Controlling MVHR preheater

[markocosic]

It gets nippy* around these parts. 

 

The MVHR will need preheating in order not to freeze or turn into MEV:

https://www.paulheatrecovery.co.uk/components/frost-protection/

https://www.ventilation-alnor.co.uk/index/support/alnor-knowledge-base/heat-recovery/preheater-mvhr.html

 

It sometimes gets warm** too.

 

 

We're fitting a ground source heat pump.

 

I think I'd like to use the heat pump as the pre-heater. Perhaps using just the ground loop as a preheater would be a better idea.

 

I'd also like to use the ground loop as a pre-cooler.

 

 

72m2 ground floor area. 48 m2 mezzanine area. Call it 100m2 useful floor area and 100 m3/hr as the base flowrate:

http://www.passivhaustrust.org.uk/UserFiles/File/Technical Papers/2018 MVHR Good Practice Guide rev 1.1.pdf

 

The MVHR unit I have (eBay special) 500+, perhaps 300

 

 

 

System air have some calculators for duct heaters / coolers:

https://www.systemair.com/support/product-selection-tools/online-selection-tools/

 

 

When it's cold a 200-3 duct heater will do 0.9 kW of heating if fed with 10C water from a ground loop:

 

 

 

Or 1.6 kW of cooling if we assume 30C incoming air and 10/15C incoming/outgoing water from a ground loop:

 

 

These seem like useful numbers to me. (1) to stop the MVHR from freezing in winter (that's the limit on how much heat you can put in before it becomes counter productive) and (2) to take the edge off in summer.

 

 

 

Questions that come to mind:

 

- Ever seen this done before?

 

- How does one control these things? (the flow on the water side - we don't want to under/over preheat in winter or pump unnecessarily in summer)

 

- Boosting the MVHR whilst the heat pump is making hot water in summer (and the return to the loop is cooler) would be fun. Need to think about how the loop is shared between GSHP and duct preheater/cooler.

 

Assume that condensate is routed into the MVHR then to drain from there.

 

I should probably think about keeping drains ice free if the MVHR runs in winter. That's going to drip slowly. A boiler condensate auto-siphon might be an idea...

 

 

*Debatable what the cold design condition should be.

**Debatable what the hot design condition should be.

 

I'm going with needing to handle -20C winter / 30C summer whilst maintaining comfort and -30C winter without breaking.

https://weatherspark.com/y/92709/Average-Weather-in-Molėtai-Lithuania-Year-Round#Figures-Temperature

https://weatherspark.com/h/y/148554/2021/Historical-Weather-during-2021-at-Vilnius-International-Airport-Lithuania#Figures-Temperature

Warm winters might not drop much below -5

https://weatherspark.com/h/y/148554/2020/Historical-Weather-during-2020-at-Vilnius-International-Airport-Lithuania#Figures-Temperature

Cold winters can be pushing -30 

https://weatherspark.com/h/y/148554/2012/Historical-Weather-during-2012-at-Vilnius-International-Airport-Lithuania#Figures-Temperature
 

[Dan F]

Duct heaters I've seen are typically used after MVHR, not before. MVHR units often have an electrical element for frost protection, although if your have an enthalpy exchanger this isnt really needed unless it gets really cold.  The other option for pre-conditioning MVHR intake is brine loop (some MVHR manufuactuers have their own product for this).  This is good to stop MVHR cooling the house too much in the winter, but the ROI for cooling purposes is terrible if you have PV and this gives you free ASHP cooling when its hot.

 

We are using a Comfopost after MVHR and it's simply controlled as an additional heating circuit for heating and cooling. Boosting MVHR would make it slightly more effective, but heating/cooling power is rather limited regardless.  It has a condensate.

 

 

Edited February 16, 2022 by Dan F

[markocosic]

Preheater is absolutely required here because it's below zero for hours at a time for the entire heating season even in a mild winter.

 

The brine loop is already needed for the ground source heat pump so extra cost for this is near zero.

 

One can't heat with the preheater (it would defeat much of the point of the mvhr) but for cooling with the MVHR in summer bypass it's as effective as it being in the outlet.

 

How is the water side of your comfopost controlled in terms of flow rate? No control just zero or maximum?

[Dan F]

3 hours ago, markocosic said:

How is the water side of your comfopost controlled in terms of flow rate? No control just zero or maximum?

Flow temp is controlled by ASHP controller as heating circuit.  Flow rate is fixed, based on pump size and configured speed.  I had to reduce pump speed on mine.

 

If I remember correctly, i think enthalpy exchanger was good down to -17c (or was it -7C) without frost protection.

 

You couldnt use same brine loop for GSHP and paasive MVHR warmung though,  could do?  You'd have to use GSHP actively, right?  Other less popular alternative is to actually run the air intake in a loop underground before entering building.

Edited February 16, 2022 by Dan F

[Marvin]

1 hour ago, Dan F said:

You couldnt use same brine loop for GSHP and paasive MVHR warmung though,  could do?  You'd have to use GSHP actively, right?  Other less popular alternative is to actually run the air intake in a loop underground before entering building.

I think that is because of the large amount of pipe surface, which would be the cooling surface, that would be required to produce significant temp difference and the depth the pipe has to be in the ground ( about the same as a GSHP coil thing.  Oh and the pipe would have to be in a different hole to the GSHP pipe.) 

[hb1982]

Heating

Typical domestic installation would use electric heater with integrated control, to maintain off-coil temperature at 3 to 5 degC.

In your case, with hot water source, you can:

1. Buy the right heater (I supposed you can buy a 150mm / 200mm dia. circular heater easily online), plenty of option on the consumer market

2. Connect to the brine circuit

3.A simple solution is to use a TRV (Thermostatic Radiator Valve) with remote temperature probe. Put the probe in the off-coil position and to turn the set-point to minimum (may be 10 degC)?

Otherwise, you may need to install a proper valve with actuator linked with a PI (Proportional- Integral) controller 

 

Cooling

More complicated. You cannot use a heating coil for cooling. I would install a dedicated cooling coil in this instance to make your control and piping easier. 

For control, you will need a PI controller with duct temperature sensor.

 

Alternatively, you can get hold with a 4-pipe FCU (Fan Coil Unit) controller, you can use a single controller to control both heating and cooling.

[markocosic]

FYI:

 

Doing little more digging it appears that shared brine loops is "a thing" and the duct heaters/coolers referenced weren't far off the mark on sizing. 

 

Here's one with decent documentation in English including guidance on temperatures, soil types, loop sizes, flow rates etc:

http://www.ght.lv/home/

https://site-51152.mozfiles.com/files/51152/__Technical_data_GHT_2020.pdf

 

(that largely answers the question on control - do it with the pump for the brine loop and you can't really over-do it)

 

The other thing I tripped over on this research fest was Card Case Pleated Panel Filters - these are (a) dirt cheap compared with poxy MVHR filters and (b) materially lower cost

https://www.energyvanguard.com/blog/path-low-pressure-drop-across-high-merv-filter/

 

As are...pizza boxes if you want a nice insulating EPP box that you can DIY duct entries, condensate drains, and indeed pleated panel filters etc into. For those not troubled by building regs at least.

https://www.thermocateringbox.com/171-thermobox-epp

 

For everybody else there's an off the shelf heating/cooling battery and pleated filter inside a pizza box for a couple of €k. 🙂

 

 

[MikeSharp01]

Not sure I have not missed something but what stops you using your GSHP to circulate water through the GHT (http://www.ght.lv/home/) and not bother with the brine loop and presumably when needed in the summer you can cool the air that same way. You are payng some energy to run the brine loop after all. The 8KwH exchanger looks like a beast you could air condition a moderate place with that although perhaps not via MVHR. The sorts of flow rates the average MVHR can manage (Say 100m³Hr in a 3 bed Passive house) that unit would only deliver about 1kW of cooling but you might squeeze that to 2kW in boost mode. 2kW would be getting close to enough for many a passive house standard places around here but there are loads of other factors aren't there.

[markocosic]

Nothing at all stops you using the brine pump Mike - hence the interest.

 

Cooling power is higher than you imagine if you assume that incoming air would otherwise be 30C.

 

You're not delivering 100m3/hr at 17C to cool a room at 23C. You're preventing the addition of 100 M3/HR at 30C. Delta 13 not 6. Worth having if it's cooling of air that's needed not removal of solar gain in a property that is otherwise already cool so to speak.