Prepping for ASHP and sizing - Heat loss calcs confusion

[mistake_not]

Hi all,

 

Contemplating to moving to a ASHP over the summer (mainly for cooling benefit from UFH and fan coils). Doing my heat calcs I have a big difference between my calculated heat loss and real world heat loss. Hoping someone will help me understand the difference.

 

Calculated heat loss is around 5.1kw using the freedom heat pump spreadsheet (attached). I used the same target temps as we have in real life, and the output figures match those generated from loopcad when I was using that.

 

Real world I got data from my smart meter. I used a night in Jan that was -2, so no solar gain etc. This real world figure gave me 3.2kw.

 

So what do I believe? I don't want to have an oversized heat pump and want to run the system on weather comp. 

 

Other info is we find we can heat the house quite nicely just running the UFH, and leave the rads off upstairs. Means downstairs is at 19.5 and upstairs at 17. 

 

Thanks 👍 😊 

Heat loss calcs 20 HCR.xlsx

[JohnMo]

7 minutes ago, mistake_not said:

Real world I got data from my smart meter. I used a night in Jan that was -2, so no solar gain etc. This real world figure gave me 3.2kw.

Not sure a single night will give you an exact heat loss rate, as any day time gains will still be there. Suspect your real heat loss is somewhere between the two figures. Plus the heat loss spreadsheet uses min BR U values not your real ones.

 

But basically your ASHP model choice will depend on actually output at you min outside temp. I would understand size not over size.

[SimonD]

32 minutes ago, mistake_not said:

So what do I believe? I don't want to have an oversized heat pump and want to run the system on weather comp. 

 

Difficult to say as there are issues with both and I wouldn't trust either of them.

If you can bear the pain of re-inputting your data, you can use OpenHeatLoss.com  (which I have developed) to complete a calc and design to current design standards. This also provides the ability to look at your ufh design in relation to heat loads etc. The only thing it doesn't do right now is Fan coil sizing 

[mistake_not]

2 hours ago, JohnMo said:

Plus the heat loss spreadsheet uses min BR U values not your real ones.

Whilst it does have that deleted, it also has custom ones and I have made sure they match the as built values.

 

2 hours ago, JohnMo said:

Not sure a single night will give you an exact heat loss rate, as any day time gains will still be there. Suspect your real heat loss is somewhere between the two figures. 

Seemed like a pretty steady output over that night though, and it's wasn't sunny days.

 

But agree it's likely somewhere in the middle.

 

My gut feeling is the ability for the heat pump to modulate will be more important to me than max value.

2 hours ago, SimonD said:

If you can bear the pain of re-inputting your data, you can use OpenHeatLoss.com  (which I have developed) to complete a calc and design to current design standards. This also provides the ability to look at your ufh design in relation to heat loads etc. The only thing it doesn't do right now is Fan coil sizing 

I had a quick play when you first posted about it. All my u-values are as built, so I'm guessing I'll end up v similar but will give it a go.

 

Does anyone know the deal with BUS / MCS? Do they have to hit heat loss calcs exactly?

[SimonD]

6 minutes ago, mistake_not said:

Does anyone know the deal with BUS / MCS? Do they have to hit heat loss calcs exactly?

 

The MCS design and installation requirements are that the Heat Pump can provide all the heating/how water requirements, which does mean the unit has to be sized according to the heat loss, but that doesn't necessarily mean that your heat pump will be over-sized. Good design also makes sure that the heat pump can modulate down far enough to deal with typical temperatures without short cycling.

[JohnMo]

43 minutes ago, mistake_not said:

Does anyone know the deal with BUS / MCS?

Someone the other day had a radiator system that currently runs at 45 degs, he was getting quotes circa £13500 before grant for a heat pump and some install works - utter bonkers.

 

But generally most R290 heat pump seems to modulate well. But not all will do cooling if you need that. Example, current Grant heat pumps physically will do cooling BUT their controller does not allow this happen. Most installers are clueless about cooling, so be prepared for plenty of BS.

[mistake_not]

Yeah, the one quote I have had (based around 5kw pump, no rad changes, leaving water tank, but including boiler rip out) is something like 7k after grant 😞.

 

Tempted to see if the sparky that did my solar can do something with me so I get BUS.... I want a very simple fully open loop weather comp system, I already have PDHW and my pipework is all big enough already.

[saveasteading]

28 minutes ago, JohnMo said:

Most installers are clueless about cooling, so be prepared for plenty of BS.

Here's some input  from me. Maybe BS as I'm going by hunch and science, not proofs.

 

I can't see how air source pumps work efficiently in reverse for domestic purposes. Of course they work as chillers in air conditioning systems but that is by air circulation. Expecting to use cooling by sucking heat from a floor slab or radiator cannot be the same.

 

In heating, we warm the concrete floor or screed and it makes our feet and lower parts cosy. It then warms the adjacent, cooler air which rises to the ceiling and this continues until the whole room volume  is warm.

 

But in reverse, the screed will cool and we will have cool feet, and will slowly cool the adjacent air, but it will stay there. The warm air above will stay put.

 

If I'm wrong on any of this please advise.

 

[SimonD]

3 minutes ago, saveasteading said:

If I'm wrong on any of this please advise.

 

You're not far off. Cooling effectiveness of ASHP isn't that great, you may get a few degrees drop during the hottest days, so it might take the house from uncomfortably hot to okay, but that's about it. If you're in a position where you need to drop time, money and resources on infrastructure for an ASHP to do cooling, you're better of just getting a split a/c unit. I made that choice.

[saveasteading]

I know that air to air works both ways.. did it in our office... ashp outside, plenum inside then ducts and fans.

If I wanted cooling I'd do that.

[JohnMo]

29 minutes ago, saveasteading said:

heating, we warm the concrete floor or screed and it makes our feet and lower parts cosy. It then warms the adjacent, cooler air which rises to the ceiling and this continues until the whole room volume  is warm.

 

But in reverse, the screed will cool and we will have cool feet, and will slowly cool the adjacent air, but it will stay there. The warm air above will stay put.

 

If I'm wrong on any of this please advise

Not really correct. UFH works on radiation (around 80-90% and 10-20% by convection) not convection alone. Our 6m high ceiling is almost the same temperature as the wall thermostat reads. No cosy feet really either, the floor is actually cooler than your body, typically only a little warmer than the room.

 

Cooling works in just the same manner, by radiation, instead of heating it basically sucks the heat from you. So although room temps may drop by a couple of degrees the house feels way cooler and more comfortable.

 

 

[JohnMo]

32 minutes ago, SimonD said:

Cooling effectiveness of ASHP isn't that great, you may get a few degrees drop during the hottest days

You can have it working exactly the same A2A with fan coils. Just flow at 6 degs and can be as cold as you want.

[saveasteading]

25 minutes ago, JohnMo said:

Cooling works in just the same manner, by radiation,

But cold isn't a thing, it's a lack of heat,  so can't radiate. Heat is a thing. In your explanation is heat radiating from the air into the floor?

 

What temperature are the floor and the liquid circulating?

[JohnMo]

40 minutes ago, saveasteading said:

But cold isn't a thing, it's a lack of heat,  so can't radiate.

The floor becomes the cold area, so the room and more importantly you radiate heat to the floor. The water picks up the heat gain from the floor.

[mistake_not]

I'm going to replace rads with fan coils upstairs. My downstairs doesn't get too hot, so a few degrees reduction via the UFH is good enough. If I get more then bonus 😄 

[SimonD]

2 hours ago, JohnMo said:

You can have it working exactly the same A2A with fan coils. Just flow at 6 degs and can be as cold as you want.

Like I said:

3 hours ago, SimonD said:

If you're in a position where you need to drop time, money and resources on infrastructure for an ASHP to do cooling,

 

If you're going to run fan coils with a flow of 6C then be prepared not only for condensate drainage throughout, but also to very carefully lag all pipework through the house etc. So it's a lot of work to make sure the system is properly installed. 

 

[SimonD]

2 hours ago, saveasteading said:

But cold isn't a thing, it's a lack of heat,  so can't radiate. Heat is a thing. In your explanation is heat radiating from the air into the floor?

 

What temperature are the floor and the liquid circulating?

 

For radiation you need to ignore the air (as that can actually hinder radiative heating and cooling). It's the radiation of heat between different bodies. For UFH the warm floor radiates heat to our bodies and the surrounding walls/floors which then radiate the heat back into the space. A similar thing happens with cooling. When the floor temperature is reduced, the walls and ceilings, and our bodies radiate heat toward the colder surface. Hence we feel cooler (even if the air isn't necessarily any cooler). It's the same reason intermittent heat isn't very good for comfort because it heats the air not solid bodies so when the heating is turned off, it feels cold very quickly because the surfaces draw heat away from the body through radiation.

Passive cooling using radiation can be very effective but it has to be carefully designed for cooling.

 

If you happen to have the infrastructure, like UFH and your heat pump has the necessary controls/sensors for relative humidity and dew point calculation, then of course you can use it. I've had conversations with the tech people at a couple of large and well known heat pump manufacturers who, whilst they confirm the heat pumps do cooling, it's not like aircon, it will temper the room temperature. In many circumstances this is okay.

 

And the other side is finding the design resources to do the cooling side properly - if heating using heat pumps is difficult, just imagine trying to find the knowledge and skill for cooling. And I'd argue that if there is any risk of condensation forming on pipes, then detailing the system is even more painstaking than insulating for heating. 

[marshian]

4 hours ago, saveasteading said:

But in reverse, the screed will cool and we will have cool feet, and will slowly cool the adjacent air, but it will stay there. The warm air above will stay put.

 

If I'm wrong on any of this please advise.

 

In the same way houses lose heat - "Hot always moves to cold" - like cold rooms steal heat from warm rooms, the outside steals heat from a heated house - it might not be as effective as say AC but the rule is just the same - warm room - cool floor the hot is going to go to cold.