3 House volume it gives us an  ACH of 0.37

 

We have tried it on a lower speed but we get condensation on the inside of the windows in the main bedroom so we'd rather have a higher ACH and no condensation and a little more energy spend heating the cooler air that is being forced into the house

[SteamyTea]

1 hour ago, waxingsatirical said:

figure out how many additional air changes I need in a night to keep the CO2 constant

Why.

Slightly raised CO₂ levels helps you sleep better.

[ProDave]

On 31/12/2024 at 19:17, SteamyTea said:

Sometimes when I go up to Aylesbury, which is frequently, it is lovely and warm, other times, really cold.  Down here it seems to have been just wet.

Distance from the coast.

 

I found that in Oxfordshire, in winter like this last week, it can get very cold, Benson was -8 earlier in the week.  And in a summer heat wave it can be one of the hottest places without the coastal influence to moderate things.

 

That "inland" effect is very evident in the mid west USA.  for my sins I had 4 weeks just outside Chicago in summer, and found it unbearably hot, you really wanted to be in an air conditioned building or car to be comfortable.  Then i went again for 2 weeks in December and found it unbearably cold.  When I mentioned the cold a local said something like "cold, this isn't cold, you wait until it gets below 0"  (he was talking Fahrenheit) I did wonder how much spring and autumn fall they had where the temperature was pleasant.  I suspect not much.

[SteamyTea]

2 hours ago, ProDave said:

I did wonder how much spring and autumn fall they had where the temperature was pleasant

Lived in Pennsylvania and they only had winter, very cold and dry, and summer, very hot and humid.

Autumn in Halifax, Nova Scotia lasts about 4 hours, goes from 18°C to -18°C during that time.

 

Currently quite warm here.

 

[MPH243]

@SteamyTea Might be warm but a bit breezy for you 98mph on scilly isles and 89mph at Culdrose.

I hope everything stays ok for you.