ASHP with underfloor heating project question

[Trojan]

Hi there, am looking for a bit of advice. Planning to renovate a victorian top floor flat with ASHP underfloor heating and am trying to work out what to go for. I'd like to be heated solely by underfloor heating to maximise the space as it's only around 40m2 total (living room/kitchen, bedroom and hallway/toilet). Also intending on keeping a gas boiler for instant DHW. 

The heat pump I think I'm after is the Ecodan PUZ-wz50vha as its the only one with comprehensive specs on what I think I'll need (over 4kw at 35 deghree flow at coldest temperatures, I'm in scotland and sub zero is fairly regular in winter, and can modulate down to half that at warmer temps). I'm working off a design using a low profile screed covered UFH that should be able to output just under 100W/m2 without burning the feet.

I'm wondering what indoor unit I can pair with the pump that won't need a tank? Struggling to find anything on the mitsubishi website.

 

Cheers

[ProDave]

No 1 question, how are you going to provide enough insulation under your floor?  That might be tricky in a flat.

 

If you don't add lots of insulation and just fit some low profile UFH, you will be paying to heat the flat below you and they will be your best friends.

[JohnMo]

29 minutes ago, Trojan said:

4kw at 35 deghree flow

 

30 minutes ago, Trojan said:

only around 40m2 total

Are you correct with your assumptions you need 4kW? Have done heat loss calculations? A 100W/m2 at 35 flow isn't going to happen. Even at 100mm centres you will need 47 deg mean flow temp. It just will not work well or cost effectively.

 

Something is not right with your calculations. As said

 

21 minutes ago, ProDave said:

you will be paying to heat the flat below you and they will be your best friends.

 

[Iceverge]

I'd be tempted to keep the gas boiler and install a few slimline vertical rads. 

 

Or if you really want an ASHP just get a good A2A unit like this. 

 

[Trojan]

Thanks for all the replies.

 

4 hours ago, ProDave said:

No 1 question, how are you going to provide enough insulation under your floor?  That might be tricky in a flat.

 

If you don't add lots of insulation and just fit some low profile UFH, you will be paying to heat the flat below you and they will be your best friends.

The plan is to use 20mm XPS polystyrene on top of the floorboards/ply. I'm not too worried about raising the height it has 3 metre ceilings.

 

3 hours ago, JohnMo said:

 

Are you correct with your assumptions you need 4kW? Have done heat loss calculations? A 100W/m2 at 35 flow isn't going to happen. Even at 100mm centres you will need 47 deg mean flow temp. It just will not work well or cost effectively.

 

Something is not right with your calculations. As said

 

 

It's a rough estimate from an online calculator, stone walls. Will need to try and get a more accurate one. The system I'm looking at is schluters ceramic comfort floor, page 88 reckons you can get 95W/m2 at 4m2 loops, meaning 3 or 4 loops per room.

 

2 hours ago, Iceverge said:

I'd be tempted to keep the gas boiler and install a few slimline vertical rads. 

 

Or if you really want an ASHP just get a good A2A unit like this. 

 

 

Sure they're good just seems a bit alien to me.

[ReedRichards]

4 hours ago, Trojan said:

specs on what I think I'll need (over 4kw at 35 degree flow at coldest temperatures, I'm in Scotland and sub zero is fairly regular in winter, and can modulate down to half that at warmer temps)

 

That lack of modulation is not very good.  if you need 4 kW output at, say, -5 C outside then you need 2 kW output at 8 C (assuming 21 C room temperature) and less than 2 kW at any higher outside temperature.   

[ProDave]

44 minutes ago, Trojan said:

The plan is to use 20mm XPS polystyrene on top of the floorboards/ply. I'm not too worried about raising the height it has 3 metre ceilings.

 

Think again, at least 100mm PIR

[Trojan]

17 minutes ago, ReedRichards said:

 

That lack of modulation is not very good.  if you need 4 kW output at, say, -5 C outside then you need 2 kW output at 8 C (assuming 21 C room temperature) and less than 2 kW at any higher outside temperature.   

Are there small heatpumps with better modulation? Ive had a look at a couple different brands (vaillant and samsung) but definitely been most impressed with the ecodans datasheet. I suppose I could add a DHW tank to sap some of the wasted heat but kind of ruins the simplicity of the plan!

 

9 minutes ago, ProDave said:

Think again, at least 100mm PIR

Unless my calculations are wrong the polystyrene has an R value of 0.625, coupled with the floorboards under it I'd imagine that'll take it over 0.75 which are the "presumed conditions" for that output. 

[PhilT]

Daikin make a 3kw R32 monobloc, the Altherma 3

Edited March 17 by PhilT

[JohnMo]

1 minute ago, Trojan said:

suppose I could add a DHW tank to sap some of the wasted heat but kind of ruins the simplicity of the plan!

DHW heating doesn't really help, because the heat pump either does heating or DHW not both at the same time. Modulation isn't really too much of an issue. As long as you have enough water capacity open the heat pump will cycle, but not short cycle.

 

The biggest issue will be supporting the flow required for all the loops you have. So will be worth checking. A 4kW heat pump will have a max flow rate of about 0.7 to 0.8m3/h.

 

A couple of fan coil units may be a better and easier solution to implement.

[JohnMo]

17 minutes ago, Trojan said:

Unless my calculations are wrong the polystyrene has an R value of 0.625, coupled with the floorboards under it I'd imagine that'll take it over 0.75 which are the "presumed conditions" for that output. 

Now take your calculations to the next level and work out the downward heat loss.

[Trojan]

1 hour ago, JohnMo said:

Now take your calculations to the next level and work out the downward heat loss.

Wouldn't know where to start! The table I'm referring to though is a performance test assuming 0.75R insulation so I take it the 95w/m2 is measured from the floor covering. 

 

1 hour ago, JohnMo said:

DHW heating doesn't really help, because the heat pump either does heating or DHW not both at the same time. Modulation isn't really too much of an issue. As long as you have enough water capacity open the heat pump will cycle, but not short cycle.

 

The biggest issue will be supporting the flow required for all the loops you have. So will be worth checking. A 4kW heat pump will have a max flow rate of about 0.7 to 0.8m3/h.

 

A couple of fan coil units may be a better and easier solution to implement.

 

How can I work out the required flow rate for each loop? A quick google gives a guideline of loop length/40 but that seems overly simplistic? I would have thought the diameter of the pipe would have to be considered. 

Assuming that's close enough with 10 loops at a maximum length of 61 metres I'd have 15.25 l/s, I think the unit I'm looking at max flow is 14.3 l/s which isn't a million miles away.

[Iceverge]

I really think UFH isn't the optimum solution here. 

 

1. Too much heat loss downwards. 

2. Too high a heat loss property requires quick warm up times. This means high flow temps means bad COP or poor boiler efficiency. 

3. Disruptive to install.

 

 

 

 

[Ferdinand]

4 hours ago, Trojan said:

Thanks for all the replies.

 

The plan is to use 20mm XPS polystyrene on top of the floorboards/ply. I'm not too worried about raising the height it has 3 metre ceilings.

 

It's a rough estimate from an online calculator, stone walls. Will need to try and get a more accurate one. The system I'm looking at is schluters ceramic comfort floor, page 88 reckons you can get 95W/m2 at 4m2 loops, meaning 3 or 4 loops per room.

 

 

Sure they're good just seems a bit alien to me.

 

Door height may well be your physical issue, and sealing a space you need as much attention to ventilation as insulation. 

Try the forum's heat calculator  as a place to start written by a member which many have found helpful - including me.
 

20mm XPS in a floor (unless maybe you control the flat below as well and treat them effectively as s ingle unit) seems ridiculously low. The last one I did (bungalow) was 100mm Rockwool under the existing floor (~equivalent to EPS) + 25mm of PIR on top - and that is less than ideal.

Rather than 0.7 R-value, normally for a floor we would be looking at 0.12 to 0.25, depending on the conditions beneath.

There may be a minimum value building regulations require you to meet.

Have you thought through what you will be doing with the walls (my suggestion from rental renovation experience 50mm->75mm PIR)?

Ferdinand

[Trojan]

8 minutes ago, Ferdinand said:

 

Door height may well be your physical issue, and sealing a space you need as much attention to ventilation as insulation. 

Try the forum's heat calculator  as a place to start written by a member which many have found helpful - including me.
 

20mm XPS in a floor (unless maybe you control the flat below as well and treat them effectively as s ingle unit) seems ridiculously low. The last one I did (bungalow) was 100mm Rockwool under the existing floor (~equivalent to EPS) + 25mm of PIR on top - and that is less than ideal.

Rather than 0.7 R-value, normally for a floor we would be looking at 0.12 to 0.25, depending on the conditions beneath.

There may be a minimum value building regulations require you to meet.

Have you thought through what you will be doing with the walls (my suggestion from rental renovation experience 50mm->75mm PIR)?

Ferdinand

 

Cheers, will have a look at that tomorrow.

Think it's U value youre taking about but probably not far different, thing I'm looking at says U value - 1.33 or R value - 0.75

I can't imagine why you would need such a high heat output if you had decent insulation so I understand there is going to be heat loss but it's surely still going to be a more efficient and comfortable place to be?

 

I haven't thought about the walls, mainly because the rooms are so small I wouldn't want to compromise with that.

[JohnMo]

8 hours ago, Trojan said:

heat loss but it's surely still going to be a more efficient and comfortable place to be?

The downward heat loss is a function of the floor U value, the delta temperature of the floor and the space below multipled by the area.

 

So space below assume 20 degs, your flat temp assume 20 degs. Area 40m2.

 

So UFH assume mean flow temp is 35.

 

Your 0.75 R value, and U value is 1/0.75 so is 1.34

 

Downwards heat loss UFH

1.34 x (35-20) x 40. Is 804W, then multiple by 24 to loss per day and by 0.27 to get cost per day £5. With a heat the CoP will be around 3, £1.70

 

Radiator, fan coil etc.

Temp Delta between your flat and downstairs zero so zero downward heat loss.

 

Having UFH will cost you you around £1.70 per heating day. So about £320 a year.

 

[Trojan]

3 hours ago, JohnMo said:

Downwards heat loss UFH

1.34 x (35-20) x 40. Is 804W, then multiple by 24 to loss per day and by 0.27 to get cost per day £5. 

 

Having UFH will cost you you around £1.70 per heating day. So about £320 a year.

That makes sense, although that will only be at the lowest outside temps. According to the table, those loops will output 60w/M2 at 25 degrees flow temp which takes that down to 260W by your formula.